What is Finite element analysis via (FLAC)? And how is it done?
Finite element analysis (FEA) could run in 2D or 3D and is used for geotechnical applications in the determination of tunnels, deep foundations, slope stability, seepage analysis, soil structural analysis, and capacity for earth retaining structures (Lee, Y.S. et al, 2008). Finite Layer Analysis of Consolidation (FLAC) finds application in the determination of settlement behavior of the material as a function of time subject to either uniform torque (non-linear moments: circular) or rectangular forces that act on a surface.
The numerical analysis using FLAC results in the determination of stress, level of displacement or pore pressure subject to user-specific points and times (Lee, C.J. et al, 2006). Thus, FLAC is a function of time and specific points. FLAC 3D can contribute to the simulation of sufficient strain behavior on a structure that is supported by the soil or continuous plastic material that can exhibit plastic properties when the material attains yield limit.
Thus, FLAC finds application in the determination of geomechanical properties of continuum problems and involves tasks like excavations, backfilling and loading (Bureau, 2006). FLAC has been determined as a FAST Lagrangian Algorithm Code (FLAC) that makes use of finite difference technique to determine geomechanical problems subject to time and specific point of stress on a surface of continuum material that exhibits plastic properties. FLAC, as (Fast Lagrangian Algorithm Code) simulates a large domain of non-linear problems (Alonso et al, 2006).
Reason for 2.2-meter fluid penetration, why not less or more than that?
Fluid penetration determines the costs incurred during drilling operations. Narrow wellbore pressure profile gap increased costs subject to increasing capacity of wellbore formation collapse profile and increasing formation fracture pressure profile and vice versa (Burlingame, 2008). In practice, standard operational procedures for instance pipe connections and drill string tripping are structured and designed to ensure minimal pressure fluctuations. Pressure fluctuations have been identified to contribute to drilling problems subject to fluid penetration capacities, wellbore collapse. If the event the fluid penetration is very high, the outcome is decreased cutting transportation (Addis et al, 2005).
This however is also dependent on the drilling fluid properties. 2.2-meter fluid penetration has been determined to be adequate to manage possibilities of increased torque reading. It makes it possible to improve pack-off conditions within the annulus section as well as manage possibilities of the increasing pressure that could stimulate formation fracture and loss of drilling fluid in the formation (Burein et al, 2006).
Under Mechanism of achieving wellbore stability Finite element analysis makes it possible to determine wellbore stability when simulated production is in progress, what is the simulated production.
A simulated production is carried out through application of field stress using flat jacks and wellbore pressure. Differential pressure is utilized subject to estimated pressure requirements (Burlingame et al, 2007). A caliper mounted inside the wellbore tests flow. The flow is stopped and restarted as the wellbore is investigated with an endoscope. Any material that is produced during stopping and restarting of the flow for example sand is taken for further analysis. The far-field stresses that are applied via the use of flat jack are increased gradually until it is possible to measure and determine largest possible wellbore deformation.
The diameter is determined by either deformation point or if the system achieves the maximum capacity. The hole re-stabilization is maintained at horizontal position during the testing period. The ratio between the vertical and horizontal positions is kept at a fixed value of two. Measurements that are taken include for instance horizontal permeability and ultrasonic velocities. It is also possible to measure axial permeability that can take place at simulated in-situ conditions (Bugden & Cassie, 2003)
Wellbore instability is influenced by the interplay of, hydraulic and thermal factors
Effects hydraulic factors
Hydraulic factors that impact wellbore instability are a function of hydraulic simulation. Different hydraulic factors have been identified to affect wellbore instability. These factors include the geometry of the drill pipe and the rate at which pumping action takes place. The rate of pumping determines the quantity of the pressure exerted by the fluid on the wall of the pipes. Higher rates decrease the pressure of the fluid while lower rates increase the pressure of the fluid. Other factors that affect wellbore instability as a function of hydraulic factors include the wellbore penetration rate as well as the processes used in cleaning the wellbore.
Sustainability of the speed of drill pipes also impacts the wellbore instability. In real-time wellbore stability forecasts, hydraulic factors play a lead role in establishment of visualization that makes it possible to conduct and implement wellbore cleaning (Cala & Kowalski, 2008).
This is important during wellbore construction. Hydraulic effects impact prevention or pretreatment and correction processes or remediation. It involves pretesting of the mud. This is done before implementing drilling in especially high-risk lost circulations areas. The volume factor is more considered than weight factors. Remediation materials should be available on the site. Pre-treatment in hydraulic-based procedures is better than remediation. This helps to manage problems of lost circulations. It uses oil-based fluids as opposed to water-based fluids (Byrne et al, 2006)
Evaluation of Thermal Effects
Thermal effects affect level of fluid exposure, level of flow of the fluid and influence on the applied stresses (Byrne et al, 2006). Thermal loading affects capacity of the rock to lose heat (radiation or conduction) or the capacity to maintain heat. The ability to either dissipate or maintain heat affects the mechanical strength of the rock that in turn affects the deformation level of the rock. Thermal effects influence the physical properties of the rock through thermal conductivity properties, level of thermal expansion and hence heat capacity. Thermal effects influence thermal fracturing and the ability to model secondary recovery processes (Cai, 2008)
Grain scale Discrete Element Modeling, define and how is it done
Discrete Element Modeling is carried out by use of a Discrete element algorithm (Brummer et al, 2003). This is a numerical process that is applied towards determination of solutions in wellbore engineering. It is adapted based on shape thus either rigid or deformed. Discrete element modeling is applied where it is not possible to apply convectional continuum-based procedures due to influence of discontinuous behavior (Cala & Kowalski, 2008).
