Geomechanical Analysis Research Articles

Characterization of organic content, brittleness index, and geomechanical properties of the Eocene Cambay Shales — Insights from the Ankleshwar oil field in western India

Shale resource assessment involves a detailed characterization of organic and geomechanical parameters for better insights on the reservoir properties and classifying areas of economic yield. To assess the Eocene Younger Cambay Shale (YCS) Group of the Ankleshwar field, western India, for feasible shale resource play, we have applied a multistage screening methodology that combines an estimation of organic richness, brittleness index (BI), and geomechanical analyses. The estimated thermal maturity and average total-organic-carbon (TOC) contents are within the range of 0.8–1.0 and 1.8 wt%, respectively. These estimates are comparable to the reported core-based measurements. BI based on the mineralogical composition reveals that the YCS intervals of marine origin fall into the “less-ductile” to “brittle” zone, whereas the elastic property-based estimated BI falls into the “less-brittle” to “high brittle” zone. We have established a field relationship between the BI and the shale volume, and we also deciphered the effect of TOC content on the rock-elastic properties. Pore pressure in the shales is slightly above the hydrostatic gradient (10.5–11.5 MPa/km). The estimated average fracture pressure of 18.5 MPa/km, together with the BI of moderately ductile to less-brittle behavior suggests that the studied shales are capable of withstanding substantial strain while hydrofracturing for effective production. We demonstrate an expedient example to characterize a potential shale unit within a producing hydrocarbon field using the drilled wells with limited or no core data.

Unlocking flow pathways in complex carbonate reservoirs: Benefits of an integrated subsurface study from the Cretaceous Mauddud Formation, North Kuwait

Despite its long production history (since 1958) and extensive reservoir stimulation by water injection, the Mauddud reservoir of the Sabiriyah field (SAMA) in northern Kuwait has been plagued by relatively disappointing oil production increase, low oil-by-water replacement ratios, and early water-breakthrough observations. The work described in this paper indicates that reservoir complexity and associated permeability heterogeneity are at the root cause of this and are the result of the interplay between sedimentology, diagenesis and the structural evolution of the area.A multi-disciplinary study involving structural geology, sedimentology, diagenesis, reservoir engineering and geomechanics identified the geological processes responsible for the development of principal high-permeability (High-K) elements in SAMA. Their 3D distribution has been characterized and reservoir property ranges have been established, based on integrated evaluation of static (e.g., seismic, core, borehole image, wireline logs, and drilling information such as losses) and dynamic information (e.g., production and injection logging tools, well test information, pressure data, historical production data and tracer information). Feedback loops between geological realizations and dynamic simulations allowed the testing of various concepts and property ranges with learnings implemented in updates of the geological realizations. This approach increases the confidence level in the conceptual geological models.Three principal plumbing elements contribute to the reservoir and property heterogeneity in the SAMA reservoir. Firstly, relatively thin (one to several decimeters) cemented zones characterized by low matrix porosity but significant secondary vuggy and fracture porosity with high permeability occur in two distinct stratigraphic intervals in the reservoir. These intervals occur near sequence boundaries when the carbonates likely experienced brief periods of exposure. They are interpreted to be cemented cycle tops formed during meteoric diagenesis and represent key flow zones for the initial hydrocarbon production as well as for later high-rate production of formation water and injected water, as identified by production/injection logging tools. Faults and associated fault (damage) zones with abundant fractures are the second key plumbing element. Core recovery and observations from wells located closely to a fault zone identified from detailed seismic interpretation highlight the occurrence of abundant fractures. Tracer information clearly identifies faults as preferential migration pathways between injector and producer wells. Thirdly, induced fracture growth near injectors is an important process in the context of High K development. Hall plots identified that injectivity often increases near injectors after a certain period of injection. Geomechanical analysis suggests that induced fracture growth is plausible due to reduction in total horizontal stress due to the combined effect of depletion and water-injection-induced cooling of the reservoir.Reservoir simulations validate the requirement of presence of these three key elements in the plumbing diagram. Yet, dynamic sensitivity testing shows that the three processes each have a variable impact on flow in the reservoir, depending on local factors such as the nearby presence of faults and the development of the cemented cycle tops. Dynamic simulations further indicate that (1) porosity in the cemented cycle tops is likely low, in line with core observations and out of sync with upscaled porosities and (2) lateral extent of individual cemented cycle tops is in the order of few hundred meters, but final connectivity is enhanced by the stacking of different cemented cycle tops and their concentration in two distinct zones in the Mauddud.Armed with the insight from core analyses and dynamic model results, recommendations are made to improve oil production in SAMA: (1) inject away from the zones and faults, and (2) reduce the injection pressure, thus limiting the likelihood of “new” induced fractures and faults forming that may connect to “old” geologic fracture and faults and cause early-water-breakthrough and thus ineffective oil sweep. Reservoir stimulation via matrix injection, rather than under fracturing conditions, may reduce the initial oil production boost but will most likely lead to significantly better ultimate recovery over the long life of SAMA.

