Geochemical Models of Carbon Sequestration Based from the Natural Analog of the Moxa Arch, Wyoming, Usa
Capture and storage of carbon dioxide in deep underground geologic formations (geologic carbon sequestration) is currently the most advanced technology for reducing or mitigating anthropogenic carbon dioxide emissions. There are a number of scientific and engineering challenges associated with injection and storage of large amounts of CO2 in geologic formations. Understanding the chemical reactions between reservoir rocks, aqueous fluids, and supercritical carbon dioxide ± other gasses is one of the major challenges. By studying analogous situations we can identify the important chemical reactions that will have to be included in engineering models. While various modern processes including CO2 injection for EOR have been proposed as analogs, they lack the long duration that will occur in sequestration. However, there are natural analogs in oil fields and gas deposits where carbon dioxide has been stored over geologic time scales.
The Moxa Arch is a large-scale anticlinal structural feature located in the southern end of the greater Green River Basin, Wyoming with estimated gas reserves of 170 TCF in the Paleozoic sections. Current gas compositions range from 