A Numerical Study of Thermal-Hydraulic-Mechanical Simulation With Application of Thermal Recovery in Fractured Shale-Gas Reservoirs

Abstract

Shale gas is playing an important role in transforming global energy markets with increasing demands for cleaner energy in the future. One major difference in shale gas reservoirs is that a considerable amount of gas is adsorbed. Up to 85% of the total gas within shale may be found adsorbed on clay and kerogen. How much of the adsorbed gas can be produced has a significant impact on ultimate recovery. Even with improving fracturing and horizontal well technologies, average gas recovery factors in U.S. shale plays is only ~30% with primary depletion. Adsorbed gas can be desorbed by lowering pressure and raising temperature, reservoir flow capacity can be also influenced by temperature, so there is a big prize to be claimed using thermal stimulation techniques to enhance recovery. To date, not much work has been done on thermal stimulation of gas shale reservoirs.In this article, we present general formulations to simulate gas production in fractured shale gas reservoirs for the first time, with fully coupled thermal-hydraulic-mechanical (THM) properties. The unified shale gas reservoir model developed in this study enable us to investigate multi-physics phenomena in shale gas formations. Thermal stimulation of fractured gas reservoirs by heating propped fractures is proposed and investigated. This study provides some fundamental insight into real gas flow in nano-pore space and gas adsorption/desorption behavior in fractured gas shales under various in-situ conditions and sets a foundation for future research efforts in the area of enhanced recovery of shale gas reservoirs.We find that thermal stimulation of shale gas reservoirs has the potential to enhance recovery significantly by enhancing the overall flow capacity and releasing adsorbed gas that cannot be recovered by depletion, but the process may be hampered by the low rate of purely conductive heat transfer, if only the surfaces of hydraulic fractures are heated.

A Fully Coupled Numerical Model for Microwave Heating Enhanced Shale Gas Recovery

“…It is noted that the heat capacity of rock is at least 100 times larger than that of gas, and the formation porosity is normally small in shale reservoirs. Therefore, C eq and κ eq are dominated by the properties of formation rock [22]. K g and α g are the gas bulk modulus and the gas thermal expansion coefficient, respectively.…”

“…Some studies [20,21] estimate that the adsorbed gas shares are 20% to 85% of the total gas content. Therefore, enhancing gas desorption will inevitably increase shale gas recovery [9,10,22,23]. Existing experimental studies reported the effect of temperature on shale gas adsorption [24,25].…”

“…This indicates that thermal stimulation through changing the shale gas desorption behavior has the potential to enhance shale gas recovery, thus termed the thermal treatment technique. This thermal treatment technique enhances shale gas desorption and has been numerically investigated [9,10,22]. In the above numerical simulations, the fixed temperature boundary (over 200 • C) is generally used without considering the heat injection process.…”

“…The FHT of shale gas reservoirs mainly induces the following mechanisms. Firstly, it accelerates the desorption of methane in the matrix, especially for organic-rich rocks [8][9][10]22,23]. Secondly, it prevents water blocking evaporating blocked water and dehydrating the clay structure [1,8].…”

Thermal simulation of shale gas recovery involving the use of microwave heating

“…Thermal stimulation can account for about 60% of oil production in our world today (Chekhonin et al, 2012).Though several studies have been conducted on how to improve heavy oil/shale oil recovery by elevating the formation temperature, very few have considered the implementation of similar thermal techniques as a way of enhancing gas recoveries (Yue et al, 2015). Wang (2016) demonstrated the efficiency of thermal stimulation largely depended on the gas adsorption and rock properties of the formation. Thus by altering the gas adsorption/desorption behaviour through thermal stimulation, more gas could be recovered from hydraulically fractured shale gas reservoirs.…”