Geomechanical Calculations Research Articles

Characterization of low-frequency distributed acoustic sensing signals in hydraulic fracturing stimulation – A coupled flow-geomechanical simulation approach

Low-frequency distributed acoustic sensing (LF-DAS) is one of the promising diagnostic techniques for detecting and characterizing hydraulic fractures. LF-DAS signals can capture fracture hits and the strain field around the hydraulic fracture. However, the interpretation of field LF-DAS data can be challenging due to the complexity of the underground conditions. This study develops a fracture propagation model to simulate the hydraulic fracturing process. The modelling results are analyzed to examine patterns and trends observed in interpreting field LF-DAS data. The fracture propagation model, coupled with the flow and geomechanical computations, is implemented in the MATLAB Reservoir Simulation Toolbox (MRST). The flow and geomechanical calculations are discretized by the finite volume and the virtual element methods, respectively. The hydraulic fracture is set to propagate along a prescribed path with a specific propagation or activation criterion. The accuracy of our model is validated against the KGD analytical solutions for the leak-off-viscosity, storage-viscosity and leak-off-toughness dominated regimes. The simulated stress and strain features are consistent with those interpreted from field LF-DAS signals. Several case studies and sensitivity analyses demonstrate the approach's utility and examine fracture interference, closure, and stress shadowing effects. The modelling work facilitates interpreting field measurement data by investigating characteristics of fracture hits from adjacent wells. The modelling method provides insights into fracture interference and its implications on optimal designs during hydraulic fracturing stimulation.

System of directed loading and rupture of borehole walls for stress assessment in rock mass

Reliable information on the stress–strain behavior of rock mass is one of the top-priority tasks within applied geomechanics and geocontrol. This information enjoys wide application at the project stages and during implementation of all process flows included in mine construction, mineral mining and operation of various-purpose underground structures. Intelligence on the quality and quantity characteristics of stresses in the crust takes a perceptible part in prediction of possible dynamic events, as well as in selection and optimization of solutions on the technique and technology in ground control and stability of mining system structures. The developed procedure of directed loading and rupture of borehole walls for the stress assessment in rock mass can decrease labor content of measurements and increase their timeliness, and can provide the objective information on natural stresses to predict hazardous events induced by rock pressure and to backup geomechanical calculations with the required parameters. Considering the fact that the value of pressure applied by the developed system on the borehole walls in appearance of the Kaiser effect conforms with the value of stresses in intact rock mass, the proposed procedure can enable automated processing of data to eliminate errors and reduce the analysis time.The study was supported by the Foundation for Promotion of Small Businesses in Sci-Tech Environment (Foundation for Promotion of Innovations), Agreement No. 14893GU/2019 dated December 18, 2019, MISIS, Moscow.The author appreciates participation of the Director of the Research Center for Applied Geomechanics and Convergent Technologies in Mining, College of Mining, NUST MISIS, Doctor of Engineering Sciences, Professor V. A. Eremenko, Consulting Advisor to the Chairman of the Board of Global Asia Management, Kyrgyzstan, Doctor of Physics and Mathematics, Professor V. A. Mansurov and the Head of the Geotechnical Engineering Department of Vertex Gold Company, Kyrgyzstan, E. A. Babkin in this study.

Induced seismicity potential based on probabilistic geomechanics for the San Juan Basin CarbonSAFE project

Although geologic carbon sequestration projects have yet to induce — or may never induce — a damaging earthquake, experiences from other deep injection industries such as hydraulic fracturing, enhanced geothermal systems, and saltwater disposal suggest that effective quantitative seismic risk assessment is necessary for deep saline carbon capture and sequestration (CCS) projects. One such imminent CCS project is the San Juan Basin CarbonSAFE Phase III program. We use Monte Carlo probabilistic geomechanical analyses combined with observations of the geologic and operational parameters of the San Juan Basin site and suggest that this project is of low-induced seismic risk. The primary analysis is split into four sections. First, we assess the literature for faults and past seismicity, and at least five faulting scenarios are directly relevant. Second, we develop and calibrate an integrated earth model for the project site. Third, we perform Monte Carlo simulations that consider reasonable uncertainties of the geomechanical parameters. Of five tested faulting scenarios, only one presents high Coulomb failure functions, but fourth, we determine the risk to be low based on the combined lack of historical seismicity, the geologic framework of the faulting scenario, and the presence of saltwater injection at the same depth as the proposed supercritical carbon dioxide injection. The most sensitive parameters in the geomechanical calculations are the fault dip and the coefficient of friction. The least sensitive are the fault strike and the orientation of the maximum horizontal principal stress.

