Browsing by Subject "Rocks--Fracture"

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Development and implementation of a narually fractured reservoir model into a fully implicit, equation-of-state compositional, parallel simulator
(2005) Naimi-Tajdar, Reza; Sepehrnoori, Kamy, 1951-; Miller, Mark A.
Naturally fractured reservoirs contain a significant amount of the world oil reserves. A number of these fields contain several billion barrels of oil. In naturally fractured reservoirs, fluids exist in two interconnected systems: the rock matrix, which usually provides the bulk of the reservoir volume, and the highly permeable rock fractures. Accurate and efficient reservoir simulation of naturally fractured reservoirs is one of the most important, challenging, and computationally intensive problems in reservoir engineering. Parallel reservoir simulators developed for naturally fractured reservoirs can effectively address the computational problem. A new accurate parallel numerical simulator for large-scale naturally fractured reservoirs, capable of modeling fluid flow in both rock matrix and fractures, has been developed. The simulator is a parallel, 3D, fully implicit, equation-of-state compositional model that uses numerical tools for solving very large, sparse linear systems arising from discretization of the governing partial differential equations. A generalized dual porosity model, the multiple-interacting-continua (MINC), has been implemented in this simulator. The matrix blocks are discretized into subgrids in both horizontal and vertical directions to offer a more accurate transient flow description in matrix blocks. We believe this implementation has led to a unique and powerful reservoir simulator that can be used by small and large oil producers to help them in design and prediction of complex gas and waterflooding processes on their desktops or a cluster of computers. Some features of this simulator, such as modeling both gas and water processes with the ability of two dimensional matrix subgridding for naturally fractured reservoirs, to the best of our knowledge are not available in any commercial simulator. For coupling of the fracture and matrix continua, no analytical approximations are made. Instead, numerical methods are used to treat the transient flow of fluid between matrix and fractures. The development was performed on a cluster of processors, which has proven to be a very efficient and convenient resource for developing parallel programs. The results were successfully verified against analytical solutions and commercial simulators (ECLIPSE, IMEX, and GEM). Excellent agreement was achieved for a variety of reservoir case studies. Applications of this model for several enhanced oil-recovery processes (including gas and water injection) are demonstrated. The effects of matrix subgridding on the accuracy of the results of the simulation runs are investigated. The study showed that in some circumstances the results of simulators without matrix subgridding generated more than 50% error in oil-recovery calculations. Simulation results using the simulator on a cluster of processors are also presented. Excellent speedups were obtained using the simulator in conjunction with solving a variety of problems.
Discrete element modeling of rock fracture behavior: fracture toughness and time-dependent fracture growth
(2006) Park, Namsu; Olson, Jon E.
Understanding the mechanics of fracture is important in oil and gas reservoirs. Applications range from the characterization of natural fractures that enhance fluid flow to the prediction of fracturing around a wellbore that can affect its integrity and stability. Two parameters that are of particular importance in the fracturing process are fracture toughness and subcritical index. There is a fair amount of experimental data on different rock types for these parameters but it is not well-known what petrographic properties control their magnitude. Also, because of sample preparation difficulty, fracture mechanics testing of weakly cemented sandstone is very challenging. In order to better understand the micro-mechanics of fracturing of clastic rocks (sandstones of various cementation), a numerical study was performed using the Discrete Element Method (DEM). DEM was employed in order to model laboratory test behavior, vii by assessing individually the sensitivity of results to volume of cement, time-dependent cement properties, grain/cement mineralogy, temperature, and confining pressure. The micro-mechanical properties of DEM (stiffness and friction of grains and stiffness, strength, and volume of cement) were determined using macroscopic uniaxial and triaxial compression tests. The time-dependent properties of subcritical crack growth were implemented by incorporating stress corrosion of inter-particle bonds. The stress corrosion rate was quantified by the activation energy and volume of quartz. The fracture toughness and subcritical index of Berea sandstone was measured and the results were extended to weaker rock by reducing the cement volume. The DEM results generally agree with laboratory experiments. As intergranular cement volume is reduced, fracture toughness and subcritical index decrease. Based on this relationship, the fracture mechanics properties of weak rocks, which are difficult to measure in the laboratory, can be predicted. Using the DEM model constrained by laboratory results, the importance of subcritical crack growth in wellbore stability problems was investigated. Wellbore instability in shale can be an immediate result of stress redistribution and increasing formation pore pressure following the removal of the rock mass in the wellbore. Additionally, because of large clay content and the potentially high chemical reactivity with drilling fluids, shale can be susceptible to time-dependent failure. Previous studies (mostly based on continuum modeling using poroelasticity) have concentrated on predicting the onset of failure. However, the use of DEM makes it possible to evaluate the progression of failure over time by tracking the propagation of the damage zone boundary