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Wellbore stability definition

Wellbore stability refers to sustainability of the borehole from falling or breaking down and is achieved through use of drilling fluid programs, casing programs and efficiencies of drilling operating procedures (Simangunson et al, 2006). Wellbore stability is achieved through identification; adoption and implementation of best practices (cf. Yongfeng et al, 2009). Sustainability of wellbore stability is influenced by appropriateness of data while drilling operations are in progress. Effectiveness of wellbore stability and predictions depends on physical properties of the rock, present stresses and pore pressures and variability of the drill model prediction (Gisolf et al, 2009).

The wellbore stability is influenced by controllable factors and natural factors (uncontrollable factors).

The engineering controllable factors that influence wellbore stability

The mud density and prevalent bottom hole pressure
Inclination and Azimuth of the well
Transient pore pressures
Chemical properties of the rock and subsequent rock-fluid interaction
Physical properties of the rock and subsequent rock-fluid interaction
Vibration level of the drill string
Thermal temperatures
Absence or presence of erosion

The engineering natural factors that influence wellbore stability

Incidents of Faults formations
Incidents of Natural fratures on rock
Incidents of increased Stresses especially high-in-situ
Incidents of increased Stressed formations for instance tectonic
Prevalence of Formations that are unconsolidated
Prevalence of induced over-pressured shale collapse
Prevalence of naturally over-pressured shale collapse
Possible prevalence of mobile formations

Mechanism of achieving wellbore stability

Wellbore stability (Weinheber et al, 2010) could be achieved through conducting finite element analysis. Finite element analysis via FLAC makes it possible to determine factors that affect depth that a fluid penetrates. The depth that a fluid penetrates is important considerable factor when carrying out horizontal drilling (cf. Cantaloube et al, 2010). Finite element analysis makes it possible to determine wellbore stability when simulated production is in progress.

Wellbore stability could be achieved by controlling coal fine content (Kristiansen, 2004) that is generated when drill bit action carries drilling fluid into cleat system. This results into plugging of the permeability pathways and contributes into formation damage.

In the event wellbore develops instability subject to pressure drawdown (Estep et al, 2010), a perforated or slotted production liner could be used. The depth of fluid penetration should be maintained at 2.2 metres (cf. Chen et al, 1998). The 2.2 metres is recommended if there are no opportunities for filter cake formation (Weinheber et al, 2010). Finite element analysis via SLAC has demonstrated a small diameter for instance 5 centimetres has a higher preference subject to its contribution in wellbore stability.

Wellbore stability (Hawkes & McLellan, 1997) that is negatively influenced by lost circulation and differential sticking should be achieved through pulling of the drill pipes. This provides opportunity for running casing. There should be management of annulus and frictional pressure losses that influence of effective circulating density (ECD). Management of ECD (Zhang et al, 2006) results into management of possible lost circulation. As a result, lower mud weights are recommended since they make it possible to achieve required bottom hole pressure that contributes into sustainability of the wellbore stability.

Advantages of wellbore stability

Decreased damage to formation due to low reactivity of the water
Minimization of clay sweeling and wellbore stability problems
Stabilization of temperatures
Reduction of waste production
Improved rate of penetration and efficiencies of circulation
Increased lubrication that decreases chances for friction
Prevention of hydrate formation

Disadvantages of wellbore stability

It increases costs
Increases time

Wellbore instability

Wellbore instability (Nguyen et al, 2007) is defined in terms of nature or mechanism the rocks respond to natural or induced overpressures about the wellbore when drilling operations are in progress. The response of a wellbore depends on concentration of stress. Wellbore instability (Chen et al, 1998) differs based on formation damage and level of interaction of the drilling fluid. Thus, wellbore instability is influenced by interplay of mechanical factors, chemical, hydraulic and thermal factors (cf. Hawkes & McLellan, 1997).

