Wellbore Stability Model Research Articles

A porothermoelastic solution for the inclined borehole in a transversely isotropic medium subjected to thermal osmosis and thermal filtration effects

Given that the hydraulic boundary conditions at the wellbore wall are identified as permeable (PBC) and impermeable (IMPBC) and that the assumption of plane strain holds, the present paper formulates an analytical solution, which is based on fully coupled linear porothermoelastic model with the thermal osmosis and thermal filtration effects, in Laplace space domain. The solution is available for the cases that the wellbore axis of inclined wellbore is perpendicular to the isotropic plane of the transversely isotropic medium under non-hydrostatic stress condition. The wellbore problem is decomposed into axisymmetric and deviatoric loading cases. The numerical results of the time-dependent distributions of field variables are obtained by virtue of performing the inversion technique for Laplace transforms. The sensitivity analyses are performed on the key parameters to thermal osmosis and thermal filtration effects. The effect of thermal filtration on temperature is also illustrated. The numerical comparative analyses in the present paper examine the influence of material anisotropy of poromechanical parameters and thermal properties, which embody in porothermoelastic-osmosis-filtration model and common porothermoelastic one, on the thermally induced pore pressure and stresses. Finally, an integrated geomechanical wellbore stability model is developed to investigate the compressive and tensile failure extents of horizontal wells. The model is involved with three aspects, i.e. the coordinates transformations pertaining to wellbore and weak plane systems, the criteria to evaluate the shear failure (including intact rock and weak plane) as well as tensile fracture, and the fully coupled porothermoelastic analytical solution. The developed porothermoelsatic analytical solution can provide a theoretical basis for multiple of aspects including the inversion of in-situ stress, geothermal reservoir stimulation and petroleum drilling design.

Probabilistic Approach in Wellbore Stability Analysis during Drilling

In oil industry, wellbore instability is the most costly problem that a well drilling operation may encounter. One reason for wellbore failure can be related to ignoring rock mechanics effects. A solution to overcome this problem is to adopt in situ stresses in conjunction with a failure criterion to end up with a deterministic model that calculates collapse pressure. However, the uncertainty in input parameters can make this model misleading and useless. In this paper, a new probabilistic wellbore stability model is presented to predict the critical drilling fluid pressure before the onset of a wellbore collapse. The model runs Monte Carlo simulation to capture the effects of uncertainty in in situ stresses, drilling trajectories, and rock properties. The developed model was applied to different in situ stress regimes: normal faulting, strike slip, and reverse faulting. Sensitivity analysis was applied to all carried out simulations and found that well trajectories have the biggest impact factor in wellbore instability followed by rock properties. The developed model improves risk management of wellbore stability. It helps petroleum engineers and field planners to make right decisions during drilling and fields’ development.

A Wellbore Stability Model for a Deviated Well in a Transversely Isotropic Formation Considering Poroelastic Effects

To analyse wellbore stability phenomena when drilling through a transversely isotropic formation such as shale, a wellbore stability model is developed based on the coordinate transformation method and complex variable elasticity theory. In order to comprehensively consider the anisotropies in the transversely isotropic formation, the model includes the followings: 1. the elastic anisotropy due to the sedimentation effect and naturally developed fractures and 2. the strength anisotropy due to the poor cementation between bedding planes and natural fractures. The model is further generalized by accounting for an arbitrary wellbore trajectory under an arbitrary in situ stress orientation. Next, the model is used in a parametric study that includes factors such as elastic anisotropy, strength anisotropy, multiple weak planes, in situ stress anisotropy, and poroelastic anisotropy, all of which can have a great influence on wellbore stability. Finally, a correction for a frequently used failure criterion has been made to ensure that the newly developed model is comprehensive and accurate for wellbore stability analyses in highly heterogeneous formations.

A fully coupled thermo-poroelastoplasticity analysis of wellbore stability

Wellbore instability problem is one of the main problems that met frequently during drilling, particularly in high temperature, high pressure (HPHT) formations. There are large amount of researches about wellbore stability in HPHT formations, which based on the thermo-poroelastic theory and some achievements were obtained; however, few studies have investigated on the fully coupled thermo-poroelastoplasticity analysis of wellbore stability, especially the analysis of wellbore stability while the filter cake formed. Therefore, it is very necessary to do some work. In this paper, the three-dimensional wellbore stability model which overall considering the effects of fully coupled thermo-poroelastoplasticity and filter cake is established based on the finite element method and Drucker-Prager failure criterion. The distribution of pore pressure, wellbore stress and plastic deformation under the conditions of different mud pressures, times and temperatures have been discussed. The results obtained in this paper can offer a great help on understanding the distribution of pore pressure and wellbore stress of wellbore in the HPHT formation for drilling engineers.

