Geomechanical Factors Research Articles

Coal mine methane utilization as a component of the complex mining concept in the context of conversion into thermal and electrical energy

The paper deals with multi-component complex mining of mineral raw materials in the context of sustainable development and its important component – coal mine methane utilization and its conversion into thermal and electrical energy. The conducted analysis of scientific and practical experience has shown the multi-parameter nature of this process and the influence of a large number of factors. The mining-geological conditions of the Western Donbas (Ukraine) are substantiated as the research object, which makes it possible to limit the variation of geomechanical factors, as well as to develop basic provisions and general principles for reasoning predictive assessments of outgassing during complete mining of coal reserves. The dependence of gas volume on specific gas content of lithotypes and the volume of their destruction related to the coal-bearing stratum shear mechanisms at different options of complete mining of the Western Donbas coal reserves, is presented. The patterns of influence of the main geomechanical factors on the propagation of disturbed rock zones are substantiated.

Geomechanical investigation of casing collapse using finite element modeling: The role of cement sheath integrity

Casing collapse has been reported in several oil and gas wells all around the world. Cement sheath failure is one of the primary factors that may result in casing collapse. The growing focus on maintaining well integrity over the long term has prompted the creation of more detailed, realistic, and accurate models. Mechanical modeling of the casing-cement-formation setting can reveal the cement's capacity to withstand in-situ stresses, which is critical to sustaining the zonal isolation of the well throughout its lifetime. To date, the literature lacks a rigorous three-dimensional mechanical model to take into account all the geomechanical factors. This study aims to investigate the role of cement sheath failure in casing collapse by developing such a model. To do so, first, a three-dimensional numerical model was built in Abaqus using input data from a collapsed casing case study. Then, the required input data into the model were determined by following a step-by-step geomechanical procedure. Furthermore, to assess the model's accuracy and quality of the mesh, the numerical model was validated against an analytical model for stress distribution around the wellbore. As a preventive measure against casing collapse in upcoming infill wells, a new cement slurry was formulated, and its mechanical characteristics were measured through triaxial tests. The numerical results of the current wellbore cement and newly designed cement were compared to assess the effect of cement sheath integrity on casing collapse. The results revealed that the casing collapse in the case study can be highly attributed to the cement sheath shear failure and migration of the over-pressured formation fluid to the micro-annuli formed at the cement interfaces with the formation and the casing due to plastic deformation. The procedure outlined in this study can be used to design the wellbore cement according to the geomechanical factors to ensure well integrity.

Płuczki wiertnicze w skomplikowanych warunkach: przegląd

The review in this article focuses on various aspects of drilling extended reach (ERD) wells. Reaching extreme depths and setting world records for deviation illustrates the importance of well design and operating strategies. Studies of articles describing various locations, including Sakhalin Island in Russia and offshore Vietnam, provide insight into ERD operations. Furthermore, the challenges of drilling in specific geological conditions, such as layered sandstones and reactive clay, are considered. Particular attention is paid to issues related to wellbore instability and drilling fluid optimization. The results of technical studies highlight the key role of maintaining wellbore stability in achieving successful ERD results. The articles emphasize the importance of understanding geomechanical factors, employing optimal mud weight, lubrication, and specialized drilling fluids to counteract instability. It demonstrates that maintaining appropriate mud weights and employing specific drilling techniques are crucial for mitigating instability-related issues. The integration of mechanical and chemical approaches is advocated for effectively managing shale-related instability. The utilization of innovative materials and fluid systems is central to the successful resolution of stability-related problems. The incorporation of micronized sealing polymers in conjunction with conventional plugging materials is detailed as an effective approach to counter wellbore instability. The synergistic combination of materials, additives and mud salinity is showcased to achieve effective shale stabilization and optimize drilling time. The authors emphasize the importance of selecting the optimal composition for each well based on experience and laboratory testing and present laboratory-tested solutions that have been successfully applied in field operations. In summary, these articles collectively offer insights into a range of strategies to combat wellbore instability. They cover the use of advanced materials, innovative fluid systems, and chemical approaches to maintain wellbore stability, improve drilling efficiency, and reduce nonproductive time.