It is done by establishing a dynamic contact topology on the material that presents discontinuity. After establishing contact topology, non-linear interactions between the bodies are determined. This results in a numerical difference. Equations of motions are applied to resolve the numerical established or determined values (Lee C.J et al, 2006).
How is sidetracking going to help? Why not just get the information from the original well
Sidetracking apply in the enlarged wellbore and is determined by geological formations that are present (Barneich et al, 2008). It involves case lining primary wellbore and drilling of enlarged wellbore by use of a case window. Sidetracking helps to reduce costs of drilling because it involves use of infrastructure that is present for example any surface equipment on site (Brummer et al, 2003). Sidetracking makes it possible to eliminate opportunities and threats presented by spacing problems through additional drain holes that are made on the reservoir. Sidetracking makes it possible to extend the life of a wellbore which increases production time. Sidetracking makes it possible to drill through troubled zones subject to presence of geologic formations (Cai, 2008)
The common indicators of preeminent failure modes include vulnerability to hydraulic fractures that may be natural or synthetic, what are the natural & synthetic?
Natural fractures are caused by earth stress and depend on the state of tectonic equilibrium and direction of forces (Lee W.F. et al, 2006). Synthetic fractures arise from impact of drilling operations, poor planning of wellbore. Synthetic fractures are caused by faults of the mining process, they are man-made (Burlingame et al, 2007).
Under Rationale for maintaining wellbore stability: it says wellbore cleaning. What does that mean?
Wellbore is prone to wellbore debris that affects the efficiencies of drilling operations (Barneich et al, 2008). Debris contributes to failures of completing drilling operations which impact negatively the organization’s bottom line. Wellbore debris has been documented to increase costs in terms of work over trips, equipment failure and risks of damaged formation (Brummer et al, 2003). Wellbore cleaning, therefore, involves process of minimizing wellbore debris that increases the costs of production. Wellbore cleaning involves the integration of a cleaning device into the wellbore. The cleaning process is achieved by rotation of the cleaning device by providing the opportunity for cleaning fluid to flow axially about the wellbore wall (Bugden & Cassie, 2003). This results in the removal of wellbore debris and any deposits that might be on the wellbore wall hence contributing to cleaning of the wellbore.
How is geomechanical modeling done?
Geomechanical modeling involves processes that enhance capacity to achieve safe mud windows (Lee, C.J. et al, 2006). The process of carrying out geomechanical modeling involves the prediction of static mud weight, determination of in-situ stresses, determination of wellbore pressure gradients, determination of wellbore orientation and determination of formation material as well as determination of wellbore strength characteristics (Byrne et al, 2006). In carrying out geomechanical modeling, it is important to determine the influence of inter-granular stresses. The mechanical strength of the rock should be determined when calculating the inter-granular stresses because both have the capability to affect wellbore instability hence are important factors to consider when carrying out geomechanical modeling.
Geomechanical modeling utilizes wellbore stability simulators that have capacity to respond to time-dependent instability incidents (Cala & Kowalski, 2008). Geomechanical modeling through wellbore stability simulators provides important data on mechanical, chemical and thermal characteristics of the rock.
Under advantages of wellbore stability
Reduction of waste production
Drilling operations result in production of waste that could contribute to environmental degradation (Ahmadi et al, 20007a). The wastes are mainly used drilling fluids, sometimes referred to as mud and drill cuttings. The wastes of drilling operations could be disposed of onshore. Onshore disposal of drilling wastes is carried out by removing drilling fluids and liquids from the drill or material reserve pits and implementing pit burial (Li & Aubertin, 2008). This results in low costs of disposal but is subject to regulations in a country on drilling waste proposals. For instance, environmental regulations don’t allow pit burial in areas where water table is high. In offshore operations, wastes are disposed of through injection into salt caverns, plugging or injection using pressure that is greater than fracture pressure (Bugden & Cassie, 2003).
Waste production is an indicator of lack of efficiencies in a process. Waste production is managed through reduction of projects activities based on environmental footprints. Reduction of waste production involves strategies like improving efficiencies and recycling materials (Clark, 2006). Recycling processes of drilling and wellbore stability processes are structured towards decreasing environmental impacts of the mining process. Waste production reduction is strategically aimed at achieving a higher score based on environmental performance indicators. Mining and drilling operations should be targeted at decreasing opportunities for solid and waste production (Cordova et al, 2008).
Reduction of waste products as part of environmental management standards contributes to decreasing of environmental footprints of drilling operations through adoption and implementation of best practices in waste disposal.