A Study of Temperature Effects in the Bunter Closure 36, a Potential Large-scale CO2 Storage Site in UK

This paper reports on numerical thermal studies of cold CO2 injection into the Bunter Closure 36 prospect, analysing temperature effects during the injection and assessing the time required by the aquifer to recover the initial natural temperature. Fluid flow properties and pressure variation in the injection area are analysed, assessing different cooling scenarios that might occur depending on the thermal properties of the geology surrounding the storage site. Different thermal modelling approaches are presented, discussing some aspects of input data and setup. Information from thermal simulation studies might feed injection design, geo-mechanical analysis, and monitoring of CO2 migration.

Application of Fracture Barrier Analysis in Well Stimulation Planning for Upper Baturaja Limestone Formation Based on Well Log & Drill Cutting Data from OBF-01 and OBF-04 Wells, Offshore Southeast Sumatra

In general, the South Sumatra Regional Stratigraphy of the Baturaja Limestone Formation facies is deposited on the Buildup Carbonate (Reef) and the Limestone Clastic Carbonate of the Baturaja Formation which grows as a buildup reef on the platform in the Basement High (Horst) underneath is the Lemat Formation volcanic deposits. Referring to the facies model in general, the Baturaja Limestone Formation, the depositional environment starts from Shelf Lagoon Open Circulation - Winnowed Edge Sand - Organic Buildup - Fore Slope - Deep Shelf Margin - Open Sea Shelf - Basin, meaning that carbonate is formed starting from pure organic Cabonate Buildup Reef without / a little sludge / mud to the Carbonate Basin where more muddy / mud is present, this condition causes clay minerals to also more and more mix with Terigenous Clastics (Quartz, feldpar). The complexity of the Baturaja Limestone Formation requires fracture barrier analysis associated with well stimulation planning in order to increase oil productivity with the appropriate method. Fracture barrier fracture analysis is an approach method to determine the depth interval that becomes a barrier in hydraulic fracturing by correlating the results of geomechanical analysis from well log data and mineralogical analysis from drill cuttings data, so that a commonly used well stimulation method can be selected, namely hydraulic fracturing, acidizing, and acid-fracturing.From the ternary diagram plot the XRD (bulk) analysis results show that the distribution of the main minerals (Quartz, Clay, Calcite) is more dominant in the ductile zone, hard to frac category. This shows that all the depth intervals in the OBF-01 and OBF-04 wells are more ductile, and are not recommended for hydraulic fracturing. From the XRD (bulk) analysis, Calcite mineral is more dominant, so for well stimulation work it is recommended to use acidizing or acid-fracturing.