Geomechanical aspects of the choice of underground mining

The purpose and the relevance of the research. The article is devoted to the current problem of choosing the optimal development system for the new field being put into operation, according to the conditions of localization of the field. The purpose of the research is to substantiate the effective technology of field development according to geomechanical conditions. Methodology. Based on the literature data, a critical analysis of the completeness of the extraction of reserves and the effectiveness of the development systems recommended for use with open goaf and ore caving is carried out. Based on the analysis of technology options, an engineering forecast of the results of their implementation is made and an alternative version of the system is recommended. Results and scope. The mining and geological conditions for the localization of a gold ore deposit and the scheme for opening its reserves are described. The characteristics of the variants of development systems recommended by the project for use, the analysis of the quality indicators of the extraction of reserves are given. An alternative technology has been proposed with the backfilling of the goaf with hardening concrete mixtures, which is distinguished by higher quality indicators of the mined ores. It is shown that the quality of ores is a function of the completeness of taking into account the geomechanical factors of exploitation of deposits. It is noted that the utilization of ore enrichment tailings is possible only if they are demetallized to certain limits, and the direction for this is indicated. An ecological and economic model for evaluating the effectiveness of the field development technology in terms of the ratio of ecological and economic indicators is proposed. Conclusions. The conditions of the considered complex-structural deposit of valuable ores are met by variants of development systems with backfilling of the worked-out space with hardening concrete mixtures, which provide control of stresses in the ore-bearing massifs and a significantly higher quality of the mined ores, the equipment and infrastructure used.

Development of a system for ranking geomechanical factors, which influence the stability of uranium mines workings

The subject of research presented in the article are processes that take into account the influence of geomechanical factors on the stability of mining workings of uranium mines. The purpose of this work is development of a system for ranking geomechanical factors by the degree of danger based on the analysis of risk factors that affect the stability of uranium mines. The work uses critical analysis and generalization of both own research results and the results of domestic and foreign authors. The geomechanical factors that affect the stability of the mining workings of uranium mines have been determined. It is shown that their influence has both a direct and an indirect effects due to the connection with mining-geological and technological parameters. The stability of the workings is significantly influenced by mining and technical factors (outcropping time, working space length, floor height, compression properties and structure of the backfill mass, chamber width, dynamics of cleaning works progress), mining and geological factors (depth of mining, thickness of the ore body, strength (deformation)) and physical and mechanical properties of the rock mass. In addition, the engineering and technical conditions of mine construction have a significant impact, in particular, the form and size of the workings, their orientation in the massif, the method of carrying out and supporting, the design and technology of fastening, etc. It is practically impossible to simultaneously take into account all factors in an analytical way, so it is necessary to select one or two main factors that are of decisive importance for the description of a specific geomechanical process. The originality of the work is the proposed ranking of geomechanical factors that affect the stability of mining workings of uranium mines. The ranking of geological, physical and mechanical factors on the stability of the chamber system elements at underground mining of uranium ores, according to the degree of attenuation of the influence, is as follows: strength of rocks → structure of the massif → angle of fall of the deposit → water content of the deposit → thickness of a seam. The ranking of the influence of support preparatory and capital workings on their stability, according to the degree of risk decrease, is as follows: no support → grid with anchors → sprayed concrete → wooden support → metal frame support with tightening → metal arches in concrete. Generalized ranking of the influence of the most significant factors on the stability of mining workings: geological, physical and mechanical factors → parameters of the development system → fastening technologies.

ПРОГНОЗ РАЗВИТИЯ ГЕОМЕХАНИЧЕСКОЙ СИТУАЦИИ ПРИ ПОВТОРНОЙ КОНСЕРВАЦИИ ШАХТЫ "ЮГО-ЗАПАДНАЯ"

The paper presents the results of the study of the probable scenario of geomechanical processes during double flooding of a mine developing steeply declining gold veins of low thickness. Analytical calculations, field measurements and finite-element modelling have been carried out. Unfavourable geomechanical factors, which may increase the probability of dangerous ground surface shifts and collapses, are determined. Linear and angular parameters of the zone of harmful effect of mining works, the level of displacement in the neighbourhood of mine workings were calculated. Natural and anthropogenic stresses at the mine before and after mothballing were assessed. The level of rock displacements in the zone of influence of mining works was established.

