Reservoir Shear Failure Analysis: A Produced Water Disposal Case Study

ABSTRACT: The connection between undrained pore pressure response to stress in shales and fault reactivation, drilling problems and microseismicity observed at significant distances from hydrocarbon reservoirs and injection zones has not been thoroughly studied and is usually disregarded in geomechanical modelling workflows. In our study, we examine to what extent the inclusion of the pore pressure response affects the amplitude of total stress changes expected to cause shear failure in several overburden and outcrop shales. We combine poroelastic pore pressure coefficients and Mohr-Coulomb failure envelope parameters estimated through triaxial laboratory experiments to model the total stress increases at failure for a wide range of loading scenarios and medium orientations. We consider both isotropic and anisotropic pore pressure parameters - we explore the differences in modelling outcomes the two sets of parameters yield, as well as the impact of the individual parameters on the expected stresses at failure. We observed that shear failure in shales is expected at significantly lower stresses changes and is plausible in a much wider range of loading paths once the undrained pressure response in considered, and therefore we postulate that it should be taken into account during injection or production operations safety assessment. 1. INTRODUCTION Injection into or production from a subsurface reservoir have been documented to lead to fault activation, microseismic activity, and borehole stability issues in the reservoir and its surroundings (e.g., Pine et al., 1983; Raleigh et al., 1976; Zoback & Harjes, 1997). In some cases, these undesired phenomena occurred in the over-and underburden at significant distances from the reservoir subjected to pore pressure change (e.g., Odonne et al., 1999; Rutqvist et al., 2008; Segall, 1989). This has triggered an ongoing discussion of the causes of these events. Until now, it was primarily associated with pore fluid migration through fractures and fault planes throughout low permeable under- and over-burden, as well as with stress transfer (e.g., Vasco et al., 2018; Verdon et al., 2011; Williams-Stroud et al., 2020; Zhou et al., 2008) caused by injection- or production-induced pore pressure changes in the reservoir (e.g., Geertsma, 1973; Morita et al., 1989; Rudnicki, 1999).

This study evaluates the in-situ stress field and the potential risk of fault reactivation and seal breach in the Mutineer and Exeter fields, Australian North West Shelf. Stress determinations are undertaken using pumping pressure test, rock mechanical, and log data from twelve wells. Subsequent geomechanical modelling uses the stress data to assess pore pressure changes that may induce slip on mapped faults cutting the region.The principal stresses are assumed to be the vertical stress (S ), and a maximum and minimum horizontal stress, SH and Sh respectively. Borehole breakouts and drilling-induced tensile fractures (DITFs) interpreted from image logs indicate SH has an average orientation of 107°N and Sh is orientated 017°N. Leak-off test data compiled from well completion reports reveal the magnitude of Sh increases with depth at a rate of 17.1 MPa/km. Density log data show Sv can be approximated by a power law function. An upper bound to SH is calculated using the frictional limit to stress beyond which faulting occurs when using a frictional coefficient of 0.6. Better constraints on the magnitude of SH are gained using rock mechanical data, knowledge of Sh and S, mud weights, and the occurrence of borehole breakouts and DITFs. Stress magnitudes show that the tectonic regime is strike-slip (sh<sv<sH).The likelihood of fault reactivation in Mutineer-Exeter is expressed as the increase in pore pressure required for fault slip. Results show that faults are non-optimally orientated for reactivation by the stress field. The likelihood of brittle seal failure due to fault reactivation is low, primarily because of non-optimally orientated faults. The creation of new faults requires greater increases in pore pressure than reactivation and is thus seen as being more unlikely. The results have implications for seal integrity, well bore stability, and the safe and successful production of the fields.

