Abstract

In hydraulic fracturing technique, one of the questions is to determine fracture initiation pressure. The resolution of this problem should account for the coupling between material deformation, fluid flow and temperature variation. A finite element solution is proposed in this paper by taking into account non linear coupling due to heat convection and plastic deformation of rocks. A specific numerical algorithm is used to treat long time injection problem. A typical example of cold water injection through a vertical well is then considered. Comparative calculations made with and without heat convection have clearly shown the key role of heat convection in diffusion of stresses, pore pressure and temperature around the well. Two particular issues, reservoir deformation induced by injected cold water (cooling and pore increase) and influence of thermal stress diffusion by convection on hydraulic fracture initiation, are examined. The main finding is that the stress and temperature diffusion is quasi completely controlled by convection. The cold water injection induces relative reservoir compaction, which can become significant (reservoir subsidence) when the injection pressure is low. Concerning fracture initiation pressure, the tangential stress evolution is also strongly controlled by heat convection. The temperature decrease (cooling) induces a significant and time dependent tensile tangential stress zone where the breakdown pressure may be much smaller than that determined without considering heat convection. Moreover, heat convection accelerates the propagation of the tensile stress zone (consequently the fracture propagation) into reservoir. Finally, in the cases studied in this paper, rock plastic deformation plays a secondary role with respect to heat convection.

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