A wellbore stability model for shale formations: Accounting for strength anisotropy and fluid induced instability

Abstract: The presence of weak planes in rock fabric along with interactions of clay minerals with aqueous fluids are the major causes of instability in shale formations. Reviewing the current state-of-the art of wellbore stability models indicates that the interaction of aqueous fluids with clay minerals has been ignored or over-simplified due to the complexity of the processes. In this paper, a diffusion-sorption model, which is considered to be a unified model for all shale types, is integrated with a wellbore stability model. The coupled transport equations are solved using a finite difference method to determine pore pressure and moisture content in the vicinity of the borehole. The wellbore stability is analyzed using the modified Jaeger's criterion, which considers shale to be an isotropic body containing one set of weakness planes. Using field data, a comparison between the anisotropic failure model and isotropic failure model is presented. The results indicate that the wellbore is less stable when bedding planes are considered. The sensitivity analysis shows that the extent of failure zone in the vicinity of borehole is primarily governed by the orientation of wellbore trajectory with respect to the bedding planes. Furthermore, it is shown that the intrinsic rock strength anisotropy can be a significant parameter for failure analysis when shear failure along the bedding plane is not a major concern. The presented model predicts the instantaneous moisture content around the borehole, which is used as an index for estimation of rock strength both in shale matrix and along weakness planes. The simulation results shows that shale-fluid interaction affects borehole stability at early times.

The influence of water-based drilling fluid on mechanical property of shale and the wellbore stability

Abstract Because of high cost and pollution of oil-based drilling fluid, the water-based drilling fluid is increasingly used now. However, bedding planes and micro-cracks are rich in shale formation. When water-based drilling fluid contacts formation rock, it causes the propagation of crack and invasion of drilling fluid, which decrease shale strength and cause wellbore instability. In this paper, we analyzed influence of water-based drilling fluid on shale strength and failure mode by mechanics experiment. Based on those experimental results, considering the effect of bedding plane and drilling time, we established modeling of wellbore stability for shale formation. The result from this model indicates that in certain azimuth of horizontal well, collapsing pressure increases dramatically due to shale failure along with bedding plane. In drilling operation, those azimuths are supposed to be avoided. This model is applicable for predication of collapsing pressure in shale formation and offers reference for choosing suitable mud weight.

Wellbore stability model based on iterative coupling method in water alternating gas injection

Ensuring wellbore integrity is the most important factor in injection well design. The water alternating gas (WAG) injection is increasingly applied globally as the effective enhanced oil recovery (EOR) method in oil wells. High injection pressure or low injection temperature could lead to compressive wellbore failure. The rock stress around the wellbore is a function of the wellbore fluid flow and it should be precisely determined to avoid the wellbore failure. The purpose of this study is to propose a method to ensure the stability of the wellbore for the WAG process using iterative coupling method. The parameters of pressures, temperature, saturations and stresses are obtained for the multiphase flow condition using mathematical modeling. In this study, finite difference method is used to solve pressure, temperature and saturations; and finite volume method is acquired to solve the rock stresses. Iterative coupling method is employed to improve the accuracy of the results. This study introduces improved iterative coupling method between flow and stress models to reduce the processing time of obtaining corrected stress and failure results. Wellbore stability model is developed to determine the maximum pressure values, which lead to wellbore failure in WAG injection process for some different boundary conditions.

