ABSTRACT
Successful explosive gas well stimulation requires a thorough understanding of explosively driven cracks under confining in-situ stresses. In a previous paper (Simha, et al 1983) the problem of explosively driven cracks was experimentally investigated to reveal the features of crack propagation. It was observed that the explosively driven crack propagation is the result of two different but overlapping phases. The first phase involving the initiation and early time crack propagation is entirely governed by the explosively generated stress transients. The rapidly decaying stress transients then lead to the second phase of crack propagation largely controlled by the in-situ stresses. The purpose of this paper is to more fully understand the characteristics of the first phase concerning the initiation and early time propagation of explosively driven cracks. Experiments are conducted with plastic models under biaxial compression and the dynamic event is observed with a high speed multiple spark gap camera of the Cranz-Schardin type. The experimental observations are utilized to propose analytical models of crack initiation under explosive loading to aid in the design of multiple fracturing necessary for successful application of modern well stimulation techniques.
INTRODUCTION
The ever growing need for oil and natural gas has led to several new sources of hydrocarbon fuels that require novel and often very costly means of extraction. In the United States gas-bearing deposits of Devonian Shale have been identified in the states of Pennsylvania, Kentucky and West Virginia along the Appalachian basin. Vast as these deposits are, only a few of the many fields have proved to be economically viable. The key factor for a well to be successful is that the shale surrounding the well bore be fractured to promote gas flow into the well bore. Towards this end various techniques have been attempted to stimulate wells. Stimulation by hydraulic fracturing has been moderately successful in this regard. However in recent times explosive stimulation has been enthusiastically investigated, although in some instances, the flow rates have actually been substantially reduced. This has been attributed to a combination of factors including stress caging and excessive fragmentation causing crack closure by fines. However among other factors the successful application of explosive stimulation involves the phenomenon of dynamic crack propagation in the presence of confining in-situ stresses. Specifically, the problem involves linking the well bore to a pre-existing fracture network where it becomes necessary to drive cracks over long lengths as well as maintain control over their paths. While the crack length is largely determined by the borehole pressure generated by the explosion and the consequent gas flow into the propagating cracks, the crack path is significantly dependent on the existing in-situ stresses. In 1980 Warpinski, et al described the results of tests conducted at the U.S. Department of Energy's Nevada test site to examine the effects of in-situ stress variation on fracture. Earlier in 1976 Dally and Fourhey reported on a simplified model to predict the crack curvature under a uniaxial stress field. Recently, an experimental investigation was conducted to extend this model to the general case of a biaxial stress field (Simha, et al 1983).