The process of strategic waste production involves ongoing review on processes applied in reduction of waste production, selection of appropriate methods and auditing of the processes to ensure processes involved in handling wastes demonstrate conformity to ethical procedures and standards (Cundall, 2006). Reduction of waste production ensures improvisation of waste production processes are implemented for instance in the event there is a deficiency of infrastructure; the drilling company cooperates with communities and customers to identify handling methods that are appropriate (Burlingame et al, 2007).
Water use in the reduction of waste production
Water has been employed in the reduction of waste produced during ongoing drilling operations. Water is used in form of Water-Based Mud (WBM). The constituents of WBM include water, clays and chemical additives that result in formation of a homogeneous blend that has various degrees of viscosity (Ahmadi et al, 2007b). The clay component is termed shale and has characteristics of a rock. The primary constituents of shale include clays that are suspended on drilling fluid, For instance Bentonite that is commonly termed as a gel (Burlingame et al, 2006). The properties of gel include capacity to achieve free-flow state when it’s being pumped.
Upon cease of pumping, the static fluids generate a gel structure that decreases viscosity and free-flow form decreases. The viscosity is increased by pumping action. Viscosity is changed through use of potassium formate that has both characteristics of ionic and non-ionic (Alexandris et al, 2006). Potassium formate has the capacity to form basis for control of fluid viscosity. It has capability to be used to control shale stability and improve the rate of drilling as a function of penetration as well as impact positively on the cooling and lubricating property of the drilling tools.
The role of seal permeable formations
The seal permeable formations are initiated by exceeding the formation pressure by the mud column pressure (Cundall & Detourney, 2008). The mud filtrate interferes with the formation which results in deposition of filter cake on the wellbore wall. The primary role of the mud is to contribute to formation of a low permeable filter cake that has capacity to restrict invasion (Borcin et al, 2006).
The process of formation of a filter cake is instrumental in influencing tight wellbore condition characteristics. A thick filter cake has been documented to contribute to insufficient logging quality as well as predisposing inability to attain accuracy when carrying out wellbore real-time stability forecasts. Thick filter cake has been identified to contribute to incidents of stuck pipes that impact negatively on wellbore stability and predispose vulnerability of drilling operations into lost circulations. This has impact of influencing formation damage. If there is vulnerability to highly permeable formations, the outcomes contribute to large pore throats that predispose invasion of formation.
This is dependent on the size of the solids. The management best practices include the use of bridging agents that have the capacity to block large openings which results in the formation of seals (Brummer et al, 2003). Use of bridging agents should be determined by proportion of the formation fractures in terms of size. Calcium carbonate has been documented as efficient bridging agent. Alternatively, ground cellulose could be used as a bridging agent.
Stabilization of temperatures
Thermal conditions influence wellbore stability. This occurs through differences in temperatures between the drilling fluid and formations that have the potential to vary near-wellbore formation temperature (Bourdeau, 2006). The non-isothermal property has capability to impact negatively on the wellbore stability during drilling operations. Control and regulation of temperature are important in managing near borehole pressure distribution. Increase in temperature influences efficiencies of cooling drilling fluids hence the stability of the wellbore (Bathurst & Zarnani, 2008). Temperature impacts on relative permeability of oil-water-based systems.
Stabilization of temperature is important in determination of wettability conditions, appropriateness of the brine chemistry applications, clay content and its efficiencies in wellbore stability and mechanism they impact on reservoir pressure. Temperature stabilization is important element due to its capacity to impact relative permeability, saturation and viscosity ratio (Castillo et al, 2006). Temperature affects different permeability like absolute permeability, effective permeability, relative permeability, relative permeability ratio and fractional water flow.
Prevention of hydrate formation
Hydrate formation impacts wellbore stability. As a result, in the event the formation pressure rises, the density of the mud should be increased proportionally. This contributes to the achievement of an equilibrium state and enhances wellbore stability (Castellani & Valente, 2006). In addition, formation pressure and mud density should be controlled and managed within specified equilibrium conditions. The control ensures there is minimal influx of pressure that may predispose wellbore blowout. Hydrate formation and its preventions should be based on mathematical calculations of hydrostatic pressures (Cassani & Mancinelli, 2006).
Hydrostatic pressure is mathematically expressed as a product of density, height and acceleration due to gravity. Since gravity and acceleration due to gravity are contented, hydrostatic pressure is inversely proportional to the height. The rationale of seeking to achieve an equilibrium state between formation pressure and density of the mud is based on the fact that if the hydrostatic pressure exceeds the formation pressure, the formation fluid cannot flow into the wellbore. The objective interest is to achieve wellbore control hence being able to ensure there is no uncontrollable flow of formation fluids into the wellbore (Clark, 2006).
Managing hydrostatic pressure is also important because it contributes to the management of stresses that are brought about by tectonic forces. Tectonic forces or tectonic movement contribute to wellbore instability. This can occur even when the formation fluid pressure is at equilibrium and is balanced (Cassani & Mancinelli, 2006). In the event it is observed that the formation pressure is sub-normal, elements like air, gas, stiff form pr low-density mud could be used to mitigate any instability problems. The density of the mud should be monitored since it contributes to the fracture of the formation.
