Fracture Propagation Driven by Fluid Outflow from a Low-permeability Aquifer

TL;DR

This paper models fracture propagation in caprock caused by fluid outflow from low-permeability aquifers during CO2 sequestration, predicting rapid fracture growth that could threaten containment.

Contribution

It introduces a combined numerical and analytical model for fracture growth driven by aquifer outflow, linking hydraulic fracture theory to CO2 sequestration safety.

Findings

Fracture propagation can be as fast as hydraulic fractures.

Hydrostatic and geostatic effects increase propagation risk.

Fractures may serve as leakage pathways for CO2.

Abstract

Deep saline aquifers are promising geological reservoirs for CO2 sequestration if they do not leak. The absence of leakage is provided by the caprock integrity. However, CO2 injection operations may change the geomechanical stresses and cause fracturing of the caprock. We present a model for the propagation of a fracture in the caprock driven by the outflow of fluid from a low-permeability aquifer. We show that to describe the fracture propagation, it is necessary to solve the pressure diffusion problem in the aquifer. We solve the problem numerically for the two-dimensional domain and show that, after a relatively short time, the solution is close to that of one-dimensional problem, which can be solved analytically. We use the relations derived in the hydraulic fracture literature to relate the the width of the fracture to its length and the flux into it, which allows us to obtain an analytical expression for the fracture length as a function of time. Using these results we predict the propagation of a hypothetical fracture at the In Salah CO2 injection site to be as fast as a typical hydraulic fracture. We also show that the hydrostatic and geostatic effects cause the increase of the driving force for the fracture propagation and, therefore, our solution serves as an estimate from below. Numerical estimates show that if a fracture appears, it is likely that it will become a pathway for CO2 leakage.