Effects of Various Parameters on Hydraulic Fracturing Geometry

Abstract

Abstract We are conducting a theoretical and experimental program on the hydraulic fracturing process. One primary objective of the program is to determine those reservoir properties or characteristics that can control the created fracture geometry. Theoretical models are applied to analyze some aspects of the dynamics of fracturing near material interfaces. The results of these calculations indicate that variation of material properties across a well-bonded interface can cause dynamic material response resulting from the fracturing, which could enhance propagation across the interface. Effects of friction also are analyzed theoretically; however, in the frictional calculations, the wave mechanics are ignored. These calculations show that frictional slip along the interface tends to draw a pressurized fracture toward the interface; this motion tends to reduce the chances of penetrating the material across the frictional interface.Small-scale laboratory experiments are performed to study the effects of frictional characteristics on hydraulic fracture growth across unbonded interfaces in rocks. Various lubricants and mechanical preparations of the interface surfaces are used to vary the coefficients of friction on the interface surfaces. It is found that the frictional shear stress that the interface surface can support determines whether a hydraulically driven crack will cross the interface. Experiments also are being performed to study the effects of pre-existing cracks, which perpendicularly intersect the unbonded interface, on hydraulic crack growth across the interface. It also is found that the presence of these pre-existing cracks impedes the propagation of the hydraulic fracture across the interface. The experimental results on the effects of friction on the interface and the effects of pre-existing cracks on hydraulic fracture penetration of interfaces are consistent with the predictions of the numerical model calculations. Introduction Massive hydraulic fracturing (MHF) is a primary candidate for stimulating production from the tight gas reservoirs in the U.S. Hydraulic fracturing has been widely used as a well completion technique for about 30 years. MHF is a more recent application that differs from hydraulic fracturing in that more fluid and proppant are pumped to create more extensive fractures in the reservoir. Application of MHF to increase production from the tight reservoirs has provided mixed and, in many cases, disappointing results - especially in lenticular reservoirs. For MHF to be successful in enhancing gas production from tight reservoirs, it is important that the fractures be created in productive reservoir rock with large drainage surfaces in the low-permeability material and conductive channels back to the wellbore. We are faced then with the problem of containing fractures in a given formation.Under the U.S. DOE's unconventional gas recovery program, Lawrence Livermore Natl. Laboratory is conducting a research program on the hydraulic fracture process. The general goal of this research is to determine if and to what extent reservoir parameters control the geometry of the created fractures. From theories implied and demonstrated, hydraulic fractures propagate perpendicular to the least principal stress. Hence, except for very shallow applications, the fractures will be primarily vertical, with the azimuthal orientation controlled by the in-situ stress. The vertical gradient in the horizontal stresses also could be a factor in the control of the shape or vertical extent of fractures. SPEJ P. 435^