Water Weakening of Chalk - Physical and Chemical Processes

Chalk is an important reservoir rock, e.g., Ekofisk and Valhall fields. In the 90s, a dramatic increase in subsidence rate at Ekofisk was experienced after seawater injection. A similar effect is also observed in the laboratory. When chalk cores experienc e stress higher than yield, they tend to creep at constant effective stress. The observed creep is dependent on the chemistry of the fluid flooded through the core, this fact has been studied extensively at the University of Stavanger. Seawater injection induces at least twice as much deformation in chalks, compared with distilled water. This project will thus link the chemical effects and the physical forces, both being important in deformational processes in chalks. Calculations performed show that wh en seawater is in equilibrium with CO2 in the atmosphere and with calcite, several minerals are supersaturated at temperatures relevant for the Ekofisk field. If these minerals precipitate in the core, calcium will be taken out of the solution and in orde r for the solution to be in equilibrium with calcite, calcite will be dissolved. To really understand the impact of the chemical effect, the development of a pore scale model is required. The model used will be a Lattice Boltzmann model with realistic po re geometries and dissolution will be estimated quantitatively, location and amount. The volume change can be estimated as a function of mineralogical change. XRD and SEM analyses will be performed to investigate the mineralogical change induced by the b rine flooded through the core. Chemical analyses will be performed to evaluate any change in the brine composition induced by the chalk core. Long term rock mechanical tests will be performed under hydrostatic stress conditions with different brines. The chemical and pore scale calculations will predict different deformation of the chalk core as a function of the minerals in the core and the fluid chemistry.