The rationale for maintaining wellbore stability
The rationale for maintaining wellbore stability lies in the determination of chemical composition and mud chemical and physical properties (Carlgon & Golden, 2008). Chemical composition and mud properties impact of wellbore stability. The mud weight should be determined and set at a point where it should balance mechanical forces. Wellbore instability presents problems in wellbore cleaning (Leem et al, 2005).
The objective interest of determination of wellbore stability is to attain sustainable size and cylindrical shape of the wellbore that could contribute to wellbore stability and reduce opportunities of wellbore instabilities (Cargon & Golden, 2008). In cases when the wellbore increases in size, the strength of the wellbore decreases. This makes it hard to stabilize the wellbore and creates the environment for low annular velocities, insufficient wellbore cleaning, problems in solid loading and insufficient formation evaluation and capacity to attain real-time wellbore forecasts (Leelasukseree et al, 2005).
“This implies any possibilities of a micro-crack of the rock because of influence of tensile failure of shear failure is attributed to concentration of stress”. (but rocks are bound to crack! How will that influence the stability?)
The lime-based mud (LBM) has been used in drilling carbon dioxide-containing formations as well as enhancing wellbore stability(Burlingame et al, 2006). Lime influences wellbore stability through change of the rock strengths subject to change of the rock chemistry. This is initiated by the chemical composition of the drilling fluid. Tests that have been conducted based on hot rolling dispersion tests and triaxial compressive tests have determined that shales that had been treated with lime continued to harden as time increased (Button et al, 2006).
The increased strengths of the rocks were determined to have been proportional to decreasing cationic exchange capacity of the rock, change in chemical composition of the rock, and reactivity of the shale with water (Bureau, 2006). Strength of a rock therefore depends on the changes in chemical composition. Lime-based mud (LBM) has capacity to decrease wellbore instability and increase wellbore stability.
Calcium hydroxide forms insoluble salts as opposed to potassium and Sodium. This results in operational problems that are characterized by increased fluid loss and eheological loss of control (Andrianopoulos et al, 2006). Lime and Sodium hydroxide have the capacity to decrease the performance of filtrate decreasing additives for instance like the carboxymethyl cellulose, starch-based and lignites based as well as some flocculates drill cuttings that might impact negatively on the viscosity and impact negatively on the quality of the filter cake (Caceres et al, 2006).
Sodium hydroxide or potassium hydroxide decreases concentration of soluble calcium hence increasing alkalinity. The rationale of increasing alkalinity is based on the capacity of increased alkalinity to destabilize wellbore. More reactive potassium hydroxide is therefore more favored to sodium hydroxide as a solution for controlling lime mud alkalinity (Cardozo & Cuisiat, 2008). Alkaline ions contribute to exchange of cations that have capability to stabilize shales.
Constituent of apparatus used to test wellbore stability
- Means for holding prismatic shaped rock sample
- Means of controlling pressure and temperature of the sample (thermostated conditions)
- Means of exposing a face of the rock sample that is not subjected to axial loading to a drilling fluid
- Means of generating ultrasonic signal associated with the sample
- Means of measuring transit tome of the ultrasonic signal so that it travels through the sample
- Means of generating a measurement signal representative when the sample is exposed to drilling fluid
Wellbore stability problems prevalence
Wellbore stability problems are experienced when carrying out oil or gas drilling operations (Barla, 2008). Incidents of wellbore instability arise when a wellbore is exposed to conditions that increase the probability of wellbore collapse. Wellbore collapse has been identified to contribute to the sticking of the pipes, as well as prevalence of unstable formations. It can also contribute to wellbore slough caving. Wellbore instability is documented to contribute to increasing in the diameter of the wellbore.
The primary documented causes of wellbore instability hence predispose wellbore formations are shales and mudstones (Antonuo, 2006). Shale is defined as a relatively impermeable formation that is made up of fine-grained sediments and reactive clays (Barbero et al, 2007). Shale formations emerge from low-strength rocks that have undergone plastic deformation.
Basis of evaluation of shales
- To determine types and amount of clays in the formation
- To determine the degree of dispersion of unstressed cuttings when the cuttings are exposed to mud
- To determine unstressed swelling of the shale when it is exposed to fluids
Competitive disadvantage of shale evaluation
The evaluation techniques for the shales do not determine rock mechanical properties hence cannot be used to predict wellbore stability (Burlingame, 2008). As a result, the evaluation cannot be relied on as effective method of determining and predicting drilling fluid that should be used or potential to determine delay.
The Triaxial compression test
Evaluation of wellbore stability as a function of mechanical strength of the rock is effectively carried out by triaxial compression tests (Cundall, 2006). In order to conduct triaxial compression tests, it is important to make use of a jacketed cylindrical sample. It should be noted that the rock sample should be axially loaded concurrently as the pressure is applied. The mechanism through which the testing is achieved may involve lack of control of rock pore fluid pressure (Dai, 2008).
