Investigation of formation heat treatment to enhance the multiscale gas transport ability of shale

Abstract

In order to achieve the most economical production of shale gas, fracture networks should be as complex as possible to connect the matrix micropores, natural fractures and induced fractures efficiently. Since shale rock is very tight, hydraulic multi-fracturing in horizontal wells has been the key technology in shale gas development. However, it is still difficult to connect small-scale pores in the matrix. Meanwhile, the recovery rate of a fracturing fluid is low, resulting in severe formation damage primarily induced by water blocking or water phase trapping. In this work, formation heat treatment is studied for the first time in gas-shale to address these issues. Samples from Longmaxi Shale are used to investigate the change in the multiscale gas transport ability after a high temperature treatment. A core sample heating system is specially designed to measure a permeability change during a high temperature treatment. Field scanning electron microscopy imaging, total organic carbon and low pressure nitrogen adsorption measurements are also implemented to analyze a change in the micro structure after the high temperature treatment. The results indicate that shale permeability increases rapidly with an increase in temperature, especially when the temperature is more than 400–500 °C, which can be described as a threshold value in a percolation model. Experimental results also indicate that the quality of the shale matrix can be dramatically improved after a high temperature treatment since there is a high quartz and organic matter content in the shale. Stimulation mechanisms of a formation heat treatment in shale gas reservoirs are described as water removing, mineral and organic matter structure changes and multiscale fracture network generation. For the field application of formation heat treatment, both its advantages and issues that still need to be addressed are deeply analyzed. It is indicated that the mid-late period of shale gas production can be extended, and problems induced by a residual fracturing fluid can be solved by formation heat treatment. Constructive field operation suggestions, which combine the advantages of electrical heating and microwave heating, are presented.