مجله ژئومکانیک نفت
سال سوم شماره 2 (تابستان 1398)

Rock fracture as a weakness plane in most cases determines the mechanical and hydraulic behavior of the rock mass. In many geomechanical projects such as exploitation of hydrocarbon reservoirs and storage of hydrocarbon materials, recognition of mechanical and hydraulic behavior of fractures is one of the most important issue of the study in design and implement. The most important parameter affecting fracture behavior is the geometry of features such as aperture and roughness. In this manuscript, these parameters are numerically studied. Using the cloud points obtained by scanning the natural fracture surfaces, the digital geometry model is made. The fluid flow is simulated in a rock fracture by solving Navier-Stokes equations, in these geometric models, using fluent software. The validity of this numerical modeling has been tested with the results of laboratory experiments, which suggests that the modeling method is correct. fluid flow in a rock fracture was investigated for a wide range of fluid flow rates and different roughness using this numerical model. The results of this numerical modeling demonstrate that the Forchheimer's relation describes well the flow of nonlinear fluid in a rock fracture. Also, the linear and nonlinear coefficients of the Forchheimer equation are estimated for each geometry. The results show that linear and nonlinear values increase with increasing the aperture.

In this study, a method for generating a pore network model based on porous media static characteristics has been presented. The method with the aim of pore size, coordination number distribution and the porosity is developed to generate an irregular pore network model. Because the model uses probability functions, it has different answers. In order to optimize model, porosity is used as target function. This irregular model has all of the desired static properties such as pore size, coordination number distribution and porosity. The model coordination number can vary from 0 to 26. An effective pore network is extracted using a new clustering algorithm. Finally, the results of an extracted pore network from the CT scan image of a synthetic silica achieved from maximal ball algorithm were used as the model inputs. The outcome of the developed model was compared with the mentioned model and statically and structural agreement was found.

Gaussian process regression, as a nonparametric probabilistic model based on Bayesian statistics, is highly capable of supporting ‎sparse features such as global anomalies. Detecting abnormal behavior from normal behavior makes Gaussian process regression ‎as an edges detector where faults may occur in the seismic data. In this study, the Gaussian process regression-based anomaly ‎detection was applied to both synthetic and real data containing normal fault to detect the fault edge. To identify the fault edges, ‎the geological layers are considered as normal interaction and the fault edge as a global anomaly which disrupts the normal ‎behavior of layers. The error of regression is analyzed to separate the fault edge. To evaluate the proposed method, it was applied ‎on a series of synthetic seismic data and a real 2D seismic section of F3 block of the North Sea containing the fault. The results ‎show the ability of this method in fault detection.‎

Considering the geomechanical characteristics of the subsurface facies in well location can play an important role in reducing the sand production from reservoirs. The main step in the geomechanical characterization in the reservoirs is to create an accurate geomechanical zoning. Due to the intrinsic spatial variability and also the high heterogeneity of the facies making such geomechanical zoning is accompanied with high uncertainty without having a reliable structural model of the reservoir facies. In recent decades, multiple-point simulations (MPS) have been considered not only as an efficient gadget to estimate the geomechanical properties in the reservoirs but also as a tool for the modeling of the heterogeneous facies. The importance of generating an accurate model of the subsurface facies has led to the development of various algorithms to improve accuracy and computational efficiency. In this paper, a new algorithm is proposed for numerical modeling of heterogeneous facies in oil reservoirs. The proposed algorithm is based on Discrete Wavelet Transform (DWT) and the Cross Correlation (CC) function. For this reason, the proposed algorithm is called CCWSIM. The accuracy and computational efficiency of the proposed algorithm are compared with another well-known MPS algorithm, called MS-CCSIM, in a two-dimensional synthetic model of the reservoir. The results of the comparison show high accuracy of the proposed algorithm (CCWSIM) in the reproduction of heterogeneous reservoir facies. In addition, the proposed algorithm is more efficient than MS-CCSIM algorithm.

One of the most important aspects of governing physical processes through rough-walled fractures is the non-linear behavior of flow. The main aim of this paper is to investigate the effect of non-linear flow on the hydraulic parameters and deviation from Darcy’s law. To reach this goal, the three-dimensional simulation of crude oil flow inside rough-walled fractures was performed by numerical solving the Navier-Stokes equations supplemented by the continuity equation through application of finite volume technique. The crude oil flow through three-dimensional space of rough-walled fractures was numerically simulated for a wide range of inlet velocity of flow rates. Then, the regime of flow, the deviation of crude oil flow through rough-walled open fractures from Darcy’s law, nonlinear relationship between hydraulic gradient and flow rate, and the effect of non-linear flow on the hydraulic aperture and permeability of fractures were investigated by analysis of crude oil flow simulation. The results of this study indicate that the crude oil flow through rough-walled fractures is non-linear; therefore the hydraulic parameters such as hydraulic aperture and permeability are not constant and highly depend on the flow rate of crude oil. In fact, hydraulic aperture and permeability of fractures decrease non-linearly by increment of flow rate, where these parameters show 10% and 20% of relative decrement, respectively. In addition, the results of regression analysis show that the Forchheimer’s law appropriately describes the behavior of crude oil flow through rough-walled fractures than Darcy’s law. Moreover, the accuracy of Forchheimer’s law is much more than 98%, but the relative error of Darcy’s law reaches to 27%.

In an oil reservoirs, the mechanical stresses, the thermal stresses and the fluid pressures can effect on each other and create a completely coupled phenomenon. Reservoir deformations due to thermal and mechanical stresses can cause the changes on effective stress and effect on the rate of production. Similarly, fluid pore pressure and temperature variations can effect on the deformation of reservoirs. Since these phenomena are mutually interacting with each other, considering the effects of temperature, fluid pore pressure and deformation on reservoir production requires the simultaneous simulation of heat, geomechanical and flow equations. In this article, first, the history of thermo-hydro-mechanical modeling is described. Then, the governing equations include three sets of mass equilibrium, momentum equilibrium and energy balance equations are presented for a non-isothermal deformable porous medium that is saturated by three phases of water, oil, and gas. These equations are related to each other and are solved in the form of partial differential equations. Due to being coupled of governing equations and the complexity of their boundary conditions, this equation is usually solved numerically. Different numerical methods have been used for solving which have different positive and negative points. Finally, the numerical solution of coupled thermo-hydro-mechanical equations is described in a finite volume and finite element methods and the examples of porous media simulation are presented. The examples show the ability of the proposed model.