Methods and Evaluation of Explosive Fracturing in Oil Shale

Abstract

Abstract The USBM developed methods for fragmenting oil shale formations with chemical explosives to prepare oil shale for in-situ retorting and to prepare oil shale for in-situ retorting and to evaluate the extent of fragmentation by various methods. A combination of three methods of explosive fracturing was used: (1) the displacement and detonation of a liquid chemical explosive in a natural fracture system, (2) the use of 60 percent dynamite to relieve stress conditions of percent dynamite to relieve stress conditions of the rock around the wellbore and (3) the use of pelletized TNT in a series of wellbore shots to pelletized TNT in a series of wellbore shots to fragment the oil shale. An attempt was made to seal or grout around the fragmented oil shale by drilling 67 holes and displacing approximately 200 bbl of neat cement in the fracture system to aid in evaluating the fragmented zone. Seven methods were used to evaluate the fragmented zone. The primary evaluation method was the recording of seismic waves to delineate the fragmented zone with results based on arrival time, amplitude and record quality. The data from the seismic measurements indicated the extensively fragmented zone was approximately 95 ft in diameter and 70 ft thick. Results of the remaining methods of evaluating fractures-including airflow tests, coring of the expected fragmented zone, elevation measurements to indicate overburden crowning, and surveys with high-speed camera, impression-packer, and down-hole camera-agreed with and supported the data from seismic measurements. Further field application of explosive fracturing and evaluation methods is needed to determine an effectively fragmented zone of oil shale for efficient in-situ retorting. Introduction The potential benefits from the development and application of explosives to prepare underground mineral and energy resources for extraction are attracting the serious attention of many engineers and scientists in government and industry. Current efforts are toward improving the fluid flow capacity of oil shale and very tight petroleum reservoirs. The purpose of explosive fracturing research in oil shale is to develop a method that will create sufficient explosive fragmentation and permeability for in-situ retorting.