Abstract

Borehole geophysical data were used to characterise a fault in an Upper Jurassic (Malm) reservoir. Free convection and bilinear flow were recognised in the fault. Moreover, the hanging wall of the fault presents better conditions for the fluid injection than the footwall. Structural dips and fractures confirm that the wellbore goes through a fault system from about 3094 m depth. The maximum horizontal stress is oriented approximately N-S and a strike-slip faulting regime was identified. A conventional analysis of slip tendency reveals that low reinjection pressures (of the order of few bars) are sufficient to produce failure of the pre-existing fractures. The movement of the reinjected water to great depths is supported by the inclination of the open hole, the low injection temperatures (lower than the undisturbed water temperature) and the steep bedding planes close to the fault. It is not known exactly how deep the fault extends into the basement, but the analysed information shows that the crystalline basement could act as a barrier to the downward movement of fluids. The combination of the movement of fluid downward and along the fault generates pressure increases in a large zone of the fault core.