An analytical model of coal wellbore stability based on block limit equilibrium considering irregular distribution of cleats

An analytical model of coal wellbore stability is established based on the block limit equilibrium method, taking the irregular distribution of cleats into account. Because coal seam is cut into discontinuous blocks with different shapes and sizes, block slip can be treated as the symbol of coal wellbore instability. According to the geometric characteristic of blocks, blocks surrounding the wellbore can be divided into 3 categorizes: type I, type II and type III. To evaluate the block slip, the stress distribution near the wellbore is determined based on the continuous medium mechanics and the fracture mechanics. Using Mohr–Coulomb criterion and limit equilibrium method, the critical conditions of block slip are derived by defining the failure coefficient, including type I, type II and type III. The proposed model is verified by comparison of the obtained results from the proposed analytical method with the discrete element method as well as the actual drilling. It can be proved that the proposed model can predict accurately block slip. Additionally, the effect of drilling fluid pressures on the wellbore stability is specially investigated. The results show that with the increasing of drilling fluid pressure, the degree of wellbore collapse may be more severe. Meanwhile, the analysis method of reasonable drilling fluid pressure for coal drilling is proposed based on the interval theory, combining the slip area of blocks. Finally, the parametric study is focused on investigating the effects of the internal friction angle of face cleats, the cohesion of face cleats and the tensile strength of butt cleats on the failure coefficients of blocks.

A model for analysis of wellbore stability considering the effects of weak bedding planes

In conventional analysis of wellbore stability, rock is usually assumed to be isotropic, but errors occur when isotropic theory is used if there are weak bedding planes that represent the properties of rock anisotropy. Therefore, in this paper, based on circumferential stresses and a new analysis of geometrical relationships of weak bedding planes and wellbore, a new wellbore stability model that considers the effects of weak bedding planes is introduced; then, a comparison is made by comparing the new model with the intrinsic rock failure model based on actual field data, which reveals that wellbore is less stable when weak bedding planes are considered, that the wellbore of cross-dip wells is less stable than that of up-dip wells and down-dip wells in the strike-slip stress regime and that wellbore quality is better in up-dip wells. Furthermore, results show that dip angles and dip directions of weak bedding planes have significant effects on wellbore stability for different inclination angles and wellbore azimuths. Regarding the effects of in-situ stress regimes, it is found that the wellbore of down-dip wells is overall more stable and that the maximum equivalent collapse density (MECD) of wells drilled along the wellbore azimuth θIA = 270–300° is relatively smaller in the normal fault stress regime; also, in the reverse fault stress regime, the MECD of both up-dip wells and wells drilled along θIA = 90–150° is relatively small. Finally, the influence of the anisotropic coefficient of horizontal stresses is also analyzed.

Numerical modeling of wellbore stability in layered rock masses

Borehole instabilities during drilling are more common in bedding plane rocks than in most other rock formations. Bedding plane rocks make up more than 80 % of rocks in siliciclastic environments, and about three quarters of borehole problems are caused by bedding plane rocks instability. The assessment of in situ stress and analysis of borehole failure due to instability and weak bedding plane represents one of the most critical factors when evaluating borehole stability that causes borehole failure. This paper is based on elastoplastic and isotropic model for stresses around the wellbore, with the aim of trying to understand the general behavior of inclined boreholes due to anisotropy. It was found that borehole collapse was caused predominantly mainly not only by shear but also by tensile failure. It is seen that bedding exposed depends not only on inclination but also on dip of the formation, attack angle, and azimuth. The numerical analyses presented in this paper were carried out using a three-dimensional numerical program. The effects of several dips and dip directions of rock mass layering and angles of well and three different field stress conditions have been investigated. It is known that the differential stress has an important influence on wellbore instability. Also, the effect of a high differential stress is exacerbated by the layer geometry and well angles. In other words, some dip and dip direction of bedding plane causes maximum displacement toward wellbore and significantly affect wellbore stability during drilling.

Effect of material anisotropy on time-dependent wellbore stability

The paper presents a numerical wellbore stability model using the finite element method (FEM). The core driver of the model uses Biot's theory of poroelasticity. The model uses a time-dependent analysis that accounts for the poroelastic effects of pore pressure and deviatoric stress, which are not accounted for in the common static linear elasticity analyses. The use of pseudo-3D FEM based on the theory of generalized plane strain, allows the freedom to analyze complex geometries such as inclined boreholes in anisotropic formation under three dimensional stress field. Shear failure prediction is demonstrated using isotropic and anisotropic failure criteria. Available analytical solutions in literature for anisotropic poroelasticity assume transversely isotropic material with the plane of isotropy always perpendicular to the borehole axis. The presented numerical model removes the above assumption and accounts for the mechanical anisotropy and formation dipping effects that are difficult to deal with using an analytical-based analysis. Finally, juxtaposition between isotropic and anisotropic analyses shows significantly dissimilar failure prediction and mud weight recommendations.