Predictions of fault reactivation and the hydraulic seal of cap rock are integral parts of the seal integrity research programme of the Banzhongbei gas storage facility (BGS) to ensure the safe storage of natural gas in subsurface reservoirs. We attempt to combine the heterogeneity of geomechanical properties, including frictional coefficient and cohesive strength, into a workflow to estimate fault reactivation risk and the maximum sustainable fluid pressure that will not induce failure of trap-bounding faults and cap rock. Based on the Griffith – Coulomb Failure criterion, fault failure modes – including tensile failure, shear failure and hybrid failure – may occur under conditions of the samein situstress field as a result of the strength heterogeneity of fault rocks, which plays a key role in the estimation of integrity of the entire fault trap. The juxtaposition of the seal on the BQ Fault with high shale gouge ratio (SGR) values makes a significant contribution to the high fluid pressure build-ups during gas injection, and its low strength properties lead to a higher reactivation risk for the Banqiao (BQ) Fault than for the B816 Fault. The increased pore-fluid pressure adjacent to the BQ Fault induced by gas injection should be lower than 9.7 MPa in order to avoid failure. Furthermore, the maximum sustainable fluid pressure of the cap rock is approximately 13.9 MPa at the locations of the injection wells.

Regional tectonic state and poro-thermo-elasticity analysis of near wellbore zone in field development plan: Utilization of an uncoupled approach

Changes in reservoir pore pressure and temperature during injection or production affect rock deformation, which, in turn, causes alteration of porosity and permeability. Specifically, an increase in pore pressure (or a decrease in rock temperature) can create significant rock deformation and increase of the shear stress that could lead to rock fracturing and microseismic activity in the reservoir. Furthermore, porosity and permeability of rocks decrease because of the pore pressure decrease during depletion. Thus, stress-dependent deformation in hydrocarbon bearing shale formations affects the production decline trends. This paper addresses these issues and presents a computationally efficient in-house numerical simulator for poroelastic dual-porosity reservoirs. The governing transport equations for the fluid flow and rock deformations use two interacting environments consisting of a continuous fracture medium and a discontinuous rock matrix medium. The transport and rock deformation equations are fully-coupled and solved numerically using a stationary coordinate frame. A proper assignment of rock frame modulus, affected by the interconnected fractures, versus the rock matrix modulus is a major focus of this work. The rock frame modulus is smaller than the matrix modulus. The numerical model is compared to a single-phase model and validated with a closed form of analytical solutions. We used field data from a fractured unconventional reservoir to assess our formulation. The field data includes flow rates and pressures during an extended production period in the Bakken formation, Williston basin in North Dakota. Finally, we conducted a sensitivity analysis to determine the effect of bulk rock deformation on reservoir performance as compared to conventional engineering approaches which utilize pore compressibility without accounting for the bulk rock deformation.

Investigation of present-day in-situ stresses and pore pressure in the south Cambay Basin, western India: Implications for drilling, reservoir development and fault reactivation

Unconventional oil and gas resources, especially shale gas resources have great potential for exploration and development in China. In Shale gas exploration and development process, reservoir pore pressure is a very important parameter, and the pore pressure prediction can improve the appraisal accuracy of project dessert. This paper analyzes several formation pore pressure calculation methods based on logging data, and optimized the “Equivalent depth method”. The preliminary results show that this method can carry out the accurate evaluation on the abnormal high pressure of the shale gas reservoirs of Fuling area with high calculation precision and application potential.

SummarySteam-assisted gravity drainage (SAGD) is one successful thermal-recovery technique applied in Alberta oil-sand reservoirs. When considering in-situ production from bitumen reservoirs, one must reduce viscosity for the bitumen to flow toward the production well. Steam injection is currently the most promising thermal-recovery method. Although steamflooding has proved to be a commercially viable way to extract bitumen from bitumen reservoirs, caprock integrity and the risk of losing steam containment can be challenging operational problems. Because permeability is low in Albertan thermal-project caprock formations, heating greatly increases the pressure on any water trapped in pores as a result of water thermal expansion. This water also sees a great increase in volume as it flashes to steam, causing a large effective-stress reduction. After this condition is established, pore-pressure increases can lead to caprock shear failure or tensile fracturing, and to subsequent caprock-integrity failure or potential casing failure. It is typically believed that low-permeability caprocks impede the transmission of pore pressure from reservoirs, making them more resistant to shear failure (Collins 2005, 2007). In considering the “thermo-hydromechanical pressurization” physics, low-permeability caprocks are not always more resistant. As the steam chamber rises into the caprock, the heated pore fluids may flash to steam. Consequently, there is a vapor region between the steam-chamber interface penetrated into the caprock and the water region within the caprock which is still at a subcritical state.This study develops equations for fluid-mass and thermal-energy conservation, evaluating the thermo-hydromechanical pressurization in low-permeability caprocks and the flow of steam and water after steam starts to be injected as part of the SAGD process. Calculations are made for both short-term and long-term responses, and evaluated thermal pressurization is compared for caprocks with different stiffness states and with different permeabilities. One can conclude that the stiffer and less permeable the caprock, the greater the thermo-hydromechanical pressurization; and that the application of SAGD can lead to high pore pressure and potentially to caprock shear, and to subsequent steam release to the surface or potential casing failure.

