Dynamic loading methods promise new modes for stimulating geological resources, as the fracture patterns they produce can be tailored by the shape and nature of the pressure pulse employed. However, selecting the type of load is a difficult task: too slow and the stimulatory effect is reduced; too fast and the resource may be negatively impacted by wellbore damage, fines creation or permeability reduction. Moreover, modeling these systems proves challenging due to the myriad of length and timescales involved, combined with the need to accommodate both the generation of new fractures and propagation of preexisting fracture networks.
GEODYN-L is a massively-parallel multi-material Lagrangian code that includes advanced contact models to simulate nonlinear wave propagation through heavily-jointed rock masses, along with material model libraries specifically developed to capture the dynamic response of geologic media. We present results using GEODYN-L to simulate dynamic stimulation of geologic resources with pre-existing fracture networks and discuss the implications of these results for enhancing fracture networks with dynamic loading techniques.
Fractures often control subsurface fluid flow and efforts to predict near-surface fluid flow typically focus on quantifying the relative influence of aperture and surface roughness on transmissivity. At greater depths, lithostatic and tectonic stresses play an increasingly important role in controlling fracture transmissivities. For the case of well-correlated fracture surfaces, such as those expected in new tensile fractures, fracture transmissivities quickly become negligible as a normal stress is applied to the fractures. However, if these fracture surfaces are displaced in shear, the fractures remain open and conductive at much higher stresses. Shear displacement of two well correlated or perfectly mated surfaces leads to anisotropy in the correlation structure of the fracture aperture field. In the presence of large normal stress, it is likely that only fractures subjected to some shear displacement will remain conductive, and thus, they are likely to exhibit some degree of anisotropy. Predicting the degree of anisotropy requires quantifying the combined influence of surface roughness, shear displacement of the fracture surfaces and the applied normal stresses. We use previously tested computational models to explore the influence of shear displacements of fracture surfaces subjected to a sequence of normal displacements (increasing stress) on transmissivity both parallel and perpendicular to the shear displacement of the surfaces. Results suggest that normal deformation of displaced fracture surfaces can lead to an order of magnitude increase in the anisotropy ratio from that observed in the unstressed fracture. These results suggest that the influence of normal stress on fracture anisotropy must be considered when implementing effective-continuum or discrete-fracture-network representations of fluid flow through fracture rock masses.
Generally, the features used to evaluate horizontal stresses (breakouts, DIFs, on-gauged sections, LOTs) only give one equation. This means that the methods that evaluate horizontal stresses by focusing at one depth cannot be used to determine the stress state as one equation is lacking. Other methods assume relations between horizontal stresses and depth and evaluate the constants of these relations for the computed stresses to match well features. They are expert-dependent. This paper presents a new method to determine horizontal stresses that takes the best of both existing methods without having their drawbacks. It assumes relations between the horizontal stress magnitudes and depth based on geology and plots all well features into one single stability diagram. Then a statistical analysis allows calculating the constants of the relations and determining which features are effectively explained by the stress state and which are not. This method can be used to determine the stress state from features observed in more than one well and its results only depend on a priori assumptions and not on a posteriori expert decisions.
Underground excavations in jointed rocks may result in various modes of rock block failures. Prediction of mode of failure and volume of unstable rock block are essential in the design of excavations in jointed rocks. One important parameter is the rock joint orientation concerning its mean orientation and the dispersion around the mean orientation. Although Fisher distribution is commonly used to evaluate the mean and dispersion of rock joint orientation, the present study establishes that the more advanced Kent distribution can better evaluate the scatter of rock joint orientation of actual field data. A probabilistic simulation is employed in the present study to conduct stability analysis of tetrahedral rock block based on rock joint mean orientation and dispersion obtained from Kent distribution. The results from probabilistic simulation are compared with the commonly used deterministic approach employing only the mean rock joint orientations. The comparison reveals that deterministic method generally gives a conservative estimation of the largest unstable block volume. However, while only a single failure mode of rock block can be identified from the deterministic analysis, the probabilistic simulation approach shows possibilities of multiple rock block failure modes due to scatter of rock joint orientation with the respective probability of occurrence for each mode of block failure.