Engineering-geological and geoecological aspects of formation of dry dumps on hydrodumps

The paper discusses the issues of increasing the dumping capacity of hydrodumps and their reclamation by filling them with dump embankments from relatively strong overburden. Without the implementation of this measure, the alluvial massifs are inaccessible for a long time for various mining and transport equipment due to the low bearing capacity of the rocks. The specifics of the formation of the composition, state and properties of rocks of alluvial massifs due to the development of fractionation and filtration consolidation processes are noted. The technology of dumping the surface of hydrodump by controlled deformation of the slopes of the first dump layer has been substantiated. A method has been developed for predicting the parameters of intrusion-replacement of alluvial rocks by bulk ones using numerical modeling of the FEM in the Euler formulation. The obtained mechanical properties and regularities of the deformation behavior of technogenic rocks were used to perform geomechanical calculations when justifying the stable parameters of the slopes of the formed natural-technical systems (NTS) “hydrodump+dump” and compaction of embankments to justify the mining-technical stage of reclamation. A geoecological assessment of the impact of the dump NTS on the natural environment has been carried out, which is expressed in the acquisition of land from circulation and the disruption of the natural landscape. The conditions for the formation of technogenic eluvium on the surface of the dump suitable for biological reclamation are considered.

Determination of horizontal stress orientation in the areas of the Tomsk region

Introduction. The World Stress Map project proves that horizontal stress orientation determination is a global task essential for the majority of geomechanical calculations. However, there is scant data on stress orientations in the territory of Russia at the moment. The task is therefore highly relevant. Research objective is to determine the orientations of maximum and minimum horizontal stresses by separate areas of the Tomsk region and build a map of horizontal stresses. Method of research. The basis for determining the orientations of horizontal stresses is the theory of drilling-induced fracture and borehole breakouts occurrence. The maximum stress orientation coincides with the drilling-induced fracture orientation, whereas the minimum stress orientation coincides with the borehole breakouts orientation or is perpendicular to the maximum stresses. Results. Research results are compiled in a summary table containing mean orientations of horizontal stresses by areas and a map of horizontal stress orientations. Conclusions. A summary map of maximum horizontal stress strike azimuths has been constructed. The stresses are of similar orientation in every well under consideration, except for a well in the North-Shingin area. The average value of maximum horizontal stress orientation has made up 337° northwest and 157° southeast.

Evaluating geomechanical effects related to the production of a Brazilian reservoir

This paper presents a coupled finite element approach for modeling geomechanical effects induced by production/injection processes in petroleum reservoirs. The module developed employs coupled- reservoir analysis using CMG IMEX® as the flow simulator and a finite element program in MATLAB® as the stress–strain simulator, in a two-way explicit partial coupling scheme. The flow and mechanical problems are coupled by the change of effective stress due to the change in pore pressure and by varying stress-dependent reservoir properties, such as pore compressibility, absolute permeability, and porosity. The coupling procedure was applied to the Namorado Field (Campos Basin, Brazil) to quantify the impact of the rock deformation on fluid recovery. Based on the cases studied, the coupled analyses predicted higher oil recovery than the conventional reservoir simulations. The results showed that the reservoir deformation can affect its performance and must be taken into account in reservoir-engineering studies depending on production strategy and reservoir stiffness. Besides, the geomechanical calculations were performed only in the coupling timesteps, reducing the computational effort and making this coupling method feasible on a field scale.

Characterizing fluid flow paths in the Hellisheidi geothermal field using detailed fault mapping and stress-dependent permeability

The Husmuli zone of the SW-Iceland Hellisheidi geothermal field is currently being used for re-injection of geothermal fluids and geothermal CO2 for its permanent storage in the form of carbonate minerals. A fully coupled hydro-thermo-mechanical numerical model was employed to investigate the coupled impacts of these complex processes on the calibration of fluid flow paths, which can have significant implications for the long-term performance of this subsurface reservoir.Employing a combination of high-resolution fault mapping with laboratory measurements of stress dependent permeability coupled into a dual porosity field-scale model, the flow paths were calibrated using results of tracer tests performed at the site using stress-dependent permeability tensors. Although vertically extended faults are the primary fluid flow paths, fractures connecting the faults can play an important role in fluid transport. As the upward flow streamlines manifest, deep geological layers can also deviate the fluid flow towards the shallower layers provoking the vertical flow of geothermal fluids. This highlights the sweet spot for sustainable flow and heat extraction in vicinity of faults intercepting the geological layers at depth of 1100 m.It was also shown that the inclusion of the geomechanical calculations in the history matching of the tracer test could lead to changes in arrival time and peak of the tracer profiles. Results of an independent tracer test were used to validate the model and to demonstrate the predictive capability of the calibrated model. This verifies the consistency of our methodology to incorporate the stress-dependent permeability. The results of this comprehensive modelling study provide insight into the likely fluid flow paths, which can have profound impact on the evaluation of various processes such as CO2 mineralisation taking place in Hellisheidi geothermal reservoir.

