Effects of Friction on Hydraulic Fracture Growth Near Unbonded Interfaces in Rocks

Abstract

Abstract Small-scale laboratory experiments were performed to study the growth of hydraulically driven fractures in the vicinity of an unbonded interface in rocks. The purpose was to evaluate under which conditions the hydraulic fractures would cross the interface. The materials used in these studies were Nugget sandstone from Utah (3 to 6% porosity) and Indiana limestone (12 to 15% porosity). The fracturing fluid was oil (viscosity appx. 300 cp) injected into the rock through high-pressure steel tubing. Prismatic blocks of the rock materials to be studied were held adjacent to one another in a hydraulic press so that a normal stress was set up across their mutual interface. Lubricants and surface roughening were used to vary the frictional properties of the interfaces. It was found that as the interface surface friction coefficient was decreased, the normal stress had to be increased for a hydraulic fracture to cross the interface. The frictional shear stress that the interface can support without slippage appears to be critical in determining fracture growth across the interface. Additional experiments were performed to evaluate the coefficient of friction for the different interface surface preparations used. These experiments demonstrated that a variation in the frictional properties along an interfacial surface in the vicinity of hydraulic fracture growth can alter the path of the fracture. The experiments also demonstrated that cracks, which intersect the interface from the side opposite the approaching hydraulic fracture, can impede fracture growth across the interface. Introduction Hydraulic fracturing and a variant - massive hydraulic fracturing (MHF) - are primary candidates for stimulating production from the tight-gas reservoirs in the U.S. Hydraulic fracturing has been used widely as a well completion technique for about 30 years. MHF is a more recent application that differs from hydraulic fracturing in that larger quantities of fluid and proppant are pumped to create more extensive fractures in the reservoirs. 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 the production of gas from tight reservoirs, it is important that the fractures be emplaced in productive reservoir rock providing large drainage surfaces in the low-permeability material and conductive channels back to the wellbore. We then are faced 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 to study the hydraulic fracture process. The general goal of this research is to determine if and to what extent the reservoir parameters control the geometry of the created fractures. These reservoir parameters include (1) the mechanical properties of the rock (i.e., elastic moduli, mechanical strength, etc.), (2) the physical state of the rock (i.e., presence of pre-existing cracks or faults, porosity, pore fluid, etc.), (3) presence of layering or interfaces between different rock strata, and (4) stress field on the rock. In addition to reservoir parameters, the growth of a hydraulically driven crack will be influenced by (1) the manner in which the driving fluid is injected into the rock, (2) the characteristics of the fracturing fluid (i.e., viscosity, presence of proppant, etc.), and (3) any chemical reaction between the fluid and rock. Previous work has shown that crack orientation is controlled primarily by the in-situ or applied stress field, with crack growth oriented perpendicular to the least principal stress. SPEJ P. 21^