Pressure-Solution at Grain-to-Grain Contact. A New Model for Basin Modelling

Pressure solution reaction leads to closer packing of the granular rocks, resulting in a porosity decrease, but the mechanisms operating at the grain scale are not well understood, mainly because different mechano-chemical processes interact during deformation. Sandstones from the Brent reservoirs located in the Northern North Sea show pressure solution features. We focused our work on the role of mass transfer in sandstone diagenesis, for which the present model of pressure solution is developed.

Pressure solution is driven by stress differences and its rate is affected by temperature, grain size, and fluid chemistry. The pressure solution process comprises three kinetic steps: dissolution of the grain at stressed grain-to-grain contacts, diffusion to stress-free grain surface, and precipitation. The present treatment of pressure solution idealizes the aggregate as a regular packing of truncated spheres, such that it is sufficient to consider a single spherical grain in contact with N identical neighbours. The grain is assumed to consist of a pure, isotropic solid substance that is soluble in the pore fluid. For conditions of temperature and pressure relevant for sedimentary basins, we show that the dissolution step is the slowest kinetic step, and therefore it controls the overall kinetic of pressure solution. Another original feature of our model lies in the fact that the geometrical model comes from studies of metallurgy. This geometrical model permits to calculate the coordination number and the contact surface areas for each sphere as a function of the porosity.

The model is applied to the Brent reservoirs from the Northern North Sea. The inputs of the model are: (1) the temperature and the effective stress histories computed with TemisSuite Software, (2) the grain size distribution of the reservoir unit. Then the model is used in post-processing of a basin modelling calculation performed with TemisSuite. The computed porosities are in good agreement with the observed porosities.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California