Günther, R.-M. (Institute for Geomechanics GmbH (IfG)) | Salzer, K. (Institute for Geomechanics GmbH (IfG)) | Popp, T. (Institute for Geomechanics GmbH (IfG)) | Lüdeling, C. (Institute for Geomechanics GmbH (IfG))
Actual problems in geotechnical design, e.g. of underground openings for radioactive waste repositories or high-pressure gas storages, require sophisticated constitutive models and consistent parameters for rock salt that facilitate reliable prognosis of stress-dependent deformation and associated damage from the initial excavation to long times. Fortunately in the long term the response of salt masses is governed by its steady state creep behavior. However, because in experiments the time necessary to reach true steady creep rates can last time periods of some few days to years, depending mainly on temperature, an innovative but simple creep testing approach is suggested. A series of multi-step tests with loading and un-loading cycles allow a more reliable estimate of stationary creep rates in a reasonable time schedule. In completion, the advanced strain-hardening approach of Günther/Salzer is used which describes all relevant deformation properties of rock salt, e.g. creep and damage induced rock failure, comprehensively within the scope of an unified creep approach. The capability of the combination of improved creep testing procedures and accompanied modelling is demonstrated by recalculating multi-step creep at different loading and temperature conditions. Thus reliable extrapolations relevant to in-situ creep rates (10-9 to 10-13 s-1) become possible.
Rock as brittle heterogeneous material exhibits inelastic deformation due to widely pre-existing microcracks in the formation from outcrops to the cores thousands miles deep underground. By the effect of the geological process, thermalmechanical loading, hydraulic fracturing, etc., the macrocracks or fractures may be developed as the collection of microcracks. However, it is difficult to obtain the geometry of microscopic natural fractures and their kinematic information by physical experimentation. In this study, the Distinct Element Method (DEM) is used because it treats the rock mass as an assembly of independent particles under certain contacts, and along with deformation and dynamic mechanism. With Distinct Element Modeling approach, the rock mass is represented by three-dimensional bonded particles, and the pre-existing discontinuities (joints) are simulated by smooth-joint contact model. The observed existing fractures geometric properties have been identified that they are fitted to theoretical probability density functions. Therefore, the stochastic microcracks (SM) system needs to be investigated with Monte Carlo simulation by applying distinct distribution functions into geometry variables, such as crack density, orientation, location; which addresses the probabilistic characterization of the nature of fracture system. The rock sample is sent to anisotropic stress field to observe its response after simulated hydraulic fracturing. The model is also developed with two different parallel laminations of rock types to exam the mineral phases effect. This paper combine the Monte Carlo simulation of stochastic microcracks with Distinct Element Modeling using 3D Particle Flow Code (PFC), and the presented synthetic rock mass can be applied in further study on the statistical natural fractures system coupling with microseismic event, stress distribution estimation, and thermo-hydro-mechanical behavior of the hydraulic fracturing.
In this study, evolution of mechanical and hydraulic properties in Berea sandstone with initial porosity of 20 %: is examined quantitatively by replicating mineral trapping process within pore spaces. The artificially accelerated mineral trapping is achieved by injecting a grout, resulting in calcite precipitation. The amount of calcite precipitated can be controlled by changing the concentrations of the grout and the total injection volume. The preliminary experiments indicate that the change of the mechanical and hydraulic properties is significant even if the calcite amount is relatively small – the values of Young’s modulus/permeability increase/decrease with increase of the calcite amount. Specifically, the permeability decreases by one order of magnitude as the initial porosity of 20 % reduces to 18 %. This intense decrease of permeability may not be able to be replicated by existing numerical models, although distribution of the calcite precipitated within rock samples should be adequately examined because uniform distribution may not be achieved.