This constitutes undrained compression tests. The process is termed an undrained test if the axial compression test is controlled. For all practical situations, the axial compression test is conducted while pressure is confined at a constant numerical value (Callisto et al, 2008). This makes it possible to determine the rock’s maximum axial distortion.
How do magnetic properties play a role in recovering the lost circulation?
The level of magnetism of a material determines its capacity to be recovered. Lost circulations are recovered based on their magnetic properties (Cala & Kowalski, 2008). The lost circulation is passed over a magnetic conveyer belt that has variable magnetic strengths. The materials that have highest magnetic properties are recovered first followed by materials that have lower magnetic properties (Cai, 2008). The magnetic conveyor does not recover materials that lack magnetic property unless static charges are induced into the materials.
LWD, DTP, DTS stand for?
LWD is the abbreviation for Logging-While-Drilling, DTP is the abbreviation for Digital Test Point while sometimes it may refer to Distal Tingling on Percussion and DTS is the abbreviation for Diametral Tensile Strength, other terms that are used include DU (Diagnosis Undetermined), DTC (Digital Test Controller) and DUS (Distinctness Uniformity and Stability (Cala et al, 2006).
LWD has contributed to the transformation of rock mechanical analysis and wellbore stability analysis into wellbore real-time forecasts and rock mechanical real-time analysis domain (Caceres et al, 2006). The transformation into real-time forecasts has been achieved by use of LWD tools. The use of LWD in real-time forecasts of rock mechanical analysis is structured on sonic logging that is achieved through use of compressional and shear slowness (Kumar et al, 2008).
The compressional and slowness sonic logs make it possible to determine and quantify wellbore stability situations in real-time. LWD has been important in withdrawal of use of wireline logs in wellbore. LWD provides equivalent quality information of wellbore stability and rock mechanical analysis in real-time (Lee, Y.S. et al, 2008). As a result, LWD has made it impossible to eliminate post-drilling analysis of rock mechanical analysis. The Real-time forecasts of wellbore stability and rock mechanical analysis rely on sonic data as a function of DTP and DTS (Lee, W.F. et al, 2006).
Drilling induced features, what are the features?
Drilling-induced features are also termed as drilling-induced characteristics that provide information on rock mechanical properties and wellbore stability measurements. The characteristics are observed as the drilling progresses (Lee, C.J. et al, 2006). For instance, when drilling deeper and under narrow-margin conditions, the mud weight design requires to be maintained between pore pressure or within shear limit (Leem et al, 2003).
It has been observed that a simple deviation in fracture gradient or alternatively a decrease in shear failure limit could have negative impacts of decreasing the number of casing strings. This is important in offshore operations. The concept of Controlled breakout is utilized in order to decrease opportunities of mud weight impacts on wellbore stability that can be determined and assessed through use of a BIAS system (Latha, 2006). Controlled breakout is managed in order to achieve manageable wellbore stability through determination of break-out geometry.
The BIAS system is used to provide real-time data on breakout that is generated by different mud weights. This facilitates in selection of the right mud weight based on mud weight density (Kutter et al, 2008). The BIAS system makes it possible to determine accurately breakout areas and make it possible to achieve Total Depth (TD) and ensure sand conditions that could contribute to high proportions of mud losses are determined and managed timely (Kumar, et al, 2008). It also ensures challenges of breakout areas don’t contribute to differential sticking and opportunities of sidetracking or contribute to abandonment of the good drilling operations.
The role of contour plotting
The BIAS system is also applied in wellbore trajectory analysis. Trajectory analysis of a wellbore plays an important role in the selection of mud weights that conform to the specific trajectory contour plotting (Cordova et al, 2008). The BIAS system makes it possible to create contour plots for trajectories hence the capacity to select appropriate mud weight and hence the determination of drilling direction (Li & Aubertin, 2008).
The BIAS system also finds applications in the determination of wellbore location of fractures, the size of breakout and location of the breakout as well as accurately determining the required strength that conforms to mechanical strength of the rock. The outcome contributes to wellbore trajectory analysis and direction determination as a well as in-situ stress directions (Byrne et al, 2006). The BIAS system also finds applications in designing under-balanced mud weights hence the formulation of wellbore stability parameters, depleted formations impact analysis and determination of rate of penetration (ROP) as well as formative assessment and evaluation (Clark, 2006).
The competitive advantage of the BIAS system
The BIAS system has the potential to calibrate the Geo-mechanical model. This is realized through simulation of drilling-induced characteristics or breakout areas as provided by the images logs (Barneich et al, 2008). It also depends on wellbore ovality and any predisposed wellbore instability incidents. Analysis of drilling reports for example presence of tight wellbore, possibility of stuck pipes and incidents of mud looseness due to variation of viscosity and depending on drilling fluid used (or properties of the WBM). The BIAS system provides global images of the calibration process as a function of simulated and observed features (Barla, 2008).
This provides a continuous depth profile. This makes it possible to identify information on NPT and Net present Value (NPV) hence mechanism through which lost circulation could be recovered. The BIAS system has capacity to be applied in both under-balanced mud weights and convectional or over-balanced mud weights (Arunakumari & Madhavi Latha, 2008).