Integrated modeling of fracturing-flowback-production dynamics and calibration on field data: Optimum well startup scenarios

We aim at the development of a general modelling workflow for design and optimization of the well flowback and startup operation on hydraulically fractured wells. Fracture flowback model developed earlier by the authors is extended to take into account several new fluid mechanics factors accompanying flowback, namely, viscoplastic rheology of unbroken cross-linked gel and coupled “fracture-reservoir” numerical submodel for influx from rock formation. We also developed models and implemented new geomechanical factors, namely, (i) fracture closure in gaps between proppant pillars and in proppant-free cavity in the vicinity of the well taking into account formation creep; (ii) propagation of plastic deformations due to tensile rock failure from the fracture face into the fluid-saturated reservoir.We carried out parametric calculations to study the dynamics of fracture conductivity during flowback and its effect on well production for the set of parameters typical of oil wells in Achimov formation of Western Siberia, Russia. The first set of calculations is carried out using the flowback model in the reservoir linear flow regime. It is obtained that the typical length of hydraulic fracture zone, in which tensile rock failure at the fracture walls occurs, is insignificant. In the range of rock permeability in between 0.01 mD and 1 D, we studied the effect of non-dimensional governing parameters as well as bottomhole pressure drop dynamics on oil production. We obtained a map of pressure drop regimes (fast, moderate or slow) leading to maximum cumulative oil production. The second set of parametric calculations is carried out using integrated well production modelling workflow, in which the flowback model acts as a missing link in between hydraulic fracturing and reservoir commercial simulators. We evaluated quantitatively effects of initial fracture aperture, proppant diameter, yield stress of fracturing fluid, pressure drop rate and proppant material type (ceramic and sand) on long-term well production beyond formation linear regime. The third set of parametric calculations is carried out using the flowback model history-matched to field data related to production of four multistage hydraulically fractured oil wells in Achimov formation of Western Siberia, Russia. On the basis of the matched model we evaluated geomechanics effects on fracture conductivity degradation. We also performed sensitivity analysis in the framework of the history-matched model to study the impact of geomechanics and fluid rheology parameters on flowback efficiency.

Substantiating the optimization solutions for the mine working fastening system interaction with the enclosing rock mass

Purpose. Determination of the rational interaction modes between the fastening system and the extraction working enclosing mass in the zone of stope operations influence. Methods. An algorithm for searching for optimal solutions for the interaction modes between the fastening system and the coal-bearing mass has been substantiated. The deformation-strength characteristics of the fastening system elements have been agreed. The design parameters of the support elements have been optimized according to the criterion of their equal strength. According to the optimal parameters, a methodology for calculating the function that describes the rational deformation-strength characteristic of the fastening system, depending on the mining-geological conditions, has been developed and substantiated. Findings. Computational experiments have been conducted to determine the rock mass deformation-strength characteristic. Based on the normative documents, the sizes of the natural equilibrium arch have been calculated. The adequacy of methodical principles for minimizing the load on the fastening system has been proved. The patterns for the influence of geomechanical factors on the choice of optimal parameters of the fastening system deformation-strength characteristics have been determined. A methodology for calculating the rational parameters of the fastening system and its constituent elements has been obtained. Originality. Combined studies of minimizing the load on the fastening system have been conducted. The patterns for the influence of geomechanical factors on the choice of load-bearing capacity and the yielding property value of the fastening system have been determined. Regression equations have been obtained for calculating the fastening system optimal parameters with a geomechanical index of working conditions. This enables implementation of a unified strategy for resource-saving improvement in fastening systems. Practical implications. A methodology has been developed for obtaining the weakening mass deformation-strength characteristic, depending on the depth of mine working location, the texture of the rocks in the coal-overlaying formation and its strength properties. The applicability of the methodology for the implementation of a unified strategy of resource-saving improvement of the mine working fastening systems for the Western Donbas mines has been proved.