Extraction of ore reserves from pillars in room-and-pillar mining

The geotechnical situation in mines using the open stoping methods is governed by complex geomechanical behavior of rock mass around rib and level pillars. The impact of rock pressure on underground excavations grows in this case, and dynamic events due rock pressure happen more often. The latter became the reason for implementing this research. In terms of steeply dipping ore bodies, safe mining of earlier left pillars is feasible. The best scenario for pillar extraction, considering their parameters, is caving to open and partly filled with overlying rocks stopes. The geomechanical calculations during justification of mining parameters during extraction of ore reserves from pillars have determined regular patterns in the stress–strain behavior of rocks mass, which contributes to qualitative and quantitative prediction of safe and feasible mining conditions. The zones of instability in structural elements of the geotechnology, including pillars, are detected. The author proposes the order and sequence of pillar caving to open and partly filled stopes at the admissible completeness and quality of extraction.

Substantiation of stable parameters of dumps on the basis of a geomechanical model of designed objects

Recently, an increase in the depth of opencast mining of practically all types of minerals leads to an increase in overburden volumes. The lack of space for the placement of excavated rocks leads to the optimization of technological solutions regarding dumping. Determination of the stable parameters of the dumps in specific mining and geological conditions by creating a geomechanical model of the designed dumping sites will solve this problem as much as possible. Taking into account all the difficulties and peculiarities of the variety of mining-geological and mining-technical conditions for the placement of external dumps at the stage of designing mountain objects, involving in the creation of a geomechanical block model the physicomechanical characteristics of overburden stored rocks, base soils, taking into account studies at maximum load, the moisture content of the entire dump facilities, power and angle of inclination of the base of the designed facility, climatic conditions, the intensity of the dumping will give the ability to improve the reliability of the obtained geomechanical calculations of the stability parameters of a complex engineering structure. The SURPAC system allows you to display the results of calculations and design in a visual form in real three-dimensional space. Based on the many available initial data, a set of surface maps is developed for the angle of inclination of the blade base, for elasticity, for the thickness of the weak layer, for humidity, with the allocation of zones of the most and least favorable base conditions for the dump dump, and we obtain a map of the distribution of the safety factor of the building under construction.

Seepage Analysis and Optimization of Engineering Method for Municipal Underwater Tunnel Construction in Urban Coastal Stratum

Li X.-Z.; Zhu, Y.-Q., and Zhao, J., 2019. Seepage analysis and optimization of engineering method for municipal underwater tunnel construction in urban coastal stratum. In: Gong, D.; Zhu, H., and Liu, R. (eds.), Selected Topics in Coastal Research: Engineering, Industry, Economy, and Sustainable Development. Journal of Coastal Research, Special Issue No. 94, pp. 260–263. Coconut Creek (Florida), ISSN 0749-0208.A municipal tunnel is a double-deck tunnel shared by traffic and municipal corridors. It is an underwater tunnel in the coastal stratum. Water seepage must be considered during the tunnel construction process. By establishing a three-dimensional geo-mechanical calculation model and considering the seepage, the construction process of the two construction methods, the step method and the CRD method, is analyzed. The CRD method is better in controlling tunnel deformation, tunnel seepage and pore water pressure distribution.

ESTIMATION OF KARST SINKHOLE HAZARD — DETERMINISTIC OR STOCHASTIC?

