Numerical investigation of fracturing fluid invasion into hydrate reservoirs during hydraulic-fracturing stimulation

Abstract

Hydraulic fracturing is a potentially promising stimulation technology employed for enhancing gas productivity and energy efficiency of low-permeability hydrate reservoirs. At present, most studies in the field of hydrate reservoir fracturing have mainly focused on the fracturing feasibility and recovery efficiency without considering the effects of fracturing fluids on hydrate reservoirs. However, fracturing fluid will inevitably invade hydrate reservoirs driven by high fracturing pressure, which may cause hydrate phase transitions and consequently affects fracturing. In this study, the dynamic process of fracturing fluid invasion into hydrate reservoir was simulated using the TOUGH+HYDRATE software, primarily focusing on the differences between the fracturing fluid invasion into the gas hydrate-bearing layer (GHBL), three-phase layer (TPL), and free gas layer (FGL). The results showed that a secondary hydrate column was formed within 2–10 m around the fracture by the fracturing fluid/free water and free gas during fracturing fluid invasion into the TPL and FGL, which was mainly driven by the large driving force exerted by high fracturing pressure. But the exothermic reaction of hydrate formation can provide negative feedback to the formation and expansion of the secondary hydrate column, leading to a state of phase equilibrium in the secondary hydrate formation region and the outward expansion of the secondary hydrate column in a “three-phase equilibrium” mode. It should be noted that secondary hydrate formation did not occur during fracturing fluid invasion into GHBL because of the absence of free gas, and fracturing fluid invasion into GHBL only affected reservoir temperature and pressure, which was similar to the fracturing fluid invasion into unconventional oil and gas layers. These results will provide important guidance for the potential application of hydraulic fracturing in the field tests of hydrate reservoirs and its scheme formulation.