К природо- и ресурсосберегающим технологиям подземной разработки месторождений сложной структуры

The results of long-term scientific and practical studies of the completeness of the use of mineral resources and the safety of miners in the underground mining of rock type ore deposits are presented. The goal is to improve the technologies for the extraction and processing of resources on the principles of nature and resource conservation in the context of a reduction in the mineral reserves, the use of poor metal-containing raw materials and mining tailings. Proposals are developed to improve the management of the state of ore-bearing massifs. The regularities of their behavior under conditions of volumetric compression and geomechanical balance of elements of the natural and technogenic system are considered. Recommendations are given to improve the parameters for managing the state of massifs adjusted for the geomechanical and seismic conditions and structural features of the deposits. Promising directions for further research of the problem are outlined. Their efficiency was assessed by comparing the options for the development of the situation considering the costs of redistributing and protecting the population in the zone of mining facilities influence. The role is established concerning the structures made of destroyed rocks in natural massifs in optimizing the normative strength of artificial massifs created in the developed space. There is a tendency towards stricter requirements for environmental protection and occupational safety in the mining industry. It is shown that the optimization of mining technology based on the geomechanical factors increases the safety of production and reduces the negative impact on the environment and the population of the mining region. An assessment is given of the prospects for expanding the scope of application of low-cost technologies for managing the state of rock masses in the underground development of complexly structured deposits.

Spatial orientations of hydraulically conductive shear natural fractures for an arbitrary stress state: An analytical study of governing geomechanical factors

Hydraulic conductivity of natural shear fractures is considered with regard to the current stress state of fractured rocks. The paper focuses on spatial orientations of hydraulically conductive shear fractures existing in naturally fractured fluid saturated rocks. The concept of critically stressed fractures is used for the analysis. The paper presents an algorithm and analytical solution that can be used to obtain all possible spatial orientations of critically stressed fractures for an arbitrary stress tensor. The proposed solution is explicit, providing functionality to predict the spatial orientations of hydraulically conductive fractures for rocks subjected to arbitrary stresses. The results obtained from the presented methodology can be directly used to deal with results of geomechanical modeling of naturally fractured reservoirs development. Several analyses of applying the obtained solution in practice are presented in the paper: spatial orientations of critically stressed fractures are obtained for cases of different gradual changes in stress tensor components under specific conditions, providing an understanding of the main tendencies in these spatial orientations. According to the obtained results, spatial orientations of critically stressed fractures tend to be related to directions of principal stresses, while magnitudes of principal stresses govern the shapes of zones containing poles to critically stressed fractures at stereonets. The influence of the following major geomechanical factors standing for hydraulic conductivity of natural fractures is studied: magnitudes and directions of principal stresses, friction coefficient of rock. Stereonets are used to visualize the changes in spatial orientations of hydraulically conductive fractures, caused by alteration of stress state of the rock.

Constructing a Heavy Oil Well

The article presents a description of the designs of wells intended for the production of high-viscosity oil. The main problems associated with the planning and deployments of architecture, construction of high-viscosity oil wells are described. World experience in well construction is presented. Vertical wells are usually used for primary cold production and cyclic steam or steam flooding processes. On the other hand, increased reservoir contact may require deviated, horizontal, or multilateral wells. In the case of steam-assisted gravity drainage (SAGD) and some solvent injection processes, the recovery process may require a well-placed pair of horizontal wells. Advanced drilling and real-time measurement technologies reviewed. Geo mechanical factors are studied when considering the implementation of any steam or thermal processes in the field. Examples of construction of multilateral wells in various combinations are shown depending on the field development strategy and for maximum reservoir drainage. The main recommendations for the placement of wells are proposed.