East Palu Valley flowslides induced by the 2018 MW 7.5 Palu-Donggala earthquake

The 2018 MW 7.5 Palu-Donggala, Indonesia earthquake initiated a series of catastrophic flowslides that claimed the lives of several thousand people, making it the deadliest natural disaster of 2018. Three of the most significant flowslides, which occurred in the communities of Petobo, Jono Oge, and Lolu Village, were clustered within alluvial sediments of the Palu River Valley. Post-earthquake field reconnaissance, subsurface investigation, eyewitness interviews, and geomechanical analyses indicate that the failure sequence began when the earthquake ground motion raised pore water pressures in the saturated interbedded alluvial sediments. The flowslides were not coseismic but instead occurred several minutes after earthquake shaking ended. We hypothesize that high excess pore pressures created large hydraulic gradients within the alluvial sediments, which ultimately triggered progressive liquefaction that led to a redistribution of stresses and accumulation of shear strains. The flowslides were then initiated as gravitational driving stresses began to exceed the residual strength of the liquefied soil. Given the interbedded stratigraphy, we speculate that void redistribution and the development of water films possibly contributed to low residual strengths. An adjacent unlined agricultural canal played a critical role in the failure sequence by artificially raising the groundwater level, thereby increasing the susceptibility to liquefaction and attendant strength loss.

GPU-accelerated smoothed particle finite element method for large deformation analysis in geomechanics

Particle finite element method (PFEM) is an effective numerical tool for solving large-deformation problems in geomechanics. By incorporating the node integration technique with strain smoothing into the PFEM, we proposed the smoothed particle finite element method (SPFEM). This paper extends the SPFEM to three-dimensional cases and presents a SPFEM executed on graphics processing units (GPUs) to boost the computational efficiency. The detailed parallel computing strategy on GPU is introduced. New computation formulations related to the strain smoothing technique are proposed to save memory space in the GPU parallel computing. Several benchmark problems are solved to validate the proposed approach and to evaluate the GPU acceleration performance. Numerical examples show that with the new formulations not only the memory space can be saved but also the computational efficiency is improved. The computational cost is reduced by ~70% for the double-precision GPU parallel computing with the new formulations. Compared with the sequential CPU simulation, the GPU-accelerated simulation results in a significant speedup. The overall speedup ranges from 8.21 to 11.17 for double-precision simulations. Furthermore, the capability of the GPU-accelerated SPFEM in solving large-scale complicated problems is demonstrated by modelling the progressive failure of a long slope with strain-softening soil.

Rockfall hazards of Mount Pellegrino area (Sicily, Southern Italy)

ABSTRACT A map derived by rockfall analysis at Mount Pellegrino is presented herein. The study area is affected by several phenomena of rockfall which caused numerous damage and a strong social and economic impact. Official reports and maps that give a general assessment of rockfall hazard are available in this respect, however, it would be advisable to provide a more specific cartographic support useful for land management and planning. The drafting of new maps showing the rockfall runout areas is an additional tool that may be used in conjunction with the existing maps as a means of risk mitigation and reduction. On the basis of geological, geomorphological, and geomechanical analysis and exploiting the information relating to a landslides inventory obtained by using both analytical and empirical methods, two different rockfall propagation areas were reconstructed. The final thematic map permit to appreciate the differences and similarities between the obtained runout areas.

Method for a Drainage Roadway Location Determination for an Overlying Methane Drainage System

The paper analyses possibilities to carry out methane drainage of a longwall in seam 510. A possibility to fight against the methane hazard was determined based on results of forecasting for methane emission to mine workings. A system of methane drainage was suggested for the adopted method of longwall face ventilation. The location of drainage roadway was determined for the selected methane drainage system. The paper includes a geomechanical analysis for a numerical model, which was aimed at determination of the optimum location of the drainage roadway in a seam overlying the longwall panel.

Studying ore field structure and rock properties of the Delken deposit to substantiate the optimal pit walls design

