Strontium isotope (87SR/86SR) chemistry in produced oilfield waters. The IEA Weyburn CO2 monitoring and storage project

EnCana's CO2 injection EOR project at Weyburn Saskatchewan (Canada) is the focal point of a multi-faceted research program, sponsored by the IEA GHG R&D and numerous international industrial and government partners. More than yearly strontium isotope, trace element and dissolved gas surveys were conducted by INGV in conjunction with the thrice yearly borehole fluid sampling trips performed by the Canadian partners. This paper focuses on the Sr isotope monitoring. Approximately 25 samples were collected over three years for 87Sr/86Sr analyses. At Weyburn, a water-alternating-gas (WAG) EOR technique is used to inject water and CO2 into the Mississippian Midale reservoir. 87Sr/86Sr ratios for produced fluids fall between 0.7077 and 0.7082, consistent with published values for Mississippian fluids and carbonate minerals. A small 87Sr/86Sr component of this produced fluid is derived from waters of the Cretaceous Mannville aquifer, which has been used for waterflooding EOR since 1959. The progressively more positive Sr isotope trend from 2001 to 2003 may be due to: 1) a smaller Mannville aquifer component in the water flooding process; and/or 2) the dissolution of Mississippian host rocks during the ongoing CO2 injection. Evidence that 87Sr/86Sr values are approaching those of Mississippian host-rock values may point towards zones of carbonate dissolution as a result of continuing CO2 injection. This hypothesis is strengthened by i) δ13C data; ii) preliminary “gross composition„ of dissolved gases (H2S, CO2, CH4, He, H2) and iii) by trace elements data.