Water weakening of chalk at realistic reservoir conditions

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Water injection has been used with great success in order to recover oil from North Sea reservoirs. For the Ekofisk field water flooding has made a vision of a recovery factor of 60% realistic. At the Ekofisk field over 10 m of subsidence has occurred ove r the last 30 years. Injection of brine there to maintain fluid pressure and increase oil recovery has reduced the compaction rate by one third but not eliminated it. Injections of different chemistry brines at elevated temperatures appear to cause differ ent amounts of compaction and different degrees of oil recovery. The aim of an ongoing Petromaks project "Water Weakening of Chalk - Physical and Chemical Processes" has therefore been to understand and model the impact of the chemical and the physical fo rces on the deformation of North Sea chalks. The ongoing project has demonstrated that the water chemistry greatly affects the rock mechanical strength, and it has given an improved insight into the basic mechanisms behind the water weakening phenomenon. The ongoing project has, however, been limited to the study of one aqueous phase only inside the rock. In a realistic case oil will also be present inside the pore space, and thereby introducing important capillary effects. One of the aims in this project is to perform experiments closer to realistic field conditions. The presence of CO2 as a supercritical phase will affect the rock mechanical strength, one effect could be to displace most of the water and then making the rock stronger; another effect cou ld be an increase of carbonate in the aqueous phase and thereby creating a weak acid and thus lead to dissolution. In this current project we therefore suggest to study the effect of water weakening in the presence of oil/water or CO2/water at realistic r eservoir conditions. The experimental program will thus include tests that simulate the whole lifetime of the reservoirs (Ekofisk and Valhall) and will be closely linked to the development of the numerical models.