Introduction. In order to determine a key design parameter substantiated by technical-economic and geomechanical analyses, the critical angle of slope of the nonmining wall in Delken deposit pit which depends on the properties and the state of rock building up the wall and its structures, and in order to determine the forms and the elements of mineral occurrence, the authors carried out comprehensive research consisting of the of deposit’s geological and structural features determination, including endogeneous and exogeneous jointing, and the detailed examination of physical and mechanical properties of rock and ore. Research aim is to study geological-structural properties of the deposit in order to analyze and substantiate the optimal and safe structure of pit walls when developing ore at the Delken gold deposit. Methodology. The structural features of Vodorazdelny ground were determined by carrying out the comparative analysis of geological sections and maps, structural description of core and building the scheme of rock blocking according to the level of destruction during deposit opencast. Physical and mechanical properties of rock were determined in laboratory conditions. Numerical analysis of wall stability was fulfilled with modern software. Results. The comprehensive research consisting of the analysis of mining and geological conditions of development, deposit’s geological and structural features determination, detailed examination of the properties of rock building up the slopes of the quarry have made it possible to substantiate the rational design of pit walls when developing ore reserves of the deposit. Summary. The results of the research carried out at the deposit may be applied when substantiating the safest technology of uncovering the deposit and the safest mining sequence.

Geomechanical Analysis to Avoid Serious Drilling Hazards in Zubair Oilfield, Southern Iraq

Zubair oilfield is an efficient contributor to the total Iraqi produced hydrocarbon. Drilling vertical wells as well as deviated and horizontal wells have been experiencing intractable challenges. Investigation of well data showed that the wellbore instability issues were the major challenges to drill in Zubair oilfield. These experienced borehole instability problems are attributed to the increase in the nonproductive time (NPT). This study can assist in managing an investment-drilling plan with less nonproductive time and more efficient well designing.
 To achieve the study objectives, a one dimension geomechanical model (1D MEM) was constructed based on open hole log measurements, including Gamma-ray (GR), Caliper (CALI), Density (RHOZ), sonic compression (DTCO) and shear (DTSM) wave velocities , and Micro imager log (FMI). The determined 1D MEM components, i.e., pore pressure, rock mechanical properties, in-situ principal stress magnitudes and orientations, were calibrated using the data acquired from repeated formation test (RFT), hydraulic fracturing test (Mini-frac), and laboratory rock core mechanical test (triaxial test). Then, a validation model coupled with three failure criteria, i.e., Mohr-Coulomb, Mogi-Coulomb, and Modified lade, was conducted using the Caliper and Micro-imager logs. Finally, sensitivity and forecasting stability analyses were implemented to predict the most stable wellbore trajectory concerning the safe mud window for the planned wells.
 The implemented wellbore instability analysis utilizing Mogi-Coulomb criterion demonstrated that the azimuth of 140o paralleling to the minimum horizontal stress is preferable to orient deviated and horizontal wells. The vertical and slightly deviated boreholes (1ess than 30o) are the most stable wellbores, and they are recommended to be drilled with 11.6 -12 ppg mud weight. The highly deviated and horizontal wells are recommended to be drilled with a mud weight of 12-12.6 ppg.

Slope mass rating-based analysis to assess rockfall hazard on Yogyakarta Southern Mountain, Indonesia

In the Parangtritis Beach tourism area located in the Southern Mountain of Yogyakarta, karst hills were excavated to build the main accessing road and produce some of long and very steep slopes along the sides of the road. But still, there was none of the slope reinforcement installed along the road. Meanwhile, at several nearby locations within Southern Mountain, rockfall incidents have occurred many times even caused casualties. The potential of rockfall hazard also could be found in the main road of Parangtritis Beach as the study area. The purpose of this study is to determine the rockfall hazard assessment along the main road using Slope Mass Rating (SMR) analysis with the additional parameter of the slope height and the rock block size. The necessary data obtained by direct measurement and laboratory test. Geomechanics analysis, stereographic projection analysis, and hazard parameters weighting were carried out to produce the Rockfall Hazard Zonation Map of the study area. Based on 17 measurement stations, there are 4 (four) rockfall hazard classes in the study area, i.e., very low, low, moderate, and high. The class of very low, which also included road segments without slope, has the largest percentages of 83.83%, followed by the classes of moderate, low, and high with the percentage of 7.16%, 4.28%, and 4,19%, respectively. SMR was assumed as the most significant parameter that influences the rockfall hazard zonation. Historical rockfall points were overlaid over the Rockfall Hazard Zonation Map to validate the predicted hazard zones. Since 91.23% of the rockfall occurred in the moderate and high hazard classes, the zonation map considered reliable to predict future rockfall. This study also identified several landslide potential zones and provides the recommendation of slope reinforcement to be installed in the study area.