Probabilistic-based geomechanical assessment of maximum operating pressure for an underground gas storage reservoir, NW China

One of the most cost-effective ways to increase deliverability and working gas capacity in underground gas storage (UGS) reservoirs is to operate the gas storage at higher pressure (increased delta pressure). Maximum operating pressure (MOP) for a gas storage reservoir depends on several geomechanical factors, including in-situ stresses, stresses induced by local and global pressure changes in the storage reservoir, and the stabilities of the caprock and faults. PetroChina’s Hutubi (HTB) gas storage site is located in the west gate of China’s west–east gas pipeline project. Multiple major faults intersect the gas storage reservoir and caprock, which pose safety concerns for increasing the gas storage pressure. The typical practice in an HTB storage site has been to operate gas storage pressures at levels equal to the original reservoir pressure because of concerns related to caprock integrity and fault stability. To investigate the possibilities of increasing the gas storage pressure, a comprehensive geomechanical study has been conducted. By utilizing the data from rock mechanics laboratory testing, in-situ stress measurements, 3D field-specific geomechanical models, and coupled reservoir-geomechanical simulations, this paper presents probabilistic risk assessment of fault slippage in response to injection-related increases in pore pressure. Uncertainties of relevant geomechanical parameters, such as the frictional strengths of faults, orientation, and magnitudes of in-situ stresses, have been incorporated through Monte Carlo simulations. The authors find that: (1) fault slippage is the controlling factor which determines the limit of MOP at a HTB gas storage site; (2) increasing the gas storage pressure by 12.5% above the initial reservoir pressure (34.5 MPa) leads to approximately 0.6% probability of fault instability that could provide a leakage pathway. Thus, from a geomechanical perspective, operating the storage pressure at initial reservoir pressure at a HTB gas storage field is an overly conservative approach that leads to under-utilization of the existing storage capability. This paper provides the first integrative geomechanical study for increasing MOP for large underground gas storage reservoir in China. The methodology as well as comprehensive data repository are potential sources for future geomechanical studies on similar underground gas storage projects.

Analysis of Shale Oil Productivity Difference in an 83 and Xi 233 Well Blocks in Ordos Basin

The An 83 well block and the Xi 233 well block are both shale oil blocks in the Changqing Oilfield. The main producing layer is the Chang 7 reservoir, with strong heterogeneity and no obvious oil-water interface. Both are typical low porosity, Low permeability-extra low permeability reservoirs. The An 83 block and the Xi 233 block adopt the same volume modification method for production, but the output data of the two are quite different. The output of the Xi 233 block is significantly better than the An 83 block, and the water content is also lower. Finding out the reasons for the production differences and proposing a targeted shale oil reservoir volume modification model is the key to increasing the production of Block An 83. On the basis of the analysis of production influencing factors, it combines geological factors such as gamma, interval transit time, porosity, saturation, etc., geomechanical factors such as Young’s modulus, Poisson’s ratio, horizontal stress, and Engineering factors such as displacement, liquid volume, sand ratio, etc. Using Pearson and Spearman correlation analysis methods to clarify the main controlling factors of output and capacity, on this basis, complete the research on the output difference of the two blocks. The final results show that the physical properties of the two blocks are the same, the water content is quite different, and the geomechanical basis is different. It is necessary to formulate targeted reservoir transformation and production methods based on the pore throat structure, relative permeability and geomechanical distribution characteristics of Block An 83.

Development of a methodology for assessing the expediency of mine workings decommissioning based on the geomechanical factor

Purpose. Substantiation of the methodology for predicting the state of mine workings based on the study on geomechanical processes when assessing the consequences of mine closure taking into account the entire period of their existence, during which the development of displacement with various intensity occurs in the surrounding coal-bearing mass. Methodology. The study is based on methods of analysis and synthesis, methods of comparison, abstraction, analogy, calculation and construction. The methods of mine tool observations of the manifestations of rock pressure and their processing by methods of correlation and dispersion analysis for establishing the relationship of displacements of the mine working contour with geomechanical factors were used. Findings. A possibility of stage-by-stage decommissioning of mine workings when grouping mining-and-geological conditions is substantiated. An example of calculating the displacements in a mine working during its decommissioning is presented. The given calculation expressions make it possible to assess the mine working state, taking into consideration the patterns of the geomechanical factor influence on making a technical decision on the expediency of its further operation. Originality. The patterns of the rock pressure manifestation development in sequentially abandoned mine workings have been determined. Based on the methods of correlation-dispersion analysis, the dependence of the mine working contour displacements on geomechanical factors have been revealed throughout the entire period of its existence. Practical value. A methodology for assessing the state of mine workings at the time of their decommissioning has been developed, which is an integral part of the recommendations to limit the negative influence of mine closure. The peculiarity of the methodology is in taking into account the entire period of mine workings existence, which leads to a well-grounded technical decision on the possibility of dismantling the metal structures with the complete exclusion of emergency situations.

