The objective of the present study is to provide a better understanding about the mechanism for weakening of chalk reservoirs when injecting large quantities of seawater to improve the oil recovery. The mechanical stability of chalk has often been related to capillary forces, which is known to induce an apparent cohesion between chalk grains, although several researchers have pointed out that other mechanisms need to be considered as well. In the present study, the experimental conditions have been designed to quantify the weakening of the chalk in a chemical dissolution/precipitation process, when one of the common ions, Ca2+ or CO32-, was chemically removed from the solution at 130 degrees C during an aging time of 6 weeks. Removal of one of the common ions enhances the dissolution of chalk. Three different saturating fluids were used: (1) artificial Ekofisk formation brine (EF-brine), (2) artificial seawater (SSW), and (3) modified seawater containing 4 times the sulphate concentration with respect to artificial seawater (SSW1). At the aging conditions, CaSO4(s) was about to precipitate from the SSW1 solution, causing dissolved Ca2+ ions from the chalk to be precipitated/removed as CaSO4(s) from the solution. No removal of common ions took place using EF-brine and SSW. Mechanical tests (Brazilian tests, hydrostatic yield, and K-modulus) at room temperature of the chalk cores saturated with the actual fluids showed that the strength of chalk saturated with SSW1 was 20-25% weaker than cores saturated with EF-brine and SSW. No difference in chalk strength was detected using EF-brine and SSW. The greater water weakening of cores saturated with SSW1 was discussed in terms of a chemical dissolution/precipitation process and the chemistry of the thin water film close to the intergranular contacts. (c) 2004 Elsevier B.V All rights reserved.
