Bed Preparation Concepts For True In Situ Oil Shale Processing: An Evaluation Of Current Technology

Abstract

Abstract True in situ methods to process oil shale, which by definition require no mining and depend on wellbores for access to the underground formation, will be successful only if techniques are available to transform the normally very tight oil shale seams into rubble beds that are sufficiently porous and permeable and composed of suitably sized fragments to permit effective retorting operations to be conducted. Prior experimental efforts to create such beds in comparatively deep formations, where significant void cannot be introduced by explosively lifting the overburden, have in nearly all cases employed either the wellbore springing or hydraulic/explosive fracturing concept. Only very limited success has ever been achieved in these efforts. In this paper, reasons for the lack of success are identified. For the wellbore springing concept, the inherent cylindrical geometry is the primary difficulty in that it leads to small fractured zones because of rapid stress pulse attenuation, and also to regions of residual compressive stress around the wellbores which restrict fluid flow and hinder void redistribution. Major difficulties with the hydraulic/explosive fracturing concepts are that many important operations cannot be controlled and that regions of enhanced permeability are formed only in the immediate vicinity of explosive filled hydrafractures. The future success of true in situ processing depends on the introduction of void prior to the blasting operation and deployment of explosive in a geometry that will ensure distribution of the void. INTRODUCTION AND BACKGROUND Recent shortages of liquid fuels, together with accompanying price escalations, have once again drawn attention to the fact that the world's rate of consumption of petroleum products has risen to an alarmingly high level, so high in fact that underground reserves of crude oil are in danger of being depleted in the foreseeable future. To prevent, or minimize, the potentially calamitous consequences of such an occurrence, the United States Government, formerly through such agencies as the Bureau of Mines and the Energy Research and Development Agency and more recently through the Department of Energy, has initiated and continued a variety of programs to develop alternate energy sources and to improve or optimize utilization methods for existing sources. Included has been an effort to develop the technology required before the vast oil shale deposits of the United States can be exploited. Impetus for this effort stems not only from the enormity of the oil shale reserve, but also from the fact that this resource represents one of the few, if not the only, alternate source of liquid fuel available in large enough quantities to be significant. While oil shale is found in many places within the United States, the Green River Formation in Colorado, Utah, and Wyoming is the largest, at least in terms of proven resources, and contains the equivalent of nearly two trillion barrels of oil; this quantity is about fifty times greater than the known crude oil reserves of the United States. While several methods to heat oil shale have been tried, or proposed, (e.g., flow of hot gases or superheated steam over fragmented oil shale, microwave heating, etc.) a particularly attractive method involves the burning of a carbonaceous residue left in the rock after the shale oil has been removed. One procedure to implement this processing technique, which actually has been practiced at various times for more than a century, involves (1) mining raw oil shale; (2) crushing and screening the material to obtain suitably sized fragments; (3) placing the fragments in a "retort" vessel; (4) initiating a flame at the top (typically) of the fragment pile; and (5) providing a flow of combustion and process support gases to maintain the advance of a flame front into the fragment pile. As the flame advances, the oil stale in its path is freed from the rock matrix and forced downstream by the gas flow where the products are collected.