Classification of geomechanical factors that create risks in mines

The subject of research presented in the article is geomechanical processes that create risks in the underground mining of minerals for the extraction of solid minerals. The aim of the work is to analyze the geomechanical factors that create risks in the construction and operation of underground workings in complex mining and geological conditions, development and classification according to the degree of impact on the safety of mining. In the paper, the methodological approach is used, which is recommended recommended by regulations widely used in a number of highly developed countries, in particular, the British standard "OHSAS 18001", which implements the current risk assessment and continuous correction of actions in accordance with the Schuhart - Deming PDCA cycle, as well as national standard DSTU IEC / ISO 31010: 2013 developed on the analysis of the european experience. Based on our own experience, a number of the most well-known geomechanical factors that create risks during the construction and operation of mine workings have been identified. Among them: depth of mining, geometric parameters of underground working, rock pressure, physical and mechanical properties of rocks, service life of underground working, type of support, hydrogeological structure of the massif, susceptibility to dynamic and gas-dynamic phenomena, stress-strain state of the massif, etc. These factors were classified according to the degree of risk. The factors are indicated, which require constant monitoring and development of special measures and their implementation in the production process in a limited period of time. The factors are separately highlighted, which affect the safety and economic performance of mining enterprises, but belong to the categories of "moderate" or "insignificant", and therefore require periodic monitoring and evaluation with further development of planned measures to eliminate or reduce them. Purpose of the work is to improve safety of miners. This technique is universal. It can be used to assess the operational reliability of engineering facilities under the significant influence of unstable factors of natural origin.

Prediction of penetration rate of drilling by using rock engineering system approach case study: a well in Azadegan oilfield

One of the criteria in the operational efficiency of drilling is the rate of penetration of the drill bit. Numerous factors affect the rate of penetration. Identification of the effective factors on rate of penetration may lead to a more accurate assessment of drilling time, and as a result, the controlling of operational costs. The concurrent effect of the entire influential factors as well as the differing significance of each of them on the rate of penetration makes the study and optimization of drilling operations much more complicated and difficult. Using the rock engineering systems (RES), the impact of effective operational and geomechanical factors on the rate of penetration has been assessed in this article and a model has been proposed for the prediction of the rate of penetration. Data from one of the wells within the Azadegan oilfield have been used in order to study the impact of effective factors on the rate of penetration. To this end, the effective factors on rate of penetration are initially identified and then an index called “the rate of penetration index (ROPi)” is proposed through the application of the rock engineering systems approach. This index has been calculated at four different depths along the aforementioned well. The results suggested the compliance of penetration rate predictions with field observations. Moreover, porosity and uniaxial compressive strength are the most effective factors on the rate of penetration whereas the weight of drilling fluid has the smallest impact. Finally, a classification for the penetration rate index is presented.

ОЦЕНКА СЛОЖНОСТИ ПОЛЕЙ ПРИРОДНЫХ НАПРЯЖЕНИЙ ЗОЛОТОРУДНЫХ МЕСТОРОЖДЕНИЙ ВО-СТОЧНЫХ САЯН

The article presents the main results of studies of initial stresses of three gold vein de-posits located in the Eastern Sayan Mountains: Zun-Kholbinsky, Barun-Kholbinsky, Konevin-sky, in the depth range of 120…940 m. Insitu measurements by the slot unloading method and reverse calculation of primary stresses on the basis of 49 rock pressure cases were made. Key geomechanical factors were determined, and average statistical parameters of natural stresses depending on the combination of these factors were estimated.