The Effects of Anisotropic Transport Coefficients on Pore Pressure in Shale Formations

This study investigates shale–fluid interactions through experimental approaches under simulated in situ conditions to determine the effects of bedding plane orientation on fluid flow through shale. Current wellbore stability models are developed based on isotropic conditions, where fluid transport coefficients are only considered in the radial direction. This paper also presents a novel mathematical method, which takes into account the three-dimensional coupled flow of water and solutes due to hydraulic, chemical, and electrical potential imposed by the drilling fluid and/or the shale formation. Numerical results indicate that the presence of microfissures can change the pore pressure distribution significantly around the wellbore and thus directly affect the mechanical strength of the shale.

Thermal effect on wellbore stability during drilling operation with long horizontal section

In this study, a three-dimensional wellbore stability model is presented that takes into account thermal stresses combined with an integrated circulation temperature model for horizontal well drilling, the bottom hole temperature simulation were then validated using field measurements, and compared with results for vertical wells. A subsequent analysis of temperature sensitivity revealed that the heat source term, the length of horizontal section and mud specific heat were the main reasons cause the bottom hole temperature for horizontal wells rises above the static formation temperature. Results from the wellbore stability model show that the temperature variation magnitude in horizontal well is smaller than that in vertical wells, however, the effect of thermal stress on critical mud weight window in horizontal is more sensitive. The wellbore at toe of horizontal section is more stable than that at heel of horizontal section when the bottom hole temperature exceeds the static formation temperature. This research can provide a theoretical reference for enhancing overall operational efficiency and safety for horizontal well drilling.

Uncertainty evaluation of wellbore stability model predictions

Abstract There is an increasing concern in the oil and gas industry regarding wellbore stability problems. The need to improve well operations becomes more imperative as the operators move towards more challenging and harsher environments such as ultra-deep waters and high-pressure and high-temperature (HPHT) fields. Different inconsistencies affect many previous wellbore stability analyses, resulting in incorrect results, or results that cannot be extended to other well configurations by well planners. Typical wellbore fracture and collapse models provide single point estimates of the geopressures. The model input data may be uncertain. Failure to capture these uncertainties has led to poor predictions. The purpose of this work is to investigate typical fracture and collapse models with respect to in accuracies in the input data with a stochastic method. Uncertainties in the input data, which include in-situ stresses, rock strength data, and pore pressure will be evaluated, to show how these contribute to the cumulative uncertainties in the model predictions. In this approach, the input parameters are assigned appropriate probability distributions. The distributions are then applied in the wellbore stability models. By means of Monte Carlo simulations, the uncertainties are propagated and the histograms of the outputs are generated. Two types of distributions – triangular and uniform – are applied, to see how the types of input-parameter distributions that are assumed influence the model predictions. Sensitivity analysis is also conducted. This is to ascertain the most significant input factors, which are largely responsible for the cumulative uncertainties or variabilities in the critical fracturing and collapse pressures. The proposed methodology can help in reducing many drilling problems such as circulation loss, stuck pipe, and well collapse. As a result, the industry may save much non-productive time. In addition, well planners will have improved information to make critical decisions.

Wellbore stability model for shale gas reservoir considering the coupling of multi-weakness planes and porous flow

Irregular wellbore collapse phenomena and accidents frequently occur during drilling operations in Longmaxi shale gas reservoir. Considering shale formation with natural cross beddings and fractures, we propose a multi-weakness plane instead of a single weakness plane failure model. Shale samples obtained from the Lower Silurian Longmaxi Strata of Sichuan Basin are investigated based on characterization and analysis of mineralogy, pore structure, sliding failure condition, and rock mechanics to study the impact of porous flow on jointed shale masses. Results show that Longmaxi gas shale is a brittle and fracture-prone material with poor hydrating capacity and extremely low permeability in rock matrices. Reduction of rock strength under porous flow may contribute to changes in intensity parameters of the weakness planes. Therefore, considering the failure of multi-weakness planes under porous flow, we present a wellbore stability model for shale gas reservoir. Two types of weakness plane distribution patterns are examined to discuss the effect of the occurrence, numbers, and water saturation of weakness planes. The results demonstrate that the number of weakness planes, difference in weakness plane occurrence, and diverse water saturation levels significantly affect wellbore stability during drilling.