Permeability evolution in intact rock as a result of micro-crack propagation is fundamental in understanding fluid flow within the excavation damage zone (EDZ) around underground openings. A 2D grain-based discrete element method incorporating Voronoi joint model is used to simulate the permeability evolution with progressive damage in intact rock when it is subjected to incremental mechanical loading. The rock response during uniaxial and triaxial loadings is simulated with and without taking into account the hydro-mechanical interactions between fluid and solid part of the rock. The numerical experiment results show that when modeling the rock behavior under compression, ignoring the coupled damage-flow processes results in an inaccurate prediction of mechanical properties of rock such as the peak strength and the post-peak response. The stress-strain response of the sample indicates that an increase in the applied confining stresses increases both the strain hardening range and the peak strength. In addition, the Permeability of models during triaxial loading increases by up to 2.5 orders of magnitude prior to rupture of rock. Progressive development of fractures as a result of crack accumulation leads to over 3 orders of magnitude increase in post-failure permeability compared to the permeability at the peak stress. These changes have been reported from laboratory testing results as well as in-situ permeability measurements carried out in the damaged zones in tunnel boundary. It has been demonstrated that Voronoi joint model incorporated in a DEM-based code has capability to simulate stress-induced permeability change in the brittle rocks as a result of processes such as initiation, propagation, and accumulation of cracks.
A new approach to upscaling and modeling of geomechanical properties using clusters has been set-up for Vaca Muerta Formation in the Neuquén Basin of Argentina. Using wells with core and cross-dipole logging tools, a core calibrated anisotropic model of the formation has been established. Clusters were determined from a logging suite comprising only gamma ray, compressional slowness, and bulk density in a key exploration well, and this cluster group was applied to several more wells in the study area. Using microseismic data obtained from three of the wells in the study area with two fracturing stages each, the vertical extent of microseismic events was determined, and the clusters obtained through our analysis have been upscaled over this interval using Backus averaging. All four upscaled wells show similar results by cluster for elastic stiffness coefficients and Young’s moduli, with a very tight range of values. Poisson’s ratio is more variable and a vague trend with the clusters is noticed. When compared to the core data, similar trends are observed in the stiffness coefficients and Young’s moduli. These clusters have been used as geomechanical facies to populate a 3D MEM which can be used to couple the petrophysical model for the study area, regional stress model, regional structure, and natural fracture network in order to combine the fully coupled geomechanics and flow effects in hydraulic fracturing treatments. The method we have developed allows for anisotropic properties to be applied over a wide area with limited available logging data.
Choi, J.C. (Norwegian Geotechnical Institute (NGI)) | Park, J. (Norwegian Geotechnical Institute (NGI)) | Viken, I. (Norwegian Geotechnical Institute (NGI)) | Bohloli, B. (Norwegian Geotechnical Institute (NGI)) | Skomedal, E. (Statoil ASA) | Godager, Ø (Sensor Developments AS) | Borgersen, K. (Sensor Developments AS) | Casseres, A.G. (Dong Energy A/S)
This study is to analyze pore pressure data acquired with a novel measurement system that is installed for a pilot test just outside a cased well within a sand formation in the North Sea. The system uses a wireless communication through casing to transfer measured data from well outside to inside. Long-term trend in the measured pore pressure is consistent with a formation-testing-based estimate. On the other hand, the measured pressure and temperature show rather high oscillation in short-period scale, especially during shut-ins. Finite element (FE) modeling study is performed in order to understand this behavior. We find that the oscillation is caused mostly by thermal effects in the tubing, not by any abnormal formation pressure, during injection. The FE modeling study also shows that the thermally-induced pressure oscillation can be removed, and the formation pore pressure is recovered. Finally, the FE modeling study is extended to explore potential behavior of the pressure data when the system is installed in shale formation.