Abstract: Simulation of Mineral Diagenesis in Reservoirs

Etienne Brosse, Brigitte Bazin, Yann Le Gallo, Eric Sonnenthal, Frederic Sommer

Petroleum geologists and production engineers are faced with reservoirs where porosities and permeabilities (poroperm) have been reduced by mineral phases precipitated during the geological evolution. Diagenesis of sandstones is influenced by many factors: initial composition of the sediment, burial history, composition of infiltrated waters. An appraisal of poroperm decline due to mineral diagenesis can only be accomplished by an integration of these factors.

A quantitative evaluation of diagenetic phenomena is possible using numerical modelling. A first approach to mineral reactions can be made by geochemical modelling applied to <<closed cells>>, where aqueous solution and minerals are not in equilibrium initially. The routine calculates, as a function of time, the dissolved and precipitated quantities of mineral phases, occurring as the system evolves towards equilibrium.

Illite and quartz, fcements are common in sandstones bearing feldspar, such as Middle Jurassic reservoirs of the Brent Group (East Shetland basin, North Sea) which today lie between 3500 and 4500 m depth. Results of <<closed cell>> simulations are presented, which explore the conditions of illite and silica authigenesis in this Province, particularly in terms of temperature, water composition, and kinetics (oversaturation of the water with respect to quartz, low pH).

Impervious rocks, the flow of interstitial water is a key contributer to non- equilibrium. Its role must be appraised by a <<reaction-transport>> code. A new software is presented, able to solve, at the reservoir scale, in a coupled way :1) advection of water and chemical elements in the porous volume; 2) mass balance of chemical elements in the rock volume; 3) dissolution-precipitation phenomena occurring locally (using the geochemical code above described); 4) a feedback of the mineral transformations on permeability and reactive surface areas through a <<textural>> model at the grain scale.

Simulations are presented, which show the possible role of water filtration in the illite and quartz cementation of the Brent sandstones. The effect of water velocity on the scale of transfer of chemical elements such as Si, Al, Na, K is discussed.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France