Conceptual study of thermal stimulation in shale gas formations

Abstract

Shale gas formations have become a major source of energy in recent years. Developments in hydraulic fracturing technology have made these reservoirs more accessible and productive. Apart from other dissimilarities from conventional gas reservoirs, one major difference is that a considerable amount of gas produced from these shale gas formations comes from desorption. Up to 85% of the total gas within shale can be found as an adsorbed phase on clay and kerogen, so how much adsorbed gas can be produced will have significant impact on ultimate gas recovery. The Langmuir isotherm has been widely used in industry to describe the pressure dependence of adsorbed gas. However, temperature dependent adsorption behavior and its major implications for evaluating thermal stimulation as a recovery method for shale reservoirs have not been thoroughly explored. Therefore, in order to design and analyze the thermal treatment of shale gas formations successfully, it is crucial to understand the effects of fracture heating on the shale gas adsorption and desorption phenomenon, and how can we exploit such effects to enhance shale gas recovery from hydraulically fractured reservoirs. Even though numerous research efforts have been focused on thermal recovery of shale oil, its possible application to shale gas has not been investigated.In this research, we propose a method to evaluate desorbed gas as a function of pressure and temperature in shale formations, by regression of a Bi-Langmuir model on Langmuir isotherm data. We have developed a fully coupled unconventional reservoir simulator, which is capable of capturing real gas flow in the shale matrix and in the hydraulic fracture by accounting for the effects of gas desorption and diffusion, as well as the temperature diffusion process within the matrix. This simulator enables us to investigate the effects of fracture heating on the shale gas desorption phenomenon on the global well performance and recovery. The results of this study show, for the first time in a rigorous way, that by increasing the temperature within the fracture, shale gas recovery can be improved.We have rationalized and quantified relations between the adsorption/desorption fundamental phenomena and stimulation temperature, fracture spacing, reservoir permeability and bottom hole pressure. The thermal properties of shale formations only have limited impacts on long term production. The results of this study can provide a guidance to develop a strategy to design thermal treatment in hydraulically fractured shale formations and propose the degree of thermal stimulation temperature required in a fracture to promote an economically viable return on production.