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PETROMAKS2-Stort program petroleum

Water weakening of chalk at realistic reservoir conditions

Awarded: NOK 8.3 mill.

Mechanical experiments simulating reservoir operations (seawater flooding conducted after pressure depletion) have been carried out on outcrop core plugs from Kansas and Liege chalks. Core analysis and textural descriptions have been conducted with SEM, MLA and NanoSims in order to document mineralogical changes induced by seawater flooding.A number of advances have been achieved in the field of applied geology to chalk. We have gained a better understanding of the geological differences of a variety of on-shore chalks, often used to be possible candidates for the reservoir chalk. We also have achieved a thorough study of other on-shore chalk deposits in Northern Ireland and Italy, where low to medium porosity chalk is exposed and as such learnt mo re about the applied geological parameters (geochemistry and isotope geochemistry) and about variations in chalks. Recently also systematic measurements of specific surface area (SSA) before and after seawater injections have been performed and the obs erved variations in SSA show to be an important quantitative parameter that changes during mechanical compaction (pore collapse) and chemical fluid-rock interactions such as calcite dissolutuion triggered by precipitation of secondary minerals. The aim is to further quantify how the specific surface area evolves with time as various types of outcrop chalks are exposed to seawater-like brines, and thus investigating how such variations in SSA impact the rock-fluid rate kinetics and permeability. Pore- scale modeling by use of the Lattice Boltzman model as well as Darcy scale modeling have been performed, and the results show that pore fluids affect chalk properties such as mechanical stability, permeability, and wettability. The research partner GEO in Denmark, has updated the JCR 6 database with data from the 10 last years of new experiments carried out at UiS and at other laboratories, focusing on the chemical aspects of the flooding fluid.

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.

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PETROMAKS2-Stort program petroleum