Stress field orientation controls on fault leakage at a natural CO<sub>2</sub> reservoir

Abstract. Travertine deposits present above the St. Johns Dome natural CO2 reservoir in Arizona, USA, document a long (>400 kyr) history of surface leakage of CO2 from a subsurface reservoir. These deposits are concentrated along surface traces of faults, implying that there has been a structural control on the migration pathway of CO2-rich fluids. Here, we combine slip tendency and fracture stability to analyse the geomechanical stability of the reservoir-bounding Coyote Wash Fault for three different stress fields and two interpreted fault rock types to predict areas with high leakage risks. We find that these areas coincide with the travertine deposits on the surface, indicating that high-permeability pathways as a result of critically stressed fracture networks exist in both a fault damage zone and around a fault tip. We conclude that these structural features control leakage. Importantly, we find that even without in situ stress field data, the known leakage points can be predicted using geomechanical analyses, despite the unconstrained tectonic setting. Whilst acquiring high-quality stress field data for secure subsurface CO2 or energy storage remains critical, we shown that a first-order assessment of leakage risks during site selection can be made with limited stress field knowledge.

Influence of hysteretic stress path behavior on seal integrity during gas storage operation in a depleted reservoir

In this study, we numerically investigate the influence of hysteretic stress path behavior on the seal integrity during underground gas storage operations in a depleted reservoir. Our study area is the Honor Rancho Underground Storage Facility in Los Angeles County (California, USA), which was converted into an underground gas storage facility in 1975 after 20 years of oil and gas production. In our simulations, the geomechanical behavior of the sand reservoir is modeled using two models: (1) a linear elastic model (non-hysteretic stress path) that does not take into consideration irreversible deformation, and (2) a plastic cap mechanical model which considers changes in rock elastic properties due to irreversible deformations caused by plastic reservoir compaction (hysteretic stress path). It shows that the irreversible compaction of the geological layer over geologic time and during the reservoir depletion can have important consequences on stress tensor orientation and magnitude. Ignoring depletion-induced irreversible compaction can lead to an over-estimation of the calculation of the maximum working reservoir pressure. Moreover, this irreversible compaction may bring the nearby faults closer to reactivation. However, regardless of the two models applied, the geomechanical analysis shows that for the estimated stress conditions applied in this study, the Honor Rancho Underground Storage Facility is being safely operated at pressures much below what would be required to compromise the seal integrity.

Research on Coupling Control Technology of Constant Resistance in Gob-side Entry Under Mined Gob of Recent Coal Seam Group

Taking the deep gob-side entry under the mined gob in the Nantun coal mine as the background, indoor physical mechanic property research, situ engineering geomechanics analysis, theoretical formula analysis and three-dimensional numerical simulation were adopted. The type of deformation mechanism of this roadway was confirmed. The coupling control technology of constant resistance was proposed and used in practice successfully in situ engineering. Practice was proved that the coupling technology of constant resistance based on NPR bolt could absorb the energy accumulated in surrounding rock and ensure the total safety of the roadway.

Geomechanical characterization of CO2 storage sites: A case study from a nearly depleted gas field in the Bredasdorp Basin, South Africa

Geomechanical analysis and integrity assessment of hydrocarbon reservoirs upon depletion and injection are crucial to ensure that CO2 storage projects can be safely implemented. The Bredasdorp basin in South Africa has a great potential for CO2 storage given its hugely available exploration data. However, there has not been any geomechanical characterization carried out on this basin to determine its integrity issues. The aim of this study is to provide a guideline as to how geomechanical analysis of depleted fields can be done for a safe CO2 sequestration practice. The results obtained from the geomechanical model constructed for the depth of 2570 m indicated that the magnitude of the principal vertical, minimum and maximum horizontal stresses in the field are respectively 57 MPa, 41 MPa and 42–46 MPa, indicating the presence of a normal faulting regime in the caprock and the reservoir. However, according to the pore pressure-stress coupling assessment, this normal faulting is much severe in compartment C3 of the reservoir. Fault reactivation and fracture stability were also investigated after depletion and it was found that faults in the compartments C1 and C2 are stable after depletion. However, normal faults (FNS8 and FNS9) in compartment C3 dipping SW were critically stressed and may be reactivated without a proper injection planning. Fractures in compartment C3 were also critically stressed, highlighting a great potential of leakage from this compartment upon injection. It was also revealed that the sustainable maximum fluid pressure of 25 MPa would not induce any fractures in the reservoir during CO2 storage.