The common indicators of preeminent failure modes include vulnerability to hydraulic fractures that may be natural or synthetic, what are the natural & synthetic?
Failure modes and fractures in a wellbore occur in the direction of least stress. If for instance the fracture arises from a single injection, the resulting shape is vertical plane (Antonou, 2006). If the pressure is continually applied to the formation, the fracture acquires horizontal and vertical vectors and hence grows horizontally and vertically away from the initial point of injection. In the event injection cycles are evident, series of vertical fractures result subject to different azimuths or angles on the wellbore.
Perforated or slotted production liner
These are designed to manage dogleg problems (Arunakumari & Madhari Latha, 2007). The slotted production liners find application in geothermal, water and environmental testing of the wellbore. Thread casing has also been documented to be important (Alonso et al, 2006). Filtration is the rationale of utility of perforated and slotted production liners. The slotted or perforations make it possible to pull out oils and leave sand and other drilling cuttings.
Tubular slotting
The tubular slotting should be designed to meet the required pipe strength and make it possible to achieve efficient drainage and filtering tasks (Ali & Bradshaw, 2008). The tubular slotting is developed in order to reduce opportunities of excess external pressure that contributes to varieties of wellbore failure modes. The tubular slotting makes it possible to manage costs that arise from use of long tubular in a wellbore hence achievement of a cost conservative design through use of tubular slotting (Alexandris et al, 2006). The selection of tubular slotting should be based on the capacity to manage pre-collapse elastic ovalization or plastic collapse via plastic hinges.
Rising doglegs
The rising doglegs present drilling problems. The wellbore should optimize on maintenance of verticality of operations (Burein et al, 2006). There should be design to manage BHA configuration and capacity to achieve low deviations. Doglegs increase the costs of drilling and increase the time spent drilling the wellbore. Doglegs create damage to drill strings and lining of the depth (Bureau, 2006). Doglegs present problems in completion of tubular which have been associated with increased costs and remedial drilling operations. The mechanism for maintaining verticality should be quantified through sustainability of wellbore stability and perforation tunnel stability. Perforations should not be implemented in the direction of maximum stress. There is requirement for conducting detailed design planning that should be supported by adequate geomechanical data (Callisto et al, 2008).
Initial planning phase
The initial planning phase should seek to establish wellbore stability and instability measures and approach management subject to management of high dogleg wells, extended reach drilling, wellbore that might have high HED opening area around 1.3 to 1.5 times pilot wellbore diameter, slim wellbore applications, salt drilling sections, for instance, salt entry and salt exit as well as kickoffs, prevalence of wellbore with inter-bedded sand and shale sequences that might affect wellbore stability and probability of utility of past drilling experiences (Castillo et al, 2006).
Wellbore real-time stability forecasts
Wellbore real-time stability forecasts are carried out by using LWD technology. LWD technology makes it possible for the wellbore operators to implement well placement accurately while at the same time conducting analysis of log data that predicts any drilling problems that may arise (Castellani & Valente, 2006). Wellbore real-time stability forecasts are important when dealing with horizontal and high angle wellbores subject to vertical and slanted geometries that require ongoing formative assessment and evaluation.
The complexity of the wellbore design
The complexity of the wellbore design can increase wellbore instability during drilling operations (Cargon & Golden, 2008). This requires ongoing wellbore real-time forecasts and control in order to obtain real-time wellbore stability and implement accurate wellbore placement. It is important to optimize drilling and geo-steering through use of LWD measurements. The drilling and Geo-steering optimization depend on LWD Azimuthal measurements capacities.
Azimuthal acquired data results in vital information on different sections of the wellbore (Cordova et al, 2008). The azimuthal data facilitates decision-making on the efficiencies of the drilling process. The azimuthal data is transmitted through mud-pulse telemetry. The mud-pulse telemetry makes it possible for the data to be interpreted throughout the world through use of secure internet communication technologies (Cundall & Detourney, 2008).
Role of real-time data on wellbore stability
Real-time images make it possible to make timely decisions on drilling operations. The data that is stored post-real-time wellbore stability forecasts is utilized as a reservoir for characterization and geological evaluation hence capacity to develop appraisal (Dai, 2008). Real-time wellbore stability forecasts contribute to the refinement of drilling operations hence the capacity to confirm or update predicted mechanical earth models and applications on drilling processes.
This makes it possible to conduct a comparative analysis between prediction and reality hence developing framework for remedial actions that need to be put in place before targeted zones are reached. Real-time wellbore stability forecasts make it possible to understand reservoir geology and petrophysics as well as geomechanics of the reservoir rock (Lee, C.J. et al, 2006). The geomechanical data provides further analysis that is essential towards optimization of wellbore planning and mud weigh designs.