Management of wellbore instability ought to consider a rock as a continuous material which means; failures that occur in a wellbore and affect wellbore stability should be taken as single initial failure point. While consideration for single initial failure point ought to be put into account (Maury & Sauzay, 1987), the rock ought to be considered as a discontinuous material and is subjected to stress. This means, instability of a wellbore arises due to single point failure (Hawkes & McLellan, 1997).

Despite influence of single point failure as contributor to wellbore instability, the wellbore should be capable of accommodating casing controls (Gaurina-Medimurec, 1994) and possible installation of a downhole apparatus. This is in the event of considering possibilities of noncircular and irregular layout of the rock subject to discontinuity or continuity of the materials. Wellbore instability (Maury & Sauzay, 1987) is managed well through grain scale Discrete Element Modeling (DEM).

DEM takes into account the discontinuity or continuity of the rock material. DEM is achieved through rock modeling as a function of grains that are bound by cement like content and there exists pore spaces between the grains (Gauriona-Medimurec, 1994). This implies any possibilities of a micro crack of the rock because of influence of tensile failure or shear failure is attributed to concentration of stress.

Causes of wellbore instability

Wellbore instability (Nguyen et al, 2007) is classified on the basis of mechanical or chemical properties. The mechanical factors (Gaurina-Medimurec, 1994) that predispose wellbore instability include mechanical failure of the rock which occurs around the wellbore (Yongfeng et al, 2009), high mechanical natural (Chen et al, 1998) or induced stress (Gaurina-Medimurec, 1994), low tensile strength of the rock (Hawkes & McLellan, 1997) or low shear strength of the rock (Chen et al, 1998) and implementation of inappropriate drilling practices (Nguyen et al, 2007) that impact negatively on the wellbore stability.

The chemical causes of wellbore instability are brought about by chemical interaction (Nguyen et al, 2007) of the drilling fluid and the rock subject to presence of minerals that are water soluble. Mechanical and chemical wellbore instabilities contribute into drilling complications that make the process unsustainable in terms of costs incurred in managing wellbore instability predisposing factors (Zhang et al, 2006). This implies analysis of wellbore stability during planning phase should account for economies of scale brought about by wellbore instability.

Wellbore instability (Nguyen et al, 2007) results from inaccurate determination of pore pressures. These inaccuracies are made when carrying out wellbore stability analysis and contribute into costs incurred through stuck pipes, lost wellbore, damaged formations, increased costs of drilling operations and increased incidents of wellbore lost circulation.

The major contributors for wellbore instability that are secondary to mechanical and chemical causes include washing, reaming and overpull (Nguyen et al, 2007), vulnerability of bottom hole assembly to sticking (Gaurina-Medimurec, 1994), losses of the drilling fluids (Chen et al, 1998), and failure of the casing reaching the bottom (Maury & Sauzay, 1987). The wellbore instability has been managed through drilling of a sidetrack. There is therefore need for improving geomechanical models and optimization of drilling practices in order to reduce chances of wellbore instability.

The indirect indicator for prevalence of wellbore instability

Prevalence of friction that contributes into high torque and drag forces
Rising incidents of circulating pressures
Possibilities of hanging up of drill strings casing or coiled tubing
Possibilities of rise in circulating pressures
Incidents of stuck pipes
Incidents of increasing drillstring vibrations
Possibilities of drill string failure
Incidents of control problems
Incidents of incapabability to control and run logs
Incidents of inappropriate logging responses
Incidents of leakage of gases subject to poor cementing procedures
Vulnerability of keyhole seating
Incidents characterized by rising doglegs

The direct indicators of wellbore instability

Poor calibration leading to oversized hole
Poor calibration leading into undergauge hole
Vulnerability to excess volume of cuttings
Vulnerability to excess volume of cavings
Possibilities of cavings at the surface
Possibilities of hole fall post tripping
Utilization of excess cement

Problems with wellbore stability

Problems that affect wellbore stability are based arise during initial planning phase (Wolfe et al, 2009). The planning phase fails to provide a stable design that accounts for possibilities of casing and fluid plan, nature of trajectories and challenges that affect the trajectory. This implies, unmanaged geomechanical problems that could affect the stability of the design contribute into wellbore instability. This predisposes the wellbore to collapse, lost circulation, stuck pipes and possibilities of casing collapse (Cantaloube et al, 2010).