Information technologies of geomechanical events integrated simulation

Introduction. Neither national nor foreign practice can guarantee high-quality assessment of enclosing rock mass state and reliable substantiation of rational geomechanical solutions. It is confirmed by actual accident and breakdown rates including catastrophic force majeure. So, current methods of geomechanical analysis need principal improvement. Research aim is to develop information technologies of integrated deterministic stochastic modeling within the system of assessing and ensuring the quality of geomechanical solutions, which is the set of interconnected estimates and indicators, such as efficient, available, reliable and adequate information on enclosing rock mass properties and state, compliance with the requirements of normative documents, geomechanical risks rates by the factors of breakdown, accident and environmental hazard factors, potential damage from risk aftermath, financial receipts and expenses which include dominant functions of potential damage and monitoring effectiveness. Methodology. Research has been carried out based on the simulation builder, basic software aimed at design, checkout and probation of new integrated deterministically stochastic models of geomechanical events. The models fulfill research, training and demonstration functions, reserve modeling process and output, simulation errors and drawbacks elimination by Monte Carlo method. The main feature of the builder is the availability of interconnected components, determination and simulation. Such complex provides the researcher with important complementary information: deterministic method fulfills safety functions and determines analysis results non-contradiction to the requirements of normative documentation; probabilistic method ensures objective evaluation of geomechanical risks or safe features with the possibility to obtain clear effectiveness by the factors of economic and social security, eliminates or significantly reduces the probability of unnecessary expenses. The universality of the approach to creating information technologies of geomechanical solutions simulation which implement the deterministic simulation complex based on the methods of nonparametric statistics is confirmed by the instances of simulation in multivariate statement. The scope of the investigation includes mine engineering structures of construction, open, integrated technologies of mining enterprises and specific underground development targets in various sectors of national economy. Summary. Information technologies of geomechanical events simulation based on the complex of deterministic and simulation methods and the procedures of nonparametric microstatistics make decisive contribution in the system of assessment and ensuring the quality of geomechanical solutions. The universality of IT development is noted together with their availability for a wide range of users, their simplicity and implementation efficiency

Multivariate optimization of tight core retrieval schedule based on numerical simulation. A novel approach

The quality and safety of core recovery determine the success of the petrophysical and geomechanical analyses. It becomes crucial for low-permeable (tight) formations, including shales, where rock stress-strain and strength properties drive well completion and treatment design.There are few technologies providing sample condition monitoring while lifting and protecting the core from deformation/damaging and fluid release. A feasible option is to simulate core tripping and optimize core retrieval parameters such as lifting rate, number, and duration of stops to meet operational requirements (time, cost, rate).The article presents a new approach to solve the safe core tripping problem. Firstly, we discuss the implementation issues of state-of-the-art poroelastic Laplace-inversion-based analytical solutions and numerical approximation models. Then we introduce an approach based on the poroelastic model assuming vertically transversely isotropic permeability. The method features an updated equation for decompression and failure criteria, determined by the properties of the porous media of the rock. We used the obtained solution to simulate typical core tripping scenarios and optimize the core tripping schedule for total retrieval time. Finally, we discuss the account for micro fracturing, core damage mechanics, and improvements of decompression models.In summary, the article makes several practically essential conclusions. The strength properties — not only tensile strength but also fracture toughness of the rock are crucial for finding the optimal safe core tripping regime. The poroelastic simulation shows that tensile stresses cause fracturing of the core inner region; damage initiates from existing (natural or drilling-induced) defects (microfractures, cracks). The presented core tripping algorithm, unlike the existing ones, optimizes the tripping rate-depth profile intelligently depth-by-depth and does not require calculating the whole core lifting case.The application of the approach increases the prediction accuracy, avoids overestimation of the time required for a safe core tripping, and, in principle, allows estimating the degree of core damage.