Breakout areas
Breakout areas are determined by using Logging While Drilling (LWD) Caliper measurements (Kutter et al, 2008). The LWD caliper measurements utilize three ultrasonic traducers. These measure the standoff that exists between the logging tool and the wellbore wall. An eclipse could be fitted into the ultrasonic traducer elements. This paves way for the determination of maxima and minima axes of the eclipse (Lee, W.F. et al, 2006). The logging while drilling facilitates in establishing the foundation of the orientation of the axis as well as provision of data on magnitude of stress as a function of resultant force emerging from vector intersection points between long axis and direction of minima horizontal stress that form basis for development of three-dimensional caliper images after logging has been carried out.
Breakout and impacts on natural fractures
Natural occurring fractures contribute to drilling mud loss and expose drilling into wellbore stability challenges. The natural fractures are managed by use of double porosity model (Cala et al, 2006). Double porosity model has been documented to have impact on increasing safety and saving costs. Deep drilling that are constrained by narrow margin conditions contributes to a scenario where the design mud weight has to be maintained at an equilibrium point between shear failure limit or wellbore pressure limit and probability of design mud weight losses (Addis et al, 2005).
It has been observed that deviation from the equilibrium point results in decrease in number of casing strings subject to especially deviation of shear failure limit. Controlled breakout however reduces chances of design mud weight loss, improves wellbore stability and contributes to strengthening of the wellbore and breakout geometry even when the equilibrium state cannot be maintained.
Wellbore cleaning design
The cleaning design of a wellbore should satisfy probability of the drilling mud being thixotropic by achieving gel characteristics under static conditions (Bourdeau, 2006). The thixotropic characteristics contribute to the suspension of cuttings when the mud. Cleaning of the wellbore should be structured to meet drilling fluids that demonstrate increased shear thinning characteristics or demonstrate increased viscosities (Bathurst & Zarnani, 2008).
Cleaning should utilize elevated viscosities of fluids. Studies have demonstrated that increased annular velocity improves cutting transport hence an important factor to consider. The design of wellbore cleaning should meet the required fluid density that should be used. Use of higher density fluids results insufficient cleaning of wellbore even under low annular velocities (Alonso et al, 2006).
Low annular velocities have been documented to have higher cleaning efficiencies through their capacity to increase buoyancy force that acts on suspended cuttings. The primary disadvantage is the mud is used in excess such that it doesn’t contribute to the balance between the formations pressures (pressure surrounding the roc). As a result, the mud weight should not be increased when carrying out the cleaning of the wellbore (Ali & Bradshaw, 2008). The requirement for high rotation is subject to rotary drill-string speeds that contribute to circular or centripetal components to annular flow. This results in a helical flow (helix-based flow) about the drill string. This has effect of contributing to insufficient cleaning of the wellbore.
Geomechanical modeling outputs
Geomechanical modeling provides information that helps in the determination, assessment and evaluation of wellbore stability. These efficiencies of control and monitoring of wellbore stability therefore depend on capability to generate geomechanical modeling output features. Geomechanical modeling outputs depend on the workflow of geomechanical model that is used in wellbore stability analysis (Arunakumari & Madhavi Latha, 2007).
The geomechanical modeling outputs have been identified to provide necessary data on minimum required mud weight (MRMW). In addition, geomechanical modeling outputs have been determined to provide data on unconfined compressive strength (UCS) for wellbore drilling and provide nature of the shales. Limitations in geomechanical knowledge make the drilling process difficult through problems like borehole collapse, inability to determine stuck pipes or lost circulation. It is also difficult to determine sidetracking (Li & Aubertin, 2008). This could result in delays hence increased costs of drilling.
Failure to conduct geomechanical modeling results in the inability to determine geomechanical rock strengths or in-situ stresses. Through geomechanical modeling outputs, it becomes possible to determine unconfined compressive strength (UCS), Poisson’s Ration (PR), Young’s Modulus (YM) and In-Situ Stress Magnitude (ISSM) and In-Situ Stress Direction (ISSD). These elements UCS, PR, YM, ISSM and ISSD are used in wellbore stability analysis. Data that is generated from geomechanical modeling can be generalized and used in a similar site or in drilling operations that are near the site where the Geomechanical modeling was determined and established.
Geomechanical modeling makes it possible to determine high overpressures and weaknesses of rocks and estimation of behavior of mud weights relative to drilling procedure to be used (Leem et al, 2005). Geomechanical modeling output has been documented to contribute to mitigation of challenges that contribute to failures of wellbore or propagation of fractures and direction of fracture propagation relative to the wellbore.
How are wellbore real-time stability forecasts done?
The principal focus of wellbore stability analysis is to achieve wellbore real-time stability forecasts that make it possible for the drilling operations expected challenges to be determined and approaches for the management of the formation variations to be timely determined (Cundall & Detourney, 2008). The primary objective of wellbore real-time stability forecasts is to achieve effective monitoring of drill out of float equipment, determine shifts in shoe track and identify variation of shoe casing. Shoe casing, shoe tracking and drill out float apparatus are determined in order to ensure sufficient downhole weight on the bit and corresponding torque moments measurements are optimized as drilling operations progress (Latha, 2006).