Uncertainty Evaluation of Wellbore-Stability-Model Predictions

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 166788, ’Uncertainty Evaluation of Wellbore-Stability-Model Predictions,’ John Emeka Udegbunam, Bernt Sigve Aadnoy, SPE, and Kjell Kare Fjelde, SPE, University of Stavanger, prepared for the 2013 SPE/IADC Middle East Drilling Technology Conference and Exhibition, Dubai, 7-9 October. The paper has not been peer reviewed. The purpose of this work is to investigate typical fracture and collapse models with respect to accuracies in the input data. Uncertainties in the input data will be considered to show how they contribute to the cumulative uncertainties in model predictions. In this approach, the input parameters are assigned appropriate probability distributions. The distributions are then applied in the wellbore-stability models. By means of Monte Carlo simulation, the uncertainties are propagated and outputs, which follow a probability distribution, are generated. Introduction Wellbore-stability analysis is necessary for safe drilling operations, especially now that oil and gas operators venture into more-challenging environments. A wide range of parameters is required for accurate study, many of which are subject to uncertainties caused by measurement errors. Error also can be introduced into data through the methods of interpretation used. Epistemic error, arising from imperfect human knowledge of a system, is another source of input uncertainties. Analytical models used for wellbore-stability analysis are also often associated with uncertainties. Mathematical modeling algorithms only try to approximate physical processes and are not true representations of the problems under study. The modelers should be aware of the imprecision and limitations of these physical models. Thus, output uncertainty stems from the variations in input data and uncertainties caused by wellbore-stability-modeling processes. Expected values give no information about uncertainty. Deterministic estimation of the downhole pressure limits provides only single-point values that lack variability information. Instead, probability distributions can be used. With this approach, cumulative uncertainties in the output predictions can be quantified, leading to a more-informed decision. In-Situ Stress Field For a given formation, the starting point in wellbore-stability analysis is the in-situ or pre-existing stress state. Knowledge of the stress state is key to handling borehole problems such as fracturing, lost circulation, collapse, and sand production. The in-situ stress state is normally assumed to coincide with vertical and horizontal directions. In relaxed depositional basins, the values of these horizontal stresses are usually lower than those of the vertical stress. The horizontal-stress magnitudes, however, may exceed those of the vertical stress in strongly tectonic regions. A stress state can be defined as normal-fault, reverse-fault, or strike/slip-fault state of stress. The normal-fault stress state is assumed in this work. If the magnitudes of the three principal stresses and the direction of one of the stresses are known, then the stress state can be specified. The stress concentration is usually very high around the borehole wall. This effect decreases rapidly away from the hole. At a long distance from the wellbore, the principal in-situ stresses are undisturbed and lie along their in-situ directions.

Thin-Oil-Column Extended-Reach Drilling at Sakhalin

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 163487, ’Case History of a Challenging Thin-Oil-Column Extended-Reach- Drilling Development at Sakhalin,’ by Vishwas P. Gupta and Shea R. Sanford, SPE, ExxonMobil Development Company; Randall S. Mathis and Erin K. DiPippo, SPE, Exxon Neftegas; and Michael J. Egan, AIPC, Consultant to Exxon Neftegas, prepared for the 2013 SPE/IADC Drilling Conference and Exhibition, Amsterdam, 5-7 March. The paper has not been peer reviewed. ExxonMobil, operator for the Sakhalin-1 project, planned an additional drilling campaign at the Chayvo field, Sakhalin Island, Russia. The focus of this campaign was to develop a thin-oil-column reservoir by use of extended-reach drilling (ERD). This overview describes the planning of these wells at the edge of the ERD envelope, the associated technical and operational challenges, and the complexity of targeting a thin oil column in an ERD environment. Planning of these wells involved building upon the operator’s previous experience and the use of additional tools and methods to address unique challenges. Introduction The Sakhalin-1 project comprises the Chayvo, Odoptu, and Arkutun Dagi fields off the east coast of Sakhalin Island, Russian Federation, as shown in Fig. 1. Development drilling at the Chayvo field started in 2003 with extended-reach wells drilled from an onshore wellsite pad with the Yastreb drilling rig. In 2005, further development drilling commenced from the offshore Orlan platform. In 2008, the Yastreb rig was moved approximately 75 km north to the Odoptu onshore wellsite pad and drilled nine ERD wells as part of the initial-stage development of Odoptu. Following the Odoptu campaign, the Yastreb rig was moved back to the Chayvo onshore wellsite in 2011 for further development and infill drilling of the Chayvo reservoirs. As of October 2012, 15 of the 20 longest ERD wells in the world had been drilled in the Sakhalin-1 project, with the recently completed Well Z-44 at Chayvo field setting a new measured-depth (MD) record at 12 376 m. Significant well-design parameters, tools, and techniques that enabled these challenging wells are detailed in the complete paper. The technical challenges included uncertainties in the location of the oil column and fluid contacts, the need for very accurate well placement for effective reservoir drainage and for preventing early gas/water breakthrough, completing long horizontal intervals, and high-torque/drag effects. The geologic uncertainties and wellbore-positioning challenges were addressed with a combination of a single near-horizontal pilot hole to locate the fluid contacts and a novel technique to minimize vertical-position uncertainty by use of formation-pressure measurements while drilling. Extensive torque/drag modeling and wellbore-stability modeling were performed to design the wells and equipment and define operational parameters. Completion designs were optimized to ensure successful placement in long horizontal intervals.

Influence of Porous Flow on Wellbore Stability for an Inclined Well With Weak Plane Formation

The criterion of slip condition and wellbore instability are strongly affected by porous flow in weak plane formation, which leads to wellbore instability in the formation with weak planes. Considering influence of porous flow, a mechanical model of wellbore stability was established for inclined well with weak planes under porous flow, and the influence of various hole deviation angle, hole deviation azimuth, and weak plane occurrence on wellbore stability were analyzed. The results show that porous flow has worsened wellbore instability, and inclined wellbore stability is highly dependent on the weak plane occurrence, hole deviation azimuth, and deviation angle under porous flow. The mechanical model of wellbore stability for inclined well with weak planes under porous flow is applied to sidetracked hole in an oilfield of western China.

Wellbore Stability in Oil and Gas Drilling with Chemical‐Mechanical Coupling

Wellbore instability in oil and gas drilling is resulted from both mechanical and chemical factors. Hydration is produced in shale formation owing to the influence of the chemical property of drilling fluid. A new experimental method to measure diffusion coefficient of shale hydration is given, and the calculation method of experimental results is introduced. The diffusion coefficient of shale hydration is measured with the downhole temperature and pressure condition, then the penetration migrate law of drilling fluid filtrate around the wellbore is calculated. Furthermore, the changing rules of shale mechanical properties affected by hydration and water absorption are studied through experiments. The relationships between shale mechanical parameters and the water content are established. The wellbore stability model chemical-mechanical coupling is obtained based on the experimental results. Under the action of drilling fluid, hydration makes the shale formation softened and produced the swelling strain after drilling. This will lead to the collapse pressure increases after drilling. The study results provide a reference for studying hydration collapse period of shale.

Mechanical Stability of Horizontal Wellbore Implementing Mogi-Coulomb Law

In this paper, a linear elastic constitutive model is described. The model consists of a three dimensional analyses of stress concentration around an arbitrarily oriented borehole, due to anisotropic in situ stress combined with internal wellbore pressure. Studying the principal stresses around a borehole require the consideration of three possible permutations for the principal stresses: (1) σ z ≥ σ q ≥ σ r , (2) σ q ≥ σ z ≥ s r , and (3) σ q ≥ σ r ≥ σ z . Considering the practical field conditions, in normal faulting stress regime and reverse faulting stress regime, wellbore stability analysis can be simplified by only assuming case 2 ( σ θ ≥ σ z ≥ σ r ) for the principal stresses around horizontal borehole. In strike-slip stress regime, however, all the three possible permutations for the principal stresses should be considered in wellbore stability analysis. The constitutive model in conjunction with Mogi-Coulomb law has been used to introduce a new wellbore stability model for horizontal boreholes. The developed model has improved wellbore stability analysis compared to adopting the classical Mohr-Coulomb criterion that is commonly applied. This has been verified by several typical field case studies. Key words : Wellbore stability; Borehole failure; Collapse pressure; Mogi-coulomb criterion; Horizontal drilling