Reservoir characteristics of the Bass Islands dolomite in Otsego County, Michigan: Results for a saline reservoir CO2 sequestration demonstration

Abstract

As part of a phase II plan to understand, test, and evaluate the CO2 sequestration potential for deep saline reservoirs in Michigan, a demonstration test well was completed in late 2006 in Otsego County, northern lower Michigan. The well was drilled to 3630 ft (1006 m) and open-hole logged. Selected conventional cores totaling 180 ft (55 m) were taken in the saline reservoir (Bass Islands Formation), the immediately overlying confining unit (Bois Blanc Formation), and the overlying seal (Amherstberg Formation). Additionally, 24 sidewall cores were taken in several uphole formations. The whole core was sampled every foot by drilling 2-in. (5-cm)-long and 1-in. (2.5-cm)-diameter test plugs for porosity and permeability (PP) analyses. Seventy-four horizontal plugs, 12 vertical plugs, 6 whole cores, and 17 sidewall core plugs were sent to Core Laboratories for routine PP analyses. Fifteen blue-dyed, epoxy-impregnated thin sections were made from selected PP plugs. The whole core was slabbed for examination and description of lithology, sedimentary structures, and facies characteristics. This Upper Silurian and Lower–Middle Devonian stratigraphic section was carefully examined for lithology and facies characteristics that relate to reservoir and seal properties pertinent to CO2 sequestration. The overlying primary seal (Amherstberg Formation) is a low-porosity, low-permeability limestone that is highly fossiliferous and densely cemented with calcite and chalcedony. This unit is the ultimate vertical barrier to the vertical migration of fluids. The immediately overlying confining unit (Bois Blanc Formation) is a very cherty limestone and dolostone with moderate porosity and low permeability. Some fluids may move into this unit, but very low permeability will severely restrict the vertical flow. Thin sections show abundant microporosity. The target saline reservoir interval (Bass Islands Formation) is a variably porous and permeable dolostone composed of several tidal flat cyclic packages. The Bass Islands Formation has a gross thickness of 70 ft (21 m) with a reservoir interval composed of more than 40 ft (12 m) of greater than 10% porosity and permeability zones exceeding 500 md. Average porosity over the entire Bass Islands is 12.5%. Average permeability is 22.4 md. The CO2 injection tests, using the Bass Islands section, were completed during February and March 2008. Analysis of the Bass Islands Formation in northern Michigan indicated excellent reservoir quality for injection and storage of CO2 and high-quality sealing units to prevent vertical migration. Monitoring well data conducted during and after the injection test validates preinjection reservoir simulation modeling performed at Battelle Pacific Northwest Labs using well data and rock observations from this study.