THM response of a borehole in naturally fractured media

Abstract

The response of a circular borehole to a constant fluid flow rate and a temperature step-change in a coupled hydraulic-thermal-mechanical system is formulated and quantified. A new coupled formulation is developed, based on a dual-porosity model to simulate a naturally fractured stratum under non-isothermal and two-phase fluid flow conditions. The semi-analytical closed-form solutions for pore pressure, temperature and stresses including rotational displacement effects are developed by coupling an averaged conductive heat transfer process and extended single-phase Darcy fluid flow in both fracture and matrix systems. Deformations and stress changes can be induced on the bulk skeleton during a temperature change, potentially leading to shear and tensile mechanical yield. Effective stresses on the borehole wall corresponding to either fracture or shear yield initiation are calculated and explained. In a fractured medium, tensile and shear yield may initiate along the natural fractures thus take place earlier than in a continuum, displaced from the immediate borehole wall. A negative pore pressure near the wellbore may be induced during cold fluid injection when the thermally induced back flow volumes may surpass those from the injected fluid. Thus, unexpected higher effective compressive stresses may be induced on the wellbore wall, leading to yield or formation damage for the injection well. This semi-analytical development permits rapid assessment of potential fracturing initiation and shear yield to be analyzed for various cases such as injection and production in a geothermal extraction system, fluid waste disposal including CO2 sequestration, and oil and gas related EOR processes such as an initiation stage of hydraulic fracturing.