NUMERICAL MODELLING OF PORE PRESSURE DIFFUSION IN A GEOTHERMAL RESERVOIR STIMULATION TREATMENT

A numerical porous media model is used to understand mechanisms involved in seismic events taking place during injection of high volume of water in deep rock formation.

For low productive reservoirs, stimulation treatments are an option to increase the fluid flow thus achieving a significant rate of production or injection. One of the most effective ways to increase flow is by injection of fluid at high pressure. To perform hydraulic fracturing injection of water at pressures higher than tectonic minimum stress is necessary to fracture the formation.

Different studies linked variation in pore pressure and shearing mechanisms seismicity, due to reduction in normal stress promoted by an increase in fluid pressure. Among many cases is the Matsushiro (Japan) research well, the Rangley earthquake control experiment and more recently the enhanced geothermal system (EGS) site in Basel. Various events related to impounding of water reservoirs shows correlation with pore pressure diffusion, too.

Pore pressure diffusion is the most relevant effect far from the injection point that can be seen during the stimulation time frame (usually days or weeks). Pore pressure can itself promote increase in permeability of the formations, leading to preferential diffusion pathways. Understanding the interaction between pore pressure and natural faults and fractures can help to develop better treatments, mitigating undesired and potentially harmful seismicity.

Diffusion of pore pressure in an heterogeneous medium has been investigated in this study with a finite element method, approximating conditions of hydraulic fracturing that took place in a geothermal field, comparing the model results with temporal and spatial distribution of the recorded events.

The eventual goal is to find a way to model pore pressure diffusion in a complex environment including tectonic stresses and rock properties heterogeneities.