The place of deterministic and probabilistic approaches of estimation of karst-suffusion and karst-landslide hazard is analyzed. It is evident that the field of deterministic calculations in comparison with the landslide capability assessment is turning out to be narrower because of the low availability to study the karst caverns, which are the most important elements in the sinkhole-formation processes. The sinkhole-formation hazard forms as a result of the cooperation of 2 factors in the certain area: the critical size of the zone of possible destruction of the clayly soil stratum (“screen”) over the unfilled karst cavern and the required volume of the receiving cavern or fractured karst collector where the collapsing and decompression material can be carried. At the present day it is impossible to identify in advance the presence of these factors there. In the highly researched areas with fairly simple models of sinkhole-formation processes it is advisable to test and verify the new methods of forecasting the location of the probable karst sinkholes along with the deterministic geomechanical calculations and physical laboratory modeling for sinkhole diameters estimation. In other cases the combination of zoning based on the geological and hydrogeological typing of conditions and mechanisms of sinkhole-formation and probabilistic methods gives the opportunity for more adequate forecasting of the parameters of sinkholes and probable intensity of their formation. It is also stimulates more reasonable appliance of the method of natural (natural and man-made) analogs and helps to use relevant methods for different models of geological massif in deterministic calculations. While the estimation of the sinkhole-formation hazard it is necessary to take into account the presence of the weakened zone along the perimeter of the karst sinkhole and the probable creep of the “screen” soil stratum.

From Natural Stresses in Seismic Zones to Predictions of Megaearthquake Nucleation Zones

Examines the state in the field of earthquake prediction and modern views of geomechanics on the nucleation of foci of strong earthquakes. From the point of view of tectonophysics, the problem of the mechanism of mega-earthquake occurrence is considered. It is shown that data obtained by tectonophysics methods on the natural stress magnitudes in regions with the strongest twenty-first century earthquakes are the key to its solution. Results of tectonophysical stress inversion define the main area of the mega-earthquake focus as an extended area of low and medium magnitudes of stress. This result corresponds to geomechanical concepts indicating that the state of earthquake focus depends on the Rate and State theory of friction. Numerical geomechanical calculation shows that before the formation of a large-scale brittle fracture, the fault weakens, which manifests in the acceleration of aseismic slip. In real rock, the processes of strength reduction due to increasing aseismic displacement can take years, many decades, and perhaps even centuries. Tectonophysical analysis of natural stress shows that the nucleation of an earthquake can occur both from the boundary of the focus and inside of the focus. In both cases, this small area should be a zone of high stress.

Smeared modeling of hydraulic fracture using partially coupled reservoir and geomechanics simulators

A smeared fracture type hydraulic fracture simulator is developed through numerical coupling between an in-house reservoir simulator and a geomechanical commercial software (FLAC2D). The new package falls within the category of partially decoupled model and is versatile, flexible and efficient. This approach can be used to couple any other advanced commercial fluid flow or geomechanical simulators for an accurate description of the initiation and propagation of hydraulic fractures.The paper contains a discussion of the partial coupling technique to link fluid flow and geomechanical calculations in modeling fracture initiation and propagation. The models use a common gridblock for the fracture and reservoir and use the deformation calculations to update the porosity and permeability. The method captures the interactive effects of the fracture on reservoir fluid flow and formation geomechanics through stress dependent permeability and porosity.The developed smeared fracture model can capture both tensile and shear fractures in the formation. Major features of this model include modeling poroelasticity and plasticity, matrix flow, shear and tensile fracturing with concomitant permeability enhancement, saturation-dependent permeability, stress-dependent stiffness and gradual degradation of oil sands due to dilatant shear deformation. The model has been applied to numerically simulate field size hydraulic fracturing in oil sands during cold-water injection to show the predictive capability of the simulator.

Geomechanical Estimation of Deformation Intensity above the Flooded Potash Mine

In focus of the article is minimization of aftereffects of the large-scale accident connected with the flooding of Berezniki Potash Mine in the Perm Krai. The authors construct a synthesized geomechanical model of the flooded mine to show the mining conditions, the process of salt rock dissolution and the elastoplastic behavior of deformation and failure of undermined rock mass in time. Approaches to analyzing intensity of degeneration of load-bearing elements of the open stoping system as a consequence of salt rock dissolving are proposed. The obtained estimates are the parametric framework for prediction of the ground surface deformation using 3D mathematical modeling. The geomechanical calculation results are used in the managerial decision-making on safety of the industrial and civil infrastructure within the hazardous territory.

