181 / 2024-03-11 17:22:27

Fluid pressure distribution and diffusion in crystalline rocks under pore pressure gradients

pore pressure distribution; permeability; fluid pressure field equations

Pore pressure has a major influence on the effective stress and thus on the mechanical behavior of rock. In particular, the perturbation of pore pressure in a reservoir changes the effective stress which may lead to microseismic activity. This may occur in conventional reservoirs, storage of CO_2, or deep geothermal energy extraction. Whereas the direct measurement of pore pressure in rocks is difficult, since its non-transparency of hiding inside rocks. Determining the pore pressure distribution and diffusion laws of inhomogeneous crystalline rocks under fluid pressure gradients requires measuring accurate pore pressure values and establishing fluid pressure field equations (FPFE).



To solve this problem, experiments were performed on one granite sample and one basalt sample, they were heated to 650°. The variation of the permeability and stress sensitivity factor with the effective stress was first determined by the permeability experiment, and a new type of fluid pressure sensor was used in the pore pressure gradient experiment to directly measure local pore pressure values under different pore pressure differentials.



The measurement results show that pore pressure distribution transitions from the linear state to the non-linear state with the increase of differential pressure gradient. The exponential variation of permeability with effective stress was substituted into the diffusion equation for compressible rock, which was used to obtain FPFE according to the boundary conditions set in the pore pressure gradient experiment. FPFE was used to fit the local pressure data to derive the stress sensitivity factor as a function of the effective stress. Finally, the consistency of the relationship between the stress sensitivity coefficient and the effective stress in the permeability experiment and the pore pressure gradient experiment verified the correctness of the established FPEF.