Geotechnical and engineering geology practitioners are always looking out for tools which can help understand and reduce the large uncertainty and variations in rock masses after complex geological processes. Relying on traditional interpolation techniques for geotechnical variables may lead to large uncertainty and major stability risk in the mining phase. The present paper proposes a direct and indirect methodology based on geostatistical estimation and simulation techniques to determine the expected Rock Mass Rating (RMR) and its underlying parameters, each geostatistical model identifies potential risk-prone areas in which failures could be experienced, superposing the different resulting maps allowed us to define low-risk conservative RMR model. A total of 115 underground rock blocks samples from five mining openings were examined for the rock mass quality using the RMR, Q and RMi characterization systems. Cross-validation and jack-knifing techniques showed that the proposed indirect estimation and simulation methods outperformed the more frequently used direct approach and shows a more accurate map with a low error coefficient which makes them adequate for RMR modeling. The resulting map of the indirect approach allowed taking into account the nonlinear nature, directional behavior of the RMR and its constitutive parameters which can be used to assist engineers in proposing suitable excavation techniques and an appropriate support system. The developed model help to assess different geomechanicalparameters that can use to develop numerical models that explicitly consider the rock mass heterogeneity.
Casing Collapse is one of the major problems in upstream oil industries. Millions of dollars are annually spent on repairing, rehabilitation and re-drilling wells due to the casing collapse in oil wells worldwide. Casing collapse occurs due to a variety of factors, including soft rock creep such as salt and shale, and the creation of a point load on the casing due to the lack of good cementation behind the casing, sliding motion of the soft layer that is mechanically located between the two harder layers, reservoir subsidence due to excessive harvest or other factors such as casing production and so on. Fig. 1 shows the classification of the causes of the casing collapse in terms of geomechanicalparameters (of formations) and solid mechanics.
In Senegal, rock mechanic studies began in last four years and were a very new engineering domain. So, the first stage is to characterize geological materials. It is on this way that this work is done. This paper analyzes geomechanicalparameters of sandstones of Dindifélo and basalt of Bafoundou belonging to the Proterozoic rocks domain of eastern Senegal. By Rock Mass Rating (RMR) and Geological Strength Index (GSI), sandstones of the Dindifello Cliff show fair to poor characteristics while basalts of the hills of Bafoundou are fair quality. In addition to the qualities of rock mass, Young moduli, uniaxial compressive strengths and tensile strength of rock mass are also defined using Rock Quality Designation (RQD) GSI, RMR. Hoek-Brown parameters m and a, depend both on the fracturation and the content fine in the rock. Values of mechanical parameters are different when deduced from RMR, GSI and from intact rock laboratory test. Those differences are due to variables taken ac- count. The variation depends also on the quality of the rock. Statistical analy- sis shows possible unstabilities which depend on rock mass parameters but with acceptable probability of failure. Probability of failure is the highest when deducing from Mohr criterion than from Hoek-Brown criterion.
The contour Map of the study area showing groundwater flow direction in two dimensions for Eocene limestone aquifer. The thickness of the Eocene limestone aquifer increases towards the central part of the basin . Flow pattern of the Eocene limestone aquifer system in study area revealed that groundwater flow direction was toward the South and south eastern parts of the area guided by the fractures and the preferential directions of the karst cavities. Its represent concentrations of void space in rocks. The geometry of the void space affects both the flow properties and the karst cavities depth, such as the geomechanicalparameters and characterizing hydraulically.