Assessing rockfall susceptibility in steep and overhanging slopes using three-dimensional analysis of failure mechanisms

Rockfalls strongly influence the evolution of steep rocky landscapes and represent a significant hazard in mountainous areas. Defining the most probable future rockfall source areas is of primary importance for both geomorphological investigations and hazard assessment. Thus, a need exists to understand which areas of a steep cliff are more likely to be affected by a rockfall. An important analytical gap exists between regional rockfall susceptibility studies and block-specific geomechanical calculations. Here we present methods for quantifying rockfall susceptibility at the cliff scale, which is suitable for sub-regional hazard assessment (hundreds to thousands of square meters). Our methods use three-dimensional point clouds acquired by terrestrial laser scanning to quantify the fracture patterns and compute failure mechanisms for planar, wedge, and toppling failures on vertical and overhanging rock walls. As a part of this work, we developed a rockfall susceptibility index for each type of failure mechanism according to the interaction between the discontinuities and the local cliff orientation. The susceptibility for slope parallel exfoliation-type failures, which are generally hard to identify, is partly captured by planar and toppling susceptibility indexes. We tested the methods for detecting the most susceptible rockfall source areas on two famously steep landscapes, Yosemite Valley (California, USA) and the Drus in the Mont-Blanc massif (France). Our rockfall susceptibility models show good correspondence with active rockfall sources. The methods offer new tools for investigating rockfall hazard and improving our understanding of rockfall processes.

Seismic and Geomechanical Monitoring of Transport Infrastructure on Undermined Territories

Development of deposits of water-soluble minerals is connected with the risk of the accidents caused by the destruction of the waterproof stratum, and, as a consequence, water breakthrough into the mines. Thereupon, besides the problem of the mine's loss the problem of stability of the massif uppermost part appears. This part includes the foundations of buildings and serves as the base for railways and highways, bridges, dams, pipelines and etc.At the present stage of remote geophysical techniques of monitoring and prediction of such negative natural and technic origin, processes methods of high-resolution seismic survey on reflected waves have been developed most of all. In addition to prompt acquisition of physical and geological models of the section, the results of seismic observation's interpretation are proposed to be used as a part of parametric support for geomechanical calculations, which makes possible to predict rock mass stability in the range of transport facilities located above the mines.

Статистический анализ сопоставления результатов геомеханических расчетов и геологических данных

Статистический анализ сопоставления результатов геомеханических расчетов и геологических данных

Focal Mechanisms of Mine-Induced Seismic Events an Explanation of Geomechanical Processes in the Area of Longwall 6, Seam 510 In Hard Coal Mine “Bobrek-Centrum” / Mechanizm Ognisk Indukowanych Wstrząsów Górniczych Wyjaśnieniem Procesów Geomechanicznych W Rejonie Ściany 6, Pokład 510 W Kopalni „Bobrek-Centrum”

Abstract Exploitation in a coal mine “Bobrek-Centrum” of the longwall 6 in seam 510 has led to the occurrence of very high seismic activity. From January 2011 to June 2012 took place almost 3500 tremors with the energy of 102-104 J and 95 tremors of energy more than to 105 J. In an attempt to identify the character of that seismicity, calculations of focal mechanism parameters were carried out, and according to them local stress field was determined. Three periods of exploitation of longwall 6 were distinguished which vary in type of focal mechanism. Tremors, which occurred in exploitation periods I and III were characterized by normal slip mechanism, occurred due to domination of vertical principal stresses σ1, horizontal intermediate stresses σ2 and minimal ones σ3. Such stress arrangement characterizes local state of rock mass behaviour as a result of cracking and collapse of sandstone, tremor-generating strata during advancing exploitation. In exploitation period II, of non-shearing mechanism of foci occurred. That was the period of change of longwall run from the NE-SW direction to E-W direction, that is the period of so called “slanting” of exploitation front. It can be presumed that this type of tremor mechanism could have occurred due to a sudden coal bed load by superimposed roof strata, which may have led to extreme load conditions and to a dynamic disintegration of seam part. It was confirmed by geomechanical calculations, which indicated that in the region of rockburst existence on 19.07.2011, layers which were located above and under seam 510 in area of longwall no. 6 were strongly deformed locally causing compression of certain parts of seam.

A full field simulation of the in Salah gas production and CO2 storage project using a coupled geo-mechanical and thermal fluid flow simulator

We report results from a full field simulation model of the combined production and injection reservoirs, extending from a depth of 3.5 km to ground level, and with a lateral, x and y, extent of approximately 50 km. The model couples geomechanical calculations to fluid flow with an energy transport equation. It simulates two-phase immiscible flow with four components (CH4, CO2, NaCl and H2O) in the gas and aqueous phases. CO2 may dissolve into the aqueous phase. Fractures are modelled explicitly in the grid.

Undulant rock pressure distribution along a longwall face

The paper presents measurement data on pressures in legs of a powered support in a longwall. The authors show that the rock pressure distribution along the longwall face is undulating and, based on the geomechanical calculation, find the relationship between the energy density of the rock mass and support. Based on the case study of a longwall before a rockburst, it is shown that the seam energy density decrease can initiate the rockburst.