An Analytical Method for Estimation of Thermal Fracturing Initiation During CO2 Injection in Depleted Gas Reservoirs

Abstract

During CO2 injection, cooling and pressure buildup in the near-wellbore may lead to fracturing of the reservoir rock. These fractures affect the injectivity and can have a significant impact on flow assurance. This paper presents the derivation of a quick and simple method to evaluate the onset of thermal fracturing. This paper starts with geomechanics theory and expressions of minimum horizontal stress in depleted reservoirs. It describes the relations between thermo-poro-elastic stress and the criterion for initiation of (thermal) fracturing. The thermal stress part uses a simplified analytical solution of the stresses around the wellbore taking into account differences due to near-wellbore cooling and far-field virgin temperature conditions. A similar methodology is used to accommodate for pressure difference between the near-wellbore area and the far-field. A set of geomechanical and reservoir parameters are used to set-up diagnostic plots for evaluation. Consequences of the difference between the depletion stress path and injection stress path are discussed. Finally, results are compared to numerical reservoir simulation results. A proposed thermal stress correction factor, which accounts for differences between a simple analytical solution and a full-field evaluation, is checked and adjusted by comparing analytical and numerical results. First indications show that this geometrical factor and the derived equivalent cold zone radius holds for many cases. The sensitivity of thermal fracturing for several reservoir and rock parameters is discussed. The analytical method found is quick, simple and generates equivalent results to the numerical simulator and is therefore assumed to be accurate for estimation of the moment of fracture initiation. The resulting diagnostic plots present a quick and simple alternative to geomechanical simulation for evaluating the possibility and moment of fracture initiation. This method can help in early-stage feasibility work and determine if more detailed modelling is needed.