The simulation approach used in this study utilizes the Finite Element (FE) technology, and geomechanical models will be run in order to simulate the dis- tribution of tectonic stress for further predicting fracture development and dis- tribution. This powerful tool allows for robust simulations of complex structures with non-linear material behavior or large deformation based on frictional con- tact mechanics  . The basic concept behind FEM is that the geological bodies are discretized into finite continuous elements connected by nodes. Each element is allocated with appropriate geomechanicalparameters determined for real rocks. The continuous field function for the region is first transformed into node function that incorporates the basic variables of stress, strain and dis- placement resulting from applied external forces  . All these elements are combined to obtain tectonic stress field over the entire geological bodies  . The general-purpose finite element code Ansys (version 15.0) was selected for this study because it is well suited to analyzing geomechanical problems over awide range of scales in one, two and three dimensions   . Ansys can accurately handle large strains and rotations as well as complex contact interfac- es with frictional behavior where significant sliding can occur. It also has effi- cient algorithms for solving highly nonlinear problems that result from both geometric complexity and material behavior. Finally, Ansys has a large library of built-in constitutive relationships that are appropriate for simulating the beha- vior of rock, ranging from simple elastic material models to advanced elas- tic-plastic and visco-elastic material models .
Abstract. Fluid extraction from producing hydrocarbon reservoirs can cause anthropogenic land subsidence. In this work, a 3-D finite-element (FE) geomechanical model is used to predict the land surface displacements above a gas field where displacement observations are available. An ensemble-based data assimilation (DA) algorithm is implemented that incorporates these observations into the response of the FE geomechanical model, thus re- ducing the uncertainty on the geomechanicalparameters of the sedimentary basin embedding the reservoir. The calibration focuses on the uniaxial vertical compressibility c M , which is often the geomechanical parameter to
Abstract. An analytical methodology is presented to evalu- ate rock slope stability under seismic conditions by consider- ing the geomechanical and topographic properties of a slope. The objective is to locate potential rockfall source areas and evaluate their susceptibility in terms of probability of failure. For this purpose, the slope face of a study area is discretized into cells having homogenous aspect, slope angle, rock prop- erties and joint set orientations. A pseudostatic limit equilib- rium analysis is performed for each cell, whereby the desta- bilizing effect of an earthquake is represented by a horizontal force. The value of this force is calculated by linear interpo- lation between the peak horizontal ground acceleration PGA at the base and the top of the slope. The ground acceleration at the top of the slope is increased by 50% to account for to- pographic amplification. The uncertainty associated with the joint dip is taken into account using the Monte Carlo method. The proposed methodology was applied to a study site with moderate seismicity in Sol`a de Santa Coloma, located in the Principality of Andorra. The results of the analysis are con- sistent with the spatial distribution of historical rockfalls that have occurred since 1997. Moreover, the results indicate that for the studied area, 1) the most important factor controlling the rockfall susceptibility of the slope is water pressure in joints and 2) earthquake shaking with PGA of ≤ 0.16 g will cause a significant increase in rockfall activity only if water levels in joints are greater than 50% of the joint height.
From eq. (22), we observe a smaller compaction in the reservoir than in a uniaxial model, eq. (10), which is obtainable assuming the stress path parameters are equal in the horizontal directions. With this development, reservoir subsidence for uniaxial compaction would probably yield poor results if polyaxial stress state were present. Reservoir compaction depends on the stress regime in a formation. Under a normal stress regime, the results of polyaxial compaction are significantly different from those of uniaxial compaction. The reason is that the stress-path coefficients change as the stress regimes change. The difference may be larger in a strike-slip regime or reverse stress regime. Using the data in Table 1, we obtained a compaction of 0.503 ft by applying the uniaxial model, , while the current model yielded a compaction of 0.4656 ft, which is about 7.9% smaller in value. In practical design, this can be economically catastrophic as poor reservoir management ensues.