It also provided environment for damage of the reservoir and inability to achieve required logging data which makes wellbore real time stability forecasts difficult and undependable. Some challenges that affect drilling as a function of wellbore stability problems include vulnerability to deviated wellbore, near salt, stress based areas (Simangunson et al, 2006). The stratums in tectonic zones pose problems of wellbore instability. Possibilities of intersection of weak bedding planes and unfavorable angles present problems to wellbore stability since they have capacity to provide opportunities for mud and fluid infiltration that finally contribute into collapse (Tan et al, 2004).

Methods for recovering wellbore stability

Wellbore stability could be recovered by recovering lost circulation material (Estep et al, 2010). This is based on a method for recovering lost circulation materials that involves directing fluid into the drill. The material should have sufficient magnetic property than the material being recovered (Zhang et al, 2006). This should be followed by magnetic separation of the materials based on strength of magnetism. The materials that cannot be recovered through use of magnetic separation should be separated through centrifuging or solution process. This utilized the property of different solubility properties of the materials. The materials that cannot be recovered should be disposed off safely (Tan et al, 2004).

Methods for managing the materials that cannot be recovered

The materials are disposed offshore or discharged overboard from the drilling vessel, a process that is termed as offshore discharge (Chen et al, 1998). The materials are treated by using solid control equipments. This occurs if the materials have no negative impacts on the aquatic life (Gaurina-Medimurec, 1994). The materials could be disposed off through offshore reinjection by being ground into fine materials before they are disposed. The materials could also be onshore disposed. In onshore treatment, the materials undergo thermal desoption or are used for land filling and land spreading.

Maintaining wellbore stability

Maintenance of wellbore stability is important process in drilling operations. Maintenance of wellbore stability has been based on failure studies on wellbore stability (Tan et al, 2004). Maintenance is achieved through use of managed pressure drilling. This involves use of surface pressure. Downhole problems should be reduced by appropriate casing control measures (Zhang et al, 2006). This involves casing while drilling method.

In presence of high tectonic stresses, wellbore stability should be maintained by decreasing opportunities that could result into collapse of the wellbore subject to interferences of over pressured shale. There should be balance between hydrostatic pressure and fracture pressure (Cantaloube et al, 2010). Adequate casing off of the formation damages should be done. This should be coupled with use of sufficient drilling fluid.

Preventing wellbore instability

Prevention of wellbore instability is important step in drilling operations. The approach for managing wellbore instability affects sustainability of drilling operations like cement zonal isolations (Hawkes & McLellan, 1997). Wellbore instability increases costs and affects completion. Wellbore instability should be prevented by wellbore stability forecasts through use of wellbore real time stability analysis. There should be ongoing wellbore measurements in order to determine mechanism through which drilling operations affect wellbore (Nguyen et al, 2007). Drilling operations impact on LWD measurements hence need to use real time images and drilling data in order to ensure remedial strategies for risk management are implemented timely.

Differences between onshore and offshore wellbore stability and instability

Onshore drilling is affected more by wellbore stability than offshore drilling (Nguyen et al, 2007). This means, approaches towards management of wellbore stability should be concentrated on onshore drilling than offshore. Improved approaches for management of wellbore stability in onshore drilling could result into decrease of costs in terms of well construction and production operations. This is due to higher temperatures and unconsolidated sand. Onshore drilling is affected more by presence of deposits of gas hydrates and pore structure fractures that impact negatively on the wellbore stability (Chen et al, 1998).

In onshore, possibilities of increased wellbore stresses are much higher than in offshore which impacts on wellbore stability. The problems of lost circulation are more pronounced in onshore than in offshore. As a result, incidents of stuck pipes and wellbore collapse are much evident in onshore than in offshore. Due to incidents of wellbore stability and instability in onshore drilling, it is important to carry out wellbore stability modeling (Hawkes & McLellan, 1997).