Real-time wellbore stability forecasts are carried out to ensure information on indicators for opening or closing of the wellbore enlargement device is documented and efficiencies assessed. Forecasts on wellbore real-time are done to determine bending moments, corresponding weights acting on the bit curves and achieve sidetrack monitoring via determination of bending moments and near bit inclination (Lee, W.F. et al, 2006). It is also done to determine spotlight impacts on the wellbore vibrations in real-time hence making it possible to mitigate wellbore vibrations that might impact wellbore stability.
Real-time wellbore stability forecasts initiate with differential pressure measurements that are meant to determine the opening and closing of the wellbore opening device (HED) located on the surface. Thus, real-time forecasts determined the current status of the HED thus whether it’s in close or open position (Kutter, et al, 2008).
The equipment that determines wellbore real-time stability forecasts has two pressure sensors. One of the pressure sensors is located in the wellbore hence internal and the other is external thus located in the annulus of the equipment body. An algorithm that is integrated into the system determined the difference between the internal and external and reports it as the differential pressure (Barneich et al, 2008). This is followed by determination of a basement measurement when the HED tool is in a closed position. After the HED is set to open position, a flow bypass results that results in decrease in the differential pressure. This makes it possible to determine status of the HED on the BHA.
It is important to have real-time data to decrease opportunity of losing the BHA in the wellbore through wellbore instability that might contribute to wellbore pack off and wellbore kicks (Brumer et al, 2003). This makes it possible to determine correct mud weight windows, wellbore pressure prediction, possible wellbore instability and practices that could be implemented to contribute to wellbore cleaning. The data achieved from wellbore real-time forecasts contribute to the determination of ECD values, and prediction of possible wellbore spiraling.
The strain gauge is used to determine quantifiable bending moments that facilitate in determination of wellbore drilling conditions hence information on status of wellbore stability (Bugden & Cassie, 2003). The bending moments provide vital data on micro-doglegs that might be prevalent in the wellbore trajectory. Micro-doglegs provide important data on the best practices for wellbore drilling processes and the efficiency of the implemented drilling procedures.
Wellbore real-time stability is also determined by use of mechanical specific energy (MSE). The MSE is based on law of conservation of Energy (CoE) hence ideal conditions should contribute to a state in which power input and power output are equal. Differential measurements provide losses in power input which makes it possible to determine power output hence predicting efficiency (Cassani & Mancinelli, 2006). This relies on supplied power measured in terms of WOB, RPM, and torque at the surface.
How to get earth stress analysis?
Earth stress analysis is an important element to determine when planning to conduct drilling operations (Kumar et al, 2008). Earth stress analysis makes it possible to determine the stability of earth and possibilities of influence of stresses on tectonic plates and mechanism of the tectonic impacts on wellbore stability and instability. Earth stress analysis can be implemented through studies on soil mechanics that depend on laws of statics and theory of elasticity (Cundall, 2006).
Earth stress analysis provides information on equilibrium of forces on different planes, level of symmetry and asymmetry of the forces on the planes, and mechanism different forces intersect. Theory of elasticity provides the basis for determining infinite and finite nature of the rocks.
The principle of elasticity however does not produce reliable data on earth stress analysis if conducted on irregular slopes and soil strata that have varying geomechanical properties (Cargon & Golden, 2008). This makes utility of law of statics on earth stress analysis a better approach.
Earth stress analysis relies on the Taylors theorem on statics of the stability of slopes. Use of Taylor’s Theorem involves the resolution of applied and resisting forces that act on the surface. Resolution should contribute to the determination of actuating forces like total weight, resultant neutral hydrostatic forces, and uplift forces (Castillo et al, 2006). The resultant forces help to determine the direction of all forces acting on the surface and are the sum of vector forces acting on the surface. Static laws make it possible to calculate stress and conduct analysis on the deformation of earth structures.
Software like SIGM/W makes it possible to conduct earth stress analysis on linear elastic deformation and non-linear elastic deformations. The results of earth stress analysis make it possible to model wellbore water pressure and possible dissipation that arise from the soil structure subject to response or reaction to exerted loads or weights.
Onshore drilling is affected more by the presence of deposits of gas hydrates and pore structure fractures that impact negatively on the wellbore stability, why is that?
The stability of gaseous hydrate depends on pressure and temperature. Concentration improves stability of a gaseous hydrate (Barla, 2008). The stability of gaseous hydrate offshore is higher than onshore because offshore, the concentration of gaseous hydrate exceeds the solubility of the gas. For instance, solubility of methane water (mixture of carbon dioxide and hydrogen) that is also termed as water gas, is documented to be 0.85% solubility in freshwater (Ali & Bradshaw, 2008). The solubility of methane gas onshore is not documented and is presented as the solubility of the methane water at atmospheric pressure and temperature which is negligible.
As a result, solubility factors influence the stability of gaseous hydrates. In onshore operations, gaseous hydrates have contributed to abandonment of drilling operations when they are penetrated before a blowout preventer is installed (Hart et al, 2008). The gaseous hydrates, due to instability in onshore drilling operations, may contribute to low-density fluids based on the principle of Archimedes. This has called for the requirement to conduct seismic evaluation before conducting drilling. In offshore, gaseous hydrates are not a threat because destabilization of the gaseous hydrates requires energy and time.
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