Geomechanical characteristics of potential carbon dioxide sinks and seals in the DeSoto Canyon Salt Basin, eastern Gulf of Mexico

Recent studies indicate vast storage capacity exists in Cretaceous and Paleocene–Miocene sandstones in the eastern Gulf of Mexico. Understanding the geomechanical integrity of reservoirs and associated seals is useful for reducing the risk of migration of injected CO2 out of the storage complex. In this study, we investigated the rock mechanical properties and reservoir-seal integrity of three potential reservoir units in the DeSoto Canyon Salt Basin using geophysical well logs. Geomechanical analysis shows the Cretaceous reservoir units contain competent carbonate cap rocks that arrest possible fracture propagation from injection. Results of reservoir and seal integrity analyses indicate all three potential reservoirs and associated seals are geomechanically stable. The injection pressure of CO2 should stay below the minimum horizontal stress to prevent the occurrence of tensile hydraulic fractures. The three candidate reservoir units are ranked according to reservoir geomechanics for long-term commercial carbon storage. Future work should focus on geomechanical simulation of the reservoir units and additional assessment of leakage risk to further ensure safe carbon storage during and after CO2 injection. Laboratory core tests are recommended to confirm the accuracy of the empirical geomechanical analysis of the candidate reservoir and seal rocks.

A comprehensive geomechanical method for the assessment of rockburst hazards in underground mining

Rockburst hazard in mining industry all over the world is one of the most severe hazards. It is becoming increasingly common because of the ever-growing depths of mining operations accompanied by the increasing strength of rocks. One of the most difficult issues is to predict this hazard before the mining operations, whether geophysical investigations have been conducted or not. Polish experience in this field shows that in such cases an effective solution can be the geomechanical method. Therefore, extensive studies on rockburst hazard should focus on three main aspects: (1) rock mass and rock (and coal) predisposition to rockburst–laboratory tests and empirical analyses based on lithology, (2) identification of the potential places with stress and elastic energy concentration in the rock mass within the area planned for exploitation, and (3) the assessment of the impact of mining tremors on the surface. This preliminary geomechanical analysis assesses the propensity of the rock mass to dynamic breakage and provides quantitatively the level of rockburst hazard. The paper presents Polish experience in rockburst hazard assessment with the use of geomechanical method, as well as some solutions and examples of such analyses.

Analysis of the Collapse Gradient of Deep Water Horizontal Wellbore and the Effects of Mud Chemical Activity and Variation in Water Depth

Abstract Wellbore collapse is an instability-event that occurs at low mud density and leads to unfavorable economic project, reaching billions of US dollars. Thus, it is important to accurately determine its value, especially in deepwater horizontal wellbores. The main reasons for nontrivial problems with such wellbores are evident: the shale encountered are anisotropic in nature and possess planes of weakness; they react with water-based mud, generate osmotic stresses, swell, and fall unto the wellbore bottom, thereby increasing the non-productive time. To this end, salts are added to reduce the collapse tendency, but it is not currently known what amount of salt addition maintains stability, and does not lead to wellbore fracture; in deepwater, the current trend in global warming means there is a future concern to the industry. As the climate temperature increases, more ice melts from the polar region, the seawater expands and the sea level rises. How to incorporate the corresponding effect on collapse gradient is scarcely known. This study captures the major concerns stated above into wellbore stability analysis. Following the classical approach for geomechanical analysis, Mogi-Coulomb criterion was combined with a constitutive stress equation comprising contributions from mechanical and osmotic potentials of mud and shale. A sophisticated industry model was used to consider the deepwater effect. The results show significant reduction in collapse gradient as the water depth increases, also, larger difference between the mud and shale chemical activities represents higher complexities in the wellbore. In addition, the reduction in the chemical activities of mud limited to 37.5% of the initial value can be practically safe.