Wellbore stability modeling facilitates in wellbore design by helping to determine wellbore trajectory. The process results into identification of hazards that could be incurred. This helps in risk analysis and mitigation. The wellbore stability real time forecasts are therefore important in onshore drilling and operations. Real time forecasts help in determining possibilities of breakout areas and possibilities of drill induced fractures (Maury & Sauzy, 1987). This means, adoption of wellbore real time stability forecasts plays an important role in geomechanical analysis of rock materials on basis of discontinuity or continuity hence positively impacting on the success for the drilling operation.

In onshore drilling, geomechanical responses are influenced by variation in depressurization induced gases that are produced from deposits of onshore hydrates. As a result, a reservoir wide pressure decline occurs (Nguyen et al, 2007). Due to variation in pressures, differences emerge between vertical and horizontal wellbore. In offshore, reservoir depressurization results into anisotrophic stress that contributes into shear failures. This results into decrease in the loads casing as observed in vertical wellbore. The management of the failures requires management of perforation. The problems in onshore arise due to rise of compression on the upper segments of wellbore casing. As a result, there occurs local shear failure and local shear yielding which affects bonding between grains (Yongfeng et al, 2009). The outcomes are characterized by formation of cavities within the perforations. The stability could be maintained by managing localized shear failures.

Significance of Wellbore stability forecasts

Wellbore stability forecasts are important in drilling operations because they facilitate in determination of drilling related failures (Wolfe et al, 2009). The common drilling failures arise from unstable boreholes subject to possibilities of lost circulation, stuck pipes and lack of casing control. Wellbore stability forecasts makes it possible to determine behavior of wellbore stability and utilize the knowledge that is acquired post forecasts to carry out modeling in real time (Estep et al, 2010).

Wellbore stability forecasts helps in providing real time wellbore instability incidents and their prevalence rates. It facilitates in determining mode of failure hence institute appropriate mitigation approach timely. This helps to minimize costs and pave way for time management. Wellbore stability forecasts provide opportunity for designing and development of stable trajectories and determination of mud weights hence identification of optimal equivalent circulating density (Zhang et al, 2006).

Wellbore stability forecasts and wellbore real time stability makes it possible to gain knowledge on prediction of pore pressure and management of failure modes, provides opportunity for planning and trajectory optimization as well as prediction of reservoir compaction (Cantaloube et al, 2010). The primary function of wellbore stability forecasts is to determine reservoir stimulation designs in onshore drilling, determine appropriate perforation design that could be implemented and provide necessary information on earth stress analysis. It also makes it possible to determine selection of mud weights hence facilitate in wellbore cleaning designs that could be effectively be implemented.

Advantages of conducting wellbore stability forecasts

To decrease opportunities for non-drilling time
To decrease opportunities for casing strings
To decrease production time by optimizing on operational time and production resources
To facilitate selection of the mud
To make it possible to implement appropriate wellbore cleaning design

Wellbore real time stability

Wellbore real time stability is a function of real time updates that is realized through the use of geomechanical modeling outputs (Zhang et al, 2006). The model outputs are generated based on actual geomechanical conditions as drilling operations progresses. Wellbore real time stability is fundamental in providing real time updates within a timeframe that could create environment for proactive decision making as operations processes progresses (Simangunson et al, 2006). Wellbore real time stability contributes into close to real time monitoring of drilling operations as well as providing foundation for wellbore stability modeling.

Wellbore real time stability forecasts provide opportunity for remote monitoring of drilling operations. Thus, modeling and wellbore risk management strategies are appropriately allocated (Kristiansen, 2004). This helps in improving drilling performance and safety assurance. Wellbore real time stability analysis provides real time streaming data that facilitates conceptualization of stability incidences.

