Embedded analytical-numerical simulation of fault reactivation in heterogeneous subsurface formations

Abstract

Induced seismicity refers to seismic events (earthquakes) triggered by human activities. Such events, even when characterized by relatively modest magnitudes, have the potential to jeopardize the safety of individuals, the surrounding environment, and infrastructure.

Production from hydrocarbon reservoirs can alter the in-situ stress state, leading to induced seismicity. This is reported in the Groningen field, where substantial gas production caused fault reactivation and subsequent earthquakes. Understanding events in the deep subsurface is crucial to proactively mitigate future seismic occurrences. To understand the causes of induced seismicity, the underlying physics are examined and defined in terms of relevant governing equations and models. This reveals the interconnected nature of fluid depletion, rock deformation, and fault slip. The goal of this study is to develop simulation techniques to solve these equations.

Towards this end, firstly, a finite volume embedded-numerical simulation method, called the Smoothed Enhanced Finite Volume method (sEFVM), is developed. This method is revealed to be computationally efficient for reservoir-scale modeling of heavily faulted systems and performed well in comparison to known solutions and other simulators.

However, in settings where analytical solutions indicated noncontinuous shear stress profiles, sEFVM accuracy suffers. Recognizing this limitation, a semi-analytical approach is developed, extending analytical expressions to be solved over the sEFVM mesh. This extension allows for more accurate solutions, accommodating complex reservoir and fault geometries. The semi-analytical method is successfully used to estimate the onset of fault nucleation and the magnitude of the seismic moment resulting from depletion.

The semi-analytical approach is limited to simulating fault slip up to the point of nucleation. To overcome this constraint, a hybrid method is developed. With appropriate assumptions regarding the post-nucleation state and the use of sEFVM to numerically calculate post-nucleation stresses, the hybrid method can effectively model multi-fault systems in the seismic stage assuming quasi-static behavior.

In summary, this research contributes by presenting novel computational frameworks for studying fault reactivation in faulted poroelastic media, offering insights into the complex interactions of the physics at play. The proposed embedded-numerical, semi analytical, and hybrid methods pave the way for further advancements in the field.