Some Results From Continuum Mechanics Analyses of the Hydraulic Fracturing Process

We are developing and applying theoretical, numerical models to analyze the hydraulic fracturing process. Applications include fracturing near process. Applications include fracturing near interfaces, effects of existing fractures near interfaces, pore-pressure effects, and stress-field changes due to pore-pressure effects, and stress-field changes due to embedded lenses. For a well-bonded interface, the calculations indicate that the stress-intensity factor of the leading crack tip decreases as the crack approaches a higher modulus material. As the tip crosses the interface into the higher modulus material, the stress-intensity factor abruptly increases to a higher value than it had in the lower modulus material. When the situation is reversed, the intensity factor increases as the tip approaches the interface and then abruptly decreases when the tip crosses the interface. Further calculations show that when existing cracks are present near the interface, the effects of the change in material properties across the bonded interface are reduced. In addition, our analysis shows that increases in pore pressure clue to leakage of fluids from the fracture into the surrounding media causes the stress-intensity factor to drop; a decrease in the stress-intensity factor means a reduction in the tendency to break. In other calculations we analyzed the stress-field disturbance caused by lenses of one material that are embedded in another material. These calculations show that in regions that are not tectonically relaxed, the stress field is modified by the lenses. We conclude that the fracture geometry is modified by the presence of embedded lenses under these conditions. Introduction Hydraulic fracturing and a variant - massive hydraulic fracturing (MHF) - are primary candidates for stimulating production from tight gas reservoirs in the U.S. Hydraulic fracturing has been used as a well completion technique for about 30 years, with more than 750,000 applications. MHF is a recent application, differing from hydraulic fracturing in that much larger quantities of fluid and proppant are pumped to create extensive fractures in proppant are pumped to create extensive fractures in the reservoir. Application of MHF to increase production from tight gas reservoirs has provided production from tight gas reservoirs has provided mixed and, in many cases, disappointing results.For hydraulic fracturing or 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 and conductive channels back to the wellbore. Hydraulic fracturing, when used as the standard well completion technique, results in fractures driven into the formation to overcome damage due to drilling. Although the concept of MHF is to drive extensive fractures in the reservoir, we are faced with the problem of containing those fractures in a given formation. There are several reservoir properties that have been proposed as having an effect on the control of the created fracture geometry. These include the in-situ stresses, stratigraphic layering, and preexisting faults and fractures.From theories implied and demonstrated, hydraulic fractures propagate perpendicular to the least principal stress. Hence, the azimuthal orientation of the fracture is controlled approximately by the in-situ stress field. We also know that, except for very shallow applications, the created hydraulic fractures will be primarily vertical. The vertical gradient in the horizontal stresses also could be a factor in the control of the shape or vertical extent of the fractures. P. 86