It provides opportunity for conducting sanding studies and determination of completion design through identification of uncertainties through uncertainty analysis (Estep et al, 2010. Wellbore real time stability provides vital information that is important in carrying out kick analysis, reservoir studies, provision of drilling reports and provision of geophysical data with regard to sonic, density and seismic impacts.

Software used to model well to predict wellbore stability

A variety of software is used in wellbore modeling to predict wellbore stability. The software is used based on the needs for the wellbore stability. For instance, the Borehole Integrity Analysis System (BIAS) whose copyrights are owned and controlled by Bakers Atlas has capability to compute the quantity of wellbore breakout that could be achieved subject to variation of mud weights (Simangunson et al, 2006). BIAS makes it possible to select the right mud weight that conforms to determined size of breakout. Thus, BIAS make sit possible to use lower mud weight. BIAS makes it possible to stimulate drilling induced features.

Bias provided necessary information on stuck pipe of loose mud. Another software that could be used is a window based ERDesigner (Gisolf et al, 2009). The window based ERdesigner makes it possible to generate colored reports automatically, development of a preliminary well plan. The window based ERdesigner makes it possible to determine rig type and power requirements and provides analytical data that could be used for implementation of the drilling program. DRILLWORKSPREDICT PROFESSIONAL 2004TM software family could be used.

The DRILLWORKSPREDICT PROFESSIONAL 2004TM includes software like Drillworksproject, Drillworkspressbase, Drillworks3D, DrillWorksPREDICTTM and DrillWorksGeostress. For instance DRillworksAnaseis software is known for coreection of seismic bias and makes it possible to carry out seismic processing which is improntant element in geopressure analysis. The software makes it possible to conduct real time wellbore stability analysis (Estep et al, 2010). It makes it possible to identify and correct opportunities for shear failures, fracture and lost circulation.

Data needed to predict wellbore stability

Prediction of wellbore stability requires use of sonic data either in form of DTP or DTS (Estep et al, 2010). The density related data and shale indicators are communicated. The process of data communication is carried out in real time. The rock mechanical analysis, assessment and evaluation are carried out on site through a LWD system that is installed on site (Cantaloube et al, 2010). In absence of an on site LWD system, the data could be communicated over the internet to the office for further analysis. LWD caliper measurements are done, based on logging toll and wellbore wall. Maximum and minimum axes of ellipse could be computed in order to determine orientation of the axes. The outcomes are a three dimensional wellbore caliper images. These are created after logging.

Use of high resistivity images (Simangunson et al, 2006) that are either two or three dimensional make sit possible to determine orientation of fractures. The information makes it possible to optimize on wellbore direction that could contribute into higher production. Other data elements that are important include fracture frequency, fracture size and fracture location (Zhang et al, 2006). The three elements of fracture contribute in design, remedial plan development and analysis of a reservoir and its corresponding future engineering processes. Angles of trajectory are also determined.

The failure modes and economic significance

The common failure modes that impact on wellbore stability are brought about by incidents of tensile failure (Wolfe et al, 2009), shear failure or possibilities of matrix or collapse of the pore (Zhang et al, 2006). The common indicators of preeminent failure modes include vulnerability to hydraulic fractures that may be natural or synthetic. It could also be elevated by possibilities of local mobilization of existing faults.

Failure modes could be stimulated by incidents of spalling that arise subject to incidents associated with swabbing and tripping (Wolfe et al, 2009). Failure modes in onshore drilling could be brought about by effects of drag force. Drag force has capability to contribute into increasing sand production. Failure modes have also been identified to develop from ductile formations that arise from subsalt or salt, their corresponding evaporates. Possibilities of soft shales could result into squeeze that could result into restriction of the wellbore. Incidents of swelling have higher capability of contributing into failure modes (Kristiansen, 2004).

The stability of the shale

The stability of the shale is affected by interaction of one or more several factors for instance period of exposure (Simangunson et al, 2006), the degree of filtrate chemistry applicable or degree of solution process (Tan et al, 2004), the rate of ionic transport and feasibility of dehydration. The chemical property of the shale could result into chemical instability. The outcome is characterized by impact on the mud weight and possibilities of water influx (Weinheber et al, 2010).

