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Hydraulic fracturing causes a large amount of fluid to leak off into unconventional reservoirs and the fluid is not recovered through flowback. The fluids enter the pore structure of the formation, and the presence of the invading phase blocks the insitu phase. This event reduces permeability, particularly in unconventional reservoirs. Operators shut-in the well for an arbitrary amount of time after fracturing. The time could allow the phase blocks to dissipate, increasing permeability. Conventional core flow tests and relative permeability evaluations using centrifuge drainage tests are unreliable-if not impossible-in these ultralow-permeability unconventional rocks.We present a method to measure the gas permeability on core plugs of mudstones at reservoir conditions, both before and periodically after the plug was exposed to fracture fluid. The change in gas permeability over time allows optimization of the shut-in time. In addition, the combined rate of leakoff and imbibition is measured while the material is exposed to fluid. At the conclusion of the test, the axial water profile is determined by measuring unconfined compressive strength using the mechanical scratch machine and water saturation with Karl Fisher titration.Initially, the samples are saturated with gas and have the highest permeability. As the plug is then exposed to water, water imbibes into the core forming a water block. The water is removed from the face of the plug. Immediately after the core was exposed to water, the permeability drops below the detection limit. In most core tested, the water block dissipates over time increasing the permeability. Experimental results on core plugs from two major shale plays show that imbibition and fluid loss into ultralow-permeability rock can be substantial, but these processes are also highly variable. We developed a measure of the phase block dissipation. The information gives operators a quantitative measure to determine the length of shut-in periods and for the development of other methods to minimize the damage from water imbibition.

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Proper characterization of reservoir quality of tight shales is critical for evaluating reservoir potential. These reservoir quality properties typically include hydrocarbon filled porosity, permeability, organic content and maturation, and pore pressure. Of these, permeability measurements are among the most complex to obtain, and have been subject to much discussion. Key concerns are the lack of analytic modeling, poor reliability, poor consistency, and ignoring stress sensitivity in the measurements. This paper reviews the pressure decay permeability method using crushed rock, and includes laboratory test results to validate the findings. Part of the review is a numerical model for the pressure decay method. This model includes significant processes of pipe flow in the equipment, thermal effects, diffusion into rock fragments, Klinkenberg effects, and size, shape and anisotropic permeability of the fragments.The paper shows that the measured permeability stress dependence, in tight shales, arises from coring induced microcracks. These result from failure of weak contact planes that are naturally occurring within tight shales and fail during coring and core retrieval. Permeability stress dependence in-situ is slight, as is show by compression test measurements to greater than thirty thousand psi. Crushing the rock to create small fragments for permeability measurements effectively removes these microcracks, and allows evaluation of real in-situ properties. Alternatively, closing microcracks by applying confining stress on plug samples, as routinely done for steady state and pulse decay measurements, is possible, but problematic because the critical stress required for microcrack closure changes from rock to rock facies.The pressure decay permeability method on crushed rock is shown to provide very consistent results that agree with other measurement techniques. The numerical model relates specific ranges of fragment sizes and testing conditions, to measured ranges of permeability. This allows permeability measurements and numerical model analysis for a broad range of variability in permeability that can be measured in heterogeneous tight shales. With some exceptions (e.g., Cui et.al. 2009), the fundamental understanding of the petrophysical properties of tight shales have not previously included rigorous confirmation of experimental measurements by analytical methods.

Investigation of the laws governing the behavior of complex media under the action of various external factors is necessary for solving many basic, technological, and engineering problems. An important part in such investigations belongs to methods and approaches developed by computational mechanics. For a long time, most numerical methods were based on the approaches developed within the framework of the mechanics of continuum. It should be noted that application of the methods of continuum mechanics to description of the process of deformation encounters considerable difficulties in the presence of local straining, discontinuities, intense vortex deformations, and agitation of masses. These problems are especially significant in the case of highly porous and heterogeneous materials and composites with strongly different properties of components.Discrete approaches capable of explicitly modeling the processes involving agitation of masses were developed predominantly for the investigation of granulated and friable media [1][2][3][4], in which the basic elements can be modeled by particles. For this reason, most of these investigations use the equations of motion in the form typical of the method of particles [4] and the interaction forces are calculated within the framework of the model of hard or soft spheres. However, this formalism does not provide correct description of the behavior of continuous isotropic media.The numerical method of movable cellular automata (MCA) extensively developed in recent years [5][6][7][8][9] is free of this disadvantage. While using a discrete approach, this method is based on the equations of motion, which are different from classical equations. In particular, it was shown [7] that, when the characteristic automaton size tends to zero, the MCA formalism allows a transition to the relations of continuum mechanics. The main advantage of this method is the possibility of explicitly modeling both the motion of continuous media and the agitation of masses, including the formation of discontinuities of various types (from the generation of individual defects to the main crack propagation). This circumstance for the first time provides prerequisites for jointly using discrete and continuum approaches within the framework of a common computational scheme, thus combining the advantages of both approaches for solving problems related to modeling of complex objects containing explicit zones of intense straining and fracture. This paper is devoted to the joint use of discrete and continuum approaches, which is important for the development of computational mechanics. The new approach is based on two methods successfully used in recent years. The first method, based on the continuum approach, is the finite difference method of solution of the dynamical problems of elastoplastic deformation of continuous media, and the second is the MCA method based on the discrete description.Since both methods employed in the proposed approach are well known [5,6, 10], we will only consider the questions p...