Fault slippage and its permeability evolution during supercritical CO₂ fracturing in layered formation

Abstract

Understanding the hydromechanical responses of faults during supercritical CO 2 fracturing is important for reservoir management and the design of energy extraction systems. As small faults are widespread in Chang 7 member of the Yanchang Formation, Ordos Basin, China, supercritical CO 2 fracturing operation has the potential to reactive these undetected small faults and leads to unfavorable fracking fluid migrate. In this work, we examined the role of fault slippage and permeability evolution along a small fault connecting the pay zone and the confining formation during the whole process of fracturing and production. A coupled hydromechanical model conceptualized from actual engineering results was introduced to address the main concerns of this work, including, (1) whether the existence of a undetected small fault would effectively constrain the hydraulic fracture height evolution, (2) what the magnitude of the induced microseismic events would be and (3) whether the permeability change along the fault plane would affect the vertical conductivity of the confining formation and thus increase the risk for the fracturing fluid to leak. Our results have shown that the initial hydrofracture formed at the perforation and propagated upward, once it merged with the fault surface, the existence of an undetected small fault would effectively constrain the hydraulic fracture height evolution. As fracturing continued, further slippage spread from the permeability increase zone of high permeability to shallower levels, and the extent of this zone was dependent on the magnitude of the fault slippage. At the end of extraction, the slip velocity decreases gradually to zero and the fault slippage finally reaches stabilization. In general, undetected small faults in targeted reservoir may not be the source of large earthquakes. The induced microseismic events could be considered as the sources of acoustic emission events detected while monitoring the fracturing fluid front. Due to the limited fault slippage and lower initial permeability, the CO 2 fracturing operation near undetected small faults could not conduct preferential pathway for upward CO 2 leakage or contaminate overlying shallower potable aquifers.

Numerical study of the impact of stress concentration on shale gas production

“…Some scholars have also proposed and studied the non-Darcy path of gas flow, including Knudsen diffusion and slip flow (Javadpour, 2009;Ashrafi Moghadam and Chalaturnyk, 2014;Guo et al, 2015). A numerical model conceptualized using actual engineering results was previously introduced to evaluate the propagation of hydraulic fractures and study the hydromechanical responses and permeability alternation throughout hydraulic fracturing and production (Wei et al, 2019). Therefore, it is necessary to consider the selection of a multiporosity medium model and changes to the shale gas flow regimes in the simulation of shale gas reservoir production modeling.…”

Subsurface fluid injection and energy storage

“…Wei et al [10] studied fault slippage and its permeability evolution during supercritical CO 2 fracturing in layered formation. Major result is that due to limited determined fault slippage in combination with lower initial permeability, the CO 2 fracturing operation near undetected small faults could not conduct preferential pathway for upward CO 2 leakage or contaminate overlying shallower potable aquifers.…”