A numerical investigation on deep shale gas recovery

Abstract

In recent years, exploration and development of deep shale gas (at a burial depth of 3, 500-4, 500 m) has become a hotspot in the industry. However, the state of gas storage and transporting mechanism for deep shale gas under high pressure and temperature have not been thoroughly explored, compared with its shallower counterpart. A numerical model for deep shale gas recovery considering multi-site non-isothermal excess adsorption has been established and applied using Finite Element Method. Results from the simulation reveal the following. (1) Excess desorption significantly impacts early-stage performance of deep shale gas well; the conventional way for shallower shale gas development, in which the density of adsorbed gas is not distinguished from that of free gas, overestimates the gas in place (GIP). (2) Although thermal stimulation can speed up the desorption and transporting of deep shale gas, the incremental volume of produced gas, which is impacted not only by seepage velocity but also density of gas, is insignificant, far from expectation. Only an additional 2.03% of cumulative gas would be produced under treatment temperature of 190 °C and initial reservoir temperature of 90 °C in a period of 5 years. (3) Matrix porosity, which can be measured on cores in laboratory and/or estimated by using well logging and geophysical data, is the most favorable parameter for deep shale gas recovery. With 60% increase in matrix porosity, an extra 67.25% shale gas on a daily base would be recovered even after 5-year depletion production; (4) Production rate for gas wells in shale reservoirs at 3, 500 m and 4, 500 m deep would be raised by 5.4% in a 5-year period if the depth of target interval would increase by 340 m without thermal treatment according to the numerical model proposed in the study.The paper provides a novel framework for optimizing deep shale gas development and clarifying some misunderstandings on adsorption behaviors associated with relatively high pressure and temperature.