to geometric and geomechanical rock mass characterisation. DEM-based structural analysis, performed by COLTOP-3D software (Derron et al., 2005), on available data before and after the landslide, is an important tool in identifying the structural setting that led to the failure and controlled the di- rection of the movement. COLTOP-3D uses a colour repre- sentation merging slope aspect and slope angle in order to obtain a unique colour code for each orientation of a topo- graphical element (Jaboyedoff et al., 2007, 2009). Simple analysis of DEMs allow rapid identification of structural fea- tures (joints, lineaments, faults) affecting the slope (Derron et al., 2005). The 3-D surface reconstruction is extremely useful as it enables easy identification of the main morpho- structural features from which joint set orientations and per- sistence relevant to the area of interest can be detected. Fur- thermore, these desktop analysis allow us to explore the area under investigation and thereby provide an aid in planning the field work and the mapping of structural data in inacces- sible areas.
the geomechanical analysis is not compatible with the histor- ical one. In other words, it means that the geologist has not found enough potential rock-fall sources to be in agreement with the historian, or that the hypothesis of a steady state for the global rock-fall process is not valid. But the number of possible failures recognisable in the field is usually much higher than the historical failures. In the simple case where there is only one probability class, it is obvious that the num- ber of detected potential rock-fall sources must not be too small compared with the expected mean number of rock falls. It must be kept in mind that rT is the expected mean number of rock falls for the evaluation period and that the real num- ber of rock falls which will occur may differ from this mean value, according to the Poisson law. Moreover, the method can give only an order of magnitude of the failure probabil- ity. Consequently, only if p 1 is clearly greater than 1, can the
This paper reports the results of a detailed investigation carried out on a 45 m-high sea cliff for the assessment of its overall stability by examining different triggering pro- cesses. The sea cliff, located in the Gargano National Reserve (Southern Italy), is composed of fractured and weathered limestones and chalks and is frequently affected by rock- falls. Direct traditional geomechanical field surveys and re- mote laser scanner surveys were performed. More in partic- ular, data collected from traditional field surveys in the sur- roundings of the inaccessible cliff were integrated with per- vasive joint set characterization using a terrestrial laser scan- ner (TLS)-derived high-resolution point cloud as extensively presented in the scientific literature (e.g., Sturzenegger and Stead, 2009; Oppikofer et al., 2009).
The geomechanical instability usually occurs as a result of drilling, production operation, and reservoir management activities where the induced in situ stresses result in exceeding the in situ strength of the formation. In particular, during the production phase, the decrease of pore pressure causes a concentration of stresses around the wellbore and perforation tips which, in turn, can lead to the failure of the rock. From the phenomenological viewpoint, sand production can occur when the formation does not present sufficient strength to resist destabilizing forces generated during the flow of reservoir fluid. It has been estimated that about 70% of the world’s oil and gas reserves are found in weakly-consolidated or non-consolidated strata [1-3].
millions of years [9, 10]. Based on 3D seismic data Yielding et al.  analysed the role of stratigraphic juxtaposition for the seal integrity and across fault fluid migration and concluded that there was no risk for lateral migration. Here we build on the previous work and add more details to the stratigraphic succession to improve the fault seal analysis. Using hydrocarbon industry standard tools we calculated the sealing capacity and also studied the geomechanical properties of the bounding fault with regard to fault reactivation risks.
Geomechanical and hydrological surface deformation caused by the underground hard coal extraction leads to lim- ited agricultural and natural soil use. The paper presents results of soil productivity indices determined in the area of post-mining subsidence depression near Libiąż. The algorithm of method suggested by Zhengi Hu et al.  was used for the computations. The obtained results of field and laboratory analyses indicate soils degradation due to water logging and a necessity to undertake reclamation measures to restore their previous values. Determined productivity indices (PI) characterise the selected ground as the area with poor conditions for agricultural produc- tion. The reason for low PI values is a change in the soil properties because of degradation. The methods presented in the paper may be used to determine the degree of soil degradation in the areas of hard coal mining.
c) A procedure for the determination of the geomechanical and mechanical skin factor is outlined. d) Correction ways for the pressure derivative minimum point and the intercept formed by transition unit slope and the radial flow regime under normal conditions are presented. This guarantees the application of existing equations for characterization of the naturally fractured reservoir parameters.