Impact of geomechanical processes on formation pressure during the long-term operation of gas deposits

Practical experience in the exploitation of hydrocarbon deposits confirms the presence of geomechanical processes and their significant impact on the state of gas-saturated rock massif. The purpose of the paper is to clarify the geological conditions for the formation of technogenic reservoirs and involving additional volumes of gas under the impact of geomechanical factors while operating the gas deposits. A detailed analysis of the known gas and gas condensate fields at the Eastern oil-and-gas-bearing region of Ukraine has been carried out. In particular, changes in formation pressure of in-operation hydrocarbon deposits have been determined, the values of geostatic and effective rock pressures have been calculated, the thickness of the gas column and the volume of gas reserves growth have been compared. All of the abovementioned have granted the possibility to determine the role of geomechanical processes in assisting to produce additional volumes of gas. A formation pressure increase in a number of gas condensate fields at a late stage of development confirms the action of the formation of additional sources of hydrocarbons due to the geomechanical factor. It is shown that at the first stage of the exploitation of hydrocarbon deposits, geological criteria play a decisive role in the formation of the gas-saturated massif condition. Over time, during field exploitation, geomechanical criteria are added to the geological ones, which help to involve additional volumes of gas. At the same time, the resulting increase in reserves at some deposits is up to 50% or more. It has been established that additional volumes of gas are directly proportional to the capacity of the gas-bearingness stage and the total volume of the gas-bearing structure. Multilayer deposits of sandy-aleuritic composition with a large number of productive horizons and without thick persistent reservoir-seal rocks within a single gas-bearingness stage with a hydrodynamic link between productive layers are the most promising from the viewpoint of the researched mechanism (favorable due to the formation of additional reservoirs of technogenic genesis).

An engineering approach to quantify geomechanical safety factors in UGS programs

Abstract. Underground Gas Storage (UGS) has become one of the most widely used practices to cope with seasonal peaks in energy consumption. The planning of any new UGS facility, or its upgrading to increase the working gas volume and reservoir performance, must be supported by an evaluation of possible induced effects on the environment. From a geomechanical point of view, storage activity results in a cyclic change in stress and deformation in the reservoir rock and the surrounding formations. The main environmental issues to be accounted for when natural fluid pore pressure is planned to be exceeded are the following: (a) the differential displacements at the land surface possibly mining the integrity of ground structure; (b) the integrity of the reservoir and caprock; (c) the possible reactivation of faults, if the target reservoir is located in a faulted basin; and (d) the vertical upheaval and land subsidence that can impact on the surface drainage network in low lying coastal areas. We present an original methodology for evaluating the geomechanical safety of UGS activities using an approach derived from what is traditionally applied in the structural design of buildings. A safety factor, a margin of security against risks, is defined for each of the geomechanical issues listed above. First, a 3D FE-IE numerical model is developed to reproduce the stress and displacement due to the UGS program under evaluation. Then the reservoir pressure is increased until the “failure” condition is reached allowing to evaluate how far the project designed condition is from the above limit. The proposed approach is applied to Romagna, a depleted gas reservoir in Northern Italy converted to UGS, with the aim of investigating the safety of the project to increase the reservoir pressure up to 120 % pi, where pi is the original reservoir pressure before the start of primary production. The 3D geomechanical model has been developed using recent 3D seismic data, land displacements by InSAR, lab tests on reservoir and caprock samples, in-situ Modular Formation Dynamic Tester (MDT) stress tests, and large background information acquired from other UGS reservoirs located in the same sedimentary basin. The analysis outcome has revealed that the investigated scenario is safe, with safety factor larger than 1, in the range from 1.2 to 4. The most critical condition (the smallest safety factor) has been obtained in relation to the mechanical integrity of the reservoir formation, under very conservative conditions (cohesion = 0, friction angle = 30∘).

Determining patterns of the geomechanical factors influence on the fastening system loading in the preparatory mine workings

The authors express their gratitude to the management of DTEK Coal Unit for their help in organizing the experimental research.