Influx of water as ionic component could contribute elevates instability. Water infiltration contributes into increase of wellbore pore pressure that in turn results into decrease of the strength of the shale. The outcome manifests itself due to existence of a potential gradient between the wellbore pressure and the shale pore fluid pressure (Nguyen et al, 2007). It could also result from possibilities of chemical potential difference which contributes into local gradient between the drilling and the shale pore fluids.

The smectitic shales

Smectitic shales are characterized by increasing ductility and creepiness (Yongfeng et al, 2009). Smectitic shales contribute are associated with features like decreased wellbore pressures. The wellbore pressure impacts negatively on stability through acceleration of the creepiness (Maury & Sauzay, 1987). The smectitic shales result subject to sensitivity of the mud filtrate ionic strengths that is subject to chemical properties. However opportunities for inhibition could result following sweeling and slouging of the shale (Chen et al, 1998). Smectitic shales are managed through use of salt, gilsonite and synthetic polymers that have capability to increase stability by decreasing ductility which tends to translate smectitic shales into illitic shales.

The significance of illitic shales

Illitic shales are characterized by higher concentration of quartz (Zhang et al, 2006). This makes illitic shales to demonstrate higher stability compared to smectitic shales. Due to decreased ductility property, the illitic shales exhibit brittleness and hence inert (Kristiansen, 2004). The failures that are associated with illitic shales are brought about by increased pressure drawdown. This is subject to brittleness and energy that has been stored between the pore fluids.

Costs of damage and solutions

Wellbore construction costs fall into three categories namely the lost circulation costs, stuck pipes and flat time that are encountered during casing points (Wolfe et al, 2009). Delay in responding to wellbore instability creates environment for increase of the costs due to changes in geopressures and geo-stresses. The costs with regard to wellbore stability should be determined at wellbore planning phase, wellbore real time stability forecasts as the drilling operations are ongoing and during post drilling analysis.

The costs of the damages are caused by lack of wellbore real time stability forecasts (Zhange et al, 2006). Wellbore real time stability forecast helps to identify failure criteria models hence contributing into improvement of drilling methods or proposal for implementation of improved drilling method operations. Wellbore real time stability forecasts helps to determine strength of the existing rock hence designing of improved methods of improving wellbore stability and managing wellbore instability.

The management of costs of damage should be based on impact of time and fluid chemistry of the rock material which would contribute into understanding of wellbore stability and instability (Kristiansen, 2004). The cost of damage should be a function of relationship between analytical and finite element modeling. The methods that should be implemented should be based on feasibility towards management of wellbore stability and instability subject to application or remodeling of existing models for prediction of wellbore stability (Wolfe et al, 2009).

Analysis should provide balance between impacts of temperature and interaction of streamlined workflow based on utility of stochastic modeling. The outcome of temperature and streamlined workflow should be evaluated against correlation data on the strength of the rock material. Use of stochastic modeling should be based on designing and constructing a predictive model.

Sustainability of wellbore stability

Sustainability of strategies for managing wellbore stability and wellbore instability lies in capacity to development of software that can provide close to real time pore pressure and prediction of fracture gradient (Estep et al, 2010). This would result into implementation of real time drilling productions and processes. There is need for improvement of methodologies for managing wellbore stability and instability at onshore and offshore operations. There is need for research and development on prediction of subsalt pore pressure (Weinheber et al, 2010). This should be achieved through the use of three dimensional earth modeling.

The oil and gas drilling industries should focus on maintenance of wellbore integrity. The requirement for integrity should be achieved through efforts and investment in research in order to attain casing reduction and decreased mud related expenses (Cantaloube et al, 2010). This should be based on contingency casing designs. This means identification and adoption of best practices should form core foundation for improving wellbore stability prediction and real time forecasts.

References

Cantaloube, F., Spickett, R., Yekta, K. and M. Anderson, (2010) Optimization of stimulation treatments in naturally fractured carb

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