Geomechanical characterization is vital to understand the fracture creation and propagation. The brittleness estimation can help in selection of fluid for stimulation. The layer with brittleness index greater than 40% considered brittle . The hydraulic fractures are more likely generate at zone with low tensile strength, poison’s ratio, confining pressure and high Young’s modulus and Brittleness . The main parameters used to explain elastic deformation are Young ’ s modulus and Poison ’ s ratio. Brittleness can be estimated by using elastic parameters as proposed by Rickman al., 2008. The workflow of methodology is given below in Figure-2:
modeling through the Reservoir Geomechanics Module in Petrel 2013. Model grid and simulation results (e.g. pressure) from Eclipse 300 simulator were imported into Petrel 2013. Next, the model grid was embedded in all directions. Seven layers of equal thickness were added above the model to extend the overburden to surface at 0m. Six layers of equal thickness were added underlying to extend the underburden to the depth of 2000m. Laterally, seven cells with increasing size by a factor of 1.5 were added in each direction to serve as sideburdens and this will reduce the boundary effects during simulation. The final geomechanical grid has an angle against the global axes due to the nature of simulation grid. Grid rotation was tried to make it in accordance with global axes, however, this made a great impact on the distributions of properties. The reason is because properties are assigned to each cell; rotation of the grid will relocate cells and eventually change the property distributions. So it is essential to keep the grid orientation the same as simulation model. After embedding, the total number of grid cell reaches 249480 (110*81*28). It is suggested to reduce the cell number through simulation model upscaling; however, this means a start over from the beginning. For the concerns of short period of time, upscaling was not performed, and
to calibrate reservoir parameters by assimilation of surface displacement data. This technique is based on an ensemble smoother (ES) algorithm, a derivate of the classic Kalman Filter (KF) . The ES is a Bayesian data assimilation method that, by minimizing the variance of the estimation error, merges “prior” information from a theoretical system, i.e., the mathematical model, and field data collected from the actual system in order to produce a corrected “posterior” estimate. The ES relies upon a stochastic, or Monte Carlo, simulation, in which the system uncertainty is represented by an ensemble of realizations of surface displacements obtained by “forecast” simulations using a corresponding ensemble of reservoir parameters . The update of the displacement ensemble is performed by assimilating surface displacement data collected over time. The system parameters can be simultaneously updated by incorporating them into the ensemble. Doing so allows for the local displacement data to correct not only the displacement ensemble, but also the system parameters by leveraging the displacement–parameter cross-covariance estimated in the forecast simulation.
An obligatory condition for selection of efficient management methods of gas emission into mined-out extraction areas and their parameters is deep knowledge of gas–geomechanical processes running in the bulk disturbed by underground mining. However, at present selection and background of ventilation flowcharts and management of gas emission are based on outdated regulations: Designing of ventilation of coal mines (1989), which do not reflect specific features of mining geomechanical conditions of existing extraction areas: long span goaves, high advance rates of longwall faces, which predetermines significant intensity variation of gas - geomechanical processes. Application of the mentioned regulations is unjustified and leads to difference between actual and calculated values by some orders of magnitude (Kaledina and Karpukhin, 2007).
Most of the Iranian oil and gas wells in the Persian Gulf region are pro- ducing through their natural productivity and, in the near future, the use of stimulation methods will be undoubtedly necessary. Hydraulic fracturing as a popular technique can be a stimulation candidate. Due to the absence of adequate research in this field, numerical simulation can be an appropriate method to investigate the effectiveness of hy- draulic fracturing. In the current study, the hydraulic fracturing pro- cess is simulated for a wellbore in the Persian Gulf region with Abaqus software. The main parameters that are necessary for the simulation are collected through wellbore logs and core tests. Fracturing process is studied with more emphasis on the pressure of fracturing fluid and fracture opening. Finally, several 3D fluid-solid coupling finite element models are generated and the main obtained results are compared.