To address the high demand for produced water management, a Cuttings Reinjection (CRI) well (well-1) was converted into a Produced Water Disposal (PWD) well. The study outlines the methodology, results, challenges, and cost savings associated with this conversion. The target reservoir for existing PWD wells consists of multiple layers of limestone and dolomite. An extensive study was conducted to plan the PWD wells in this target zone, supported by production logging tools (PLT) &amp; core data. Initial injectivity results indicated that well-1's performance (bare footed completion) was fair, and it was comparable to nearby well's performance, having same completion design. The injectivity test was also compared with an existing CRI well (well-2) in the same area, having different completion design. After the initial injectivity test, well-1 remained active as a CRI for five years with low injection rates, followed by another five years of being shut-in due to surface facility constraints. Given the high demand for produced water management, it was considered whether to evaluate a new PWD well locations or opt to a cost-effective option to reactivate well-1 as PWD, given the surface slots are limited. Because of this, an integrated approach was used for evaluation, focusing on geological (such as Temp logs, etc.) and geophysical data (such as seismic, etc.). Injectivity tests were performed in well-1 at different depths, guided by attribute maps. Due to poor performance results, the injectivity test was repeated after each acid stimulation job till the desired results were achieved. Following a detailed geological and geophysical evaluation, two prolific zones were identified as potential candidates for sustainable injection. Seismic reservoir characterization revealed the distribution of cavities (karst collapse features) within the target zones, which correlated well with dynamic mud losses. Based on the performance of nearby wells and results from seismically driven attributes, optimum locations were considered for new PWD wells. The attribute map also showed that well-1 targets a highly porous zone, as confirmed by the initial injectivity results. However, the first injectivity test conducted after ten years yielded extremely poor results. The injectivity test after third acid stimulation job showed slight uplift in improvement, though still lower than the initial injectivity performance. Subsequently injectivity tests were repeated after every acid stimulation job. The last injectivity test after the seventh acid job showed a two-fold improvement in water injection compared to the initial results. Based on the strong injectivity result after the seventh acid stimulation job, it was decided to activate well-1 as a PWD. Well-1 has now been active as a PWD for over a year, showing sustainable performance. This conversion resulted in saving a surface slot along with over $10MM incremental savings in CAPEX/OPEX. Converting a CRI into a PWD well is not common due to completion design constraints, injection performance, and target Formation heterogeneity. Consequently, this unique study focused on post-conversion results to validate that the well can be switched from a CRI to a PWD or vice-versa without any changes in completion design and extra cost implications.

&lt;p&gt;&lt;b&gt;Increases in rainfall-induced landsliding following large earthquake are well documented but the time frames over which this heightened hazard persists in the land scape remains poorly understood. Whilst it is well known that the presence of failed and partially slopes after earthquakes significantly reduces the rainfall thresholds required to activate slope movement, their failure susceptibility during specific storms and how this changes through time remains poorly studied. To improve knowledge in this field requires well documented slope failures following earthquakes and a detailed understanding of their potential failure mechanisms when pore pressures are elevated in the slope. The 2016 Mw 7.8 Kaikōura earthquake provides a unique opportunity to study how rainfall events following the earthquake may impact the timing and mechanisms of landslide reactivation. &lt;/b&gt;&lt;/p&gt;&lt;p&gt;This study conducted a suite of specialist triaxial cell experiments, designed to replicate varying rainfall scenarios on remoulded samples collected from two sites where numerous earthquake-induced landslides were recorded in similar Late Cretaceous to Neogene sediments with similar physical properties (the Leader Dam Landslides (LDL) and the Limestone Hill landslide (LHL)). In each experiment rainfall events were simulated using a series of different pore pressure scenarios (increases and decreases in mean effective stress) at representative field stress conditions whilst monitoring material deformation behaviour. &lt;/p&gt;&lt;p&gt;The results demonstrate that both the deformation behaviour and pore pressure required to generate failure were influenced by the previous changes in pore pressure. Samples subjected to stepped increases in pore pressure were subject to greater pre-failure deformation (dilation) and subsequently failed at lower pore pressures (higher mean effective stress) when compared to samples subjected to linear increases in pore pressure. In addition, increases in the rate of pore pressure also increased the amount of pre-failure deformation allowing failure to occur when pore pressures were lower. In contrast a sample subjected to both increases and decreases in pore pressure underwent pre-failure densification and subsequently required a larger increase in pore pressure to fail. The results demonstrate that landslide reactivation is influenced by a number of factors including the amount and rate of previous changes in pore pressure and the slope drainage history. &lt;/p&gt;&lt;p&gt;The results provide new insights into why landslide susceptibility may remain elevated for prolonged periods of time (e.g. decades) in the landscape as well as why the rainfall thresholds for site specific failures during storms may be difficult to predict. &lt;/p&gt;

Overpressure within a circular magmatic chamber embedded in an elastic half space is a widely used model in volcanology. However, this overpressure is generally assumed to be bounded by the bedrock tensile strength since gravity is neglected. Critical overpressure for wall failure is thus greater. It is shown analytically and numerically that wall failure occurs in shear rather than in tension, because the Mohr–Coulomb yield stress is less than the tensile yield stress. Numerical modelling of progressively increasing overpressure shows that bedrock failure develops in three stages: (1) tensile failure at the ground surface; (2) shear failure at the chamber wall; and (3) fault connection from the chamber wall to the ground surface. Predictions of surface deformation and stress with the theory of elasticity break down at stage 3. For wall tensile failure to occur at small overpressure, a state of lithostatic pore-fluid pressure is required in the bedrock which cancels the effect of gravity. Modelled eccentric shear band geometries are consistent with theoretical solutions from engineering plasticity and compare well with shear structures bordering exhumed intrusions. This study shows that the measured ground surface deformation may be misinterpreted when neither plasticity nor pore-fluid pressure is accounted for. Supplementary material: The numerical benchmark data are available at: http://www.geolsoc.org.uk/SUP18517 .

Low oil price and increased environmental regulations presents a new frontier for many indigenous oil and gas companies in Nigeria. In mature fields with significant water production, produced water treatment and handling could easily account for up to a third of OPEX. Underground produced water disposal is a tested approach that has been used worldwide with mixed results. Studies have been published on the subject; however, it was observed that there were no Niger Delta case studies. This paper presents SEPLAT's subsurface approach to in-field water disposal, drawing upon geological and petroleum engineering analysis coupled with learnings from over 6 years of produced water re-injection experience. Some of the areas that will be discussed include reservoir selection/screening methodology, water quality impact on permeability, produced water disposal well selection/completion, operating philosophy, general surveillance, and basic separation requirements. Thirteen reservoirs located within 2 proximal fields were screened for suitability and ranked as possible candidates for water disposal based on 8 criteria. The best 2 were then high-graded and detailed studies carried out, spanning detailed geological characterization for reservoir quality and connectivity (including quantitative interpretation), to dynamic simulation, injection well location optimization and performance prediction (for clean water). The results of core flood tests were incorporated. It is recommended that total suspended solids should not exceed 5 mg/L, with a maximum of 5 microns particle size, under matrix injection conditions while oil content should be limited to below 30-50 ppm. Tolerance for TSS can be relaxed to 10ppm – 50ppm at fracturing conditions, depending on the reservoir parameters and process systems. The knowledge of these parameters should drive the technology selection for optimum water treatment and injection.

The mechanism of wave-induced instability in a permeable seabed have been studied for more than two decades. The distinction between shear failure and liquefaction, however, has not been clearly defined. This paper presents a fundamental study on the differences in two failure modes for a fully saturated seabed of both finite and infinite thickness. The wave-induced effective stresses and pore pressure, obtained from an analytical solution of Biot’s pore-elastic consolidation theory, were employed to examine the failure modes under a two-dimensional plane strain condition. A case study is presented to examine the failure modes with respect to several parameters, such as excess pore pressure, seepage flow, seepage force, failure areas and stress path in the seabed. The conclusions obtained from this study were as follows; (1) the thickness of a permeable seabed affects the pore pressure and effective stress response to ocean waves and the failure mode of the seabed, (2) either a liquefaction or shear failure, or both, occur in the seabed, even in the saturated seabed, (3) the Mohr-Coulomb’s failure criterion, when combined with elastic stresses, can not be employed to estimate the liquefaction failure in the seabed, (4) the liquefaction can be evaluated by a criterion in terms of the excess pore pressure, (5) The liquefied zone in the seabed is significantly different from the shear failure zone. The shape beneath the seabed surface for the former is almost identical to the contour where the upward seepage flow is concentrated.

This study investigated the macroscopic physical and mechanical properties of Guiyang red clay during surcharge loading, lateral excavation and lateral unloading with axial loading, and clarified the failure mechanism of microstructure before and after shear under different stress paths of CTC, RTC and TC. Consolidated undrained triaxial shear permeability, SEM scanning, XRF fluorescence spectrum analysis and XRD diffraction tests were conducted to simulate the actual engineering conditions. The stress–strain curve, shear strength, pore water pressure variation rule and macroscopic failure mode of soil samples under different stress paths were analysed. In addition, Image Pro Plus 6.0 and PCAS were used to study the relationship between the macro mechanical properties and micro microstructure failure under different stress paths. The stress–strain curves from CTC, RTC and TC in CU tests were different, with the peak values of shear stress under the three stress paths being P-increasing, equal P-path and P-decreasing path. Moreover, the internal friction angle and cohesion of the increasing P path were higher than those of equal P path and decreasing P path, hence, the influence of stress paths on the cohesion is greater than that of internal friction angle. The pore water pressure is strongly dependent on the stress path, and the variation characteristics of pore water pressure are consistent with the change in the law of the stress–strain curve. Under the same confining pressure in the P-increasing path, the shear failure zone runs through the whole soil sample, and the shear failure zone is significant, whereas under the condition of the P-reducing path, the shear failure angle of soil sample is about 65°, 55° and 45°, and in the equal P path, the soil sample is dominated by the confining pressure, with no obvious microcrack on the surface of the soil sample. The difference is that the distribution of pores in the path of increasing P and equal P is directional, and the anisotropy rate is small, while the distribution of pores in soil samples with shear failure and before shear is random and the anisotropy rate is high.

Moisture excess in granular pavement layers can decrease pavement performance leading to moisture-related distresses. The combination of trapped water and heavy traffic loads may substantially increase pore pressure and lead to shear failure if layer hydraulic conductivity is insufficient. Currently, most pavement analysis models are unable to predict pore pressure development, so the design and evaluation methods ignore this dynamic effect which may significantly underestimate the moisture damaging effect on pavements. This paper proposes the use of Biot’s poroelasticity theory for modelling fully saturated unbound materials. The proposed methodology – Poroelastodynamic Finite Integration Technique (PEFIT) – was used to analyze axisymmetric multilayered pavement structures consisting of elastic and poroelastic layers. The model was verified and validated using a well-known commercial finite element solver and field pore pressure data, respectively. A sensitivity analysis was conducted by varying critical pavement design parameters. The PEFIT analysis of a three-layer pavement system with a poroelastic base layer has shown that a decrease in base hydraulic conductivity significantly increases the maximum pore pressure, especially for high-speed loads. Results also indicate substantial pore pressure and water movement in the base layer, which may induce a potential shear failure mechanism for unbound materials.