INTRODUCTION
A problem common to all in situ oil shale combustion processes for production of oil is to provide an adequate void volume distribution within the rock so that gas may pass through during the retorting process. One method to achieve this goal is to heave the earth's surface by detonating an explosive charge within a relatively shallow oil-shale thickness. This technique has been tested in the field by Geokinetics,Inc. in Uintah County, Utah, (Lekas, 1979). The void space distributions resulting from the Ceokinetics blasting tests have been evaluated empirically, usually by burning the retorts.
This paper presents a general explosive fragmentation model incorporating oil shale properties specific to the Geokinetics site. Modeling their blasting program using mechanical properties of the oil shale deposit as primary input can provide an inexpensive tool for optimizing blast results. Although an actual field experiment may involve the timed detonation of hundreds of explosive boreholes, understanding the mechanics and implications of a single shot hole is important as much of the phenomenology of single-hole breakage is involved in multihole fragmentation. Also, the overall environmental impact and program cost may be significantly reduced due to more efficient rubblization.
CALCULATIONAL TECHNIQUE
The modeling efforts to study single-hole fragmentation employed an explicit finite-difference computer code called STEALTH**. STEALTH (Hofmann, 1976) solves the equations for conservation of momentum, mass, and energy and is particularly well suited to modeling dynamic phenomena such as explosive effects in rock. The constitutive equations which relate induced stresses to observed ground motions may be as complicated as the data base will allow. Previous models of explosive oil shale fragmentation have been performed (Trent, et al, 1980) and an extensive program for the study of explosive stimulation of gas shales is presently under investigation (Barbour and Young, 1980).
Rock Fracture Model
The STEALTH sub-model which provides the details of the fracturing process is called CAVS (Crack And Void Strain). The model is general and has various applications (Maxwell and Reaugh, 1980), (Barbour, et al, 1980). The unique aspect of the CAVS model is that the crack aperture (void strain) changes are coupled precisely to the three dimensional stress tensor adjustments during crack opening and closing. Due to the extensive bookkeeping system within CAVS, it is capable of determining the extent of cracking next to an explosive borehole. It has been proposed that cracks immediately adjacent to the shot hole will become filled with hot explosive reaction products and subsequently open and propagate more readily than if these gases were not present. This phenomenon has been observed and studied quantitatively in the calculations. While the concept of crack communication with the borehole (and explosive gasses) is relatively straightforward, in practice the logic could be quite sophisticated since the ultimate pathway of gas travel cannot be determined a priori.
Fracture Internal Pressurization Model
Several one-dimensional borehole calculations have been carried out which have allowed the crack internal pressurization logic to be tested.