The Weyburn project : summary report for Task 3.1 : experimental geochemical studies of CO2-porewater-rock interaction

摘要

This report describes work undertaken at the British Geological Survey (BGS) that forms part of the international Weyburn Monitoring and Storage Project. This project aims to monitor and predict the behaviour of injected CO2 into the Midale reservoir at the Weyburn oil field in southern Saskatchewan, Canada, using methods that include time-lapse geophysics, modelling its subsurface distribution and migration, and simulating likely chemical interactions with the host rock. This report is a summary of Task 3.1 within the European part of the overall Weyburn project. It aims to provide a brief description of fluid chemical and mineralogical changes occurring in a series of experiments that have been conducted within the Hydrothermal Laboratory of the British Geological Survey. These experiments were undertaken to identify the geochemical changes that would result from the injection of CO2 into the Midale Formations. The experiments utilised samples of actual Midale rocks recovered from boreholes within the Weyburn field, synthetic formation water based upon measured well fluid compositions, and either CO2 or N2 as a pressurising medium.udThe experiments summarised in this report used actual samples of Midale core material and synthetic porewaters based upon actual measured well fluid compositions. The pressures and temperatures used within the experiments were representative of in-situ conditions (60°C, 150 bar [15 MPa]) and those anticipated near to injection wells (60°C, 250 bar [25 MPa]). As such, the study aims to replicate processes occurring in the deep subsurface at Weyburn as closely as possible, including those conditions that will exist even after oil production and CO2 injection have ceased.udUpon reaction with CO2, some dissolution of calcite within the Midale Marly material was identified, though it was relatively minor. By and large, the samples remained relatively unchanged, and significant disruption of the host formation appears not to take place. The Midale Evaporite showed slightly more reaction with CO2 compared to the Midale Marly, though it was still relatively minor. Some dissolution of dolomite and anhydrite was identified, together with minor aluminosilicate mineral dissolution. There was also a small amount of gypsum precipitation. Significant dissolution of the caprock formation appears not to take place. The Midale Vuggy material showed the greatest potential for reaction with CO2. Dissolution was mainly of calcite and anhydrite, but there was also a little aluminosilicate mineral dissolution. Precipitation of gypsum was widespread, with crystals at least 2.5 mm long being observed. Samples of borehole cement underwent carbonation of their outer portions upon exposure to CO2, and this was associated with an apparent reduction in porosity. The monoliths remained intact and appear to have maintained much of their original strength.