Shale Oil Survival in Underground Retorting By Combustion

Abstract

Abstract A simplified mathematical model is developed to determine the amount of oil that survives passage to a hot fracture boundary in an underground retorting process in oil shale. Combustion at or near a fractured surface, heat conduction, oxygen diffusion, reaction of oil and oxygen, and perpendicular oil flow by diffusion and perpendicular oil flow by diffusion and convection toward the hot boundary are considered. Appropriate assumptions are employed to arrive at upper bounds on the predicted recovery. Significant amounts of oil production are unlikely when the fracture temperature approaches 2000 degrees F. However, if the temperature is 1200 degrees F. or lower, the calculated upper bound is 64%. When more realistic assumptions are invoked, the computed results indicate no oil recovery. The effects of oil migration velocity and the kinetic parameters were studied. Predicted recoveries were highly sensitive to the activation energy for temperatures below 2000 degrees F We conclude that underground combustion in oil shale employing hydraulically or electrically created fractures apparently is not technically feasible unless appropriate means of reducing temperatures are employed. It appears that massive rock breakage is necessary to achieve substantial amounts of oil production. Introduction The possibility of underground retorting of oil shale appears attractive since mining, crushing, and spent shale disposal are not involved. However, it is necessary to induce a flow path in the rock since oil shale usually has no primary permeability and porosity (vugular porosity excluded). To porosity (vugular porosity excluded). To accomplish this, hydraulic fracturing, nuclear detonation, and electro-fracturing have been suggested. A number of thermal processes, such as hot fluid injection, forward combustion, reverse combustion, and conduction heating, can presumably be conducted within a single fracture presumably be conducted within a single fracture or system of fractures. Other methods have been proposed which combine two or more of these basic processes. The success of any of the above processes will necessarily hinge on whether or not sufficient quantities of the liquid product can be recovered. In most cases, the system of fractures created in the shale must serve as the flow path for the produced as well as the injected fluids. Some or all of the produced fluids will frequently pass through high temperature zones to reach the fractures. To make intelligent performance predictions, it is important to know if the residence time and temperature levels will result in severe reduction in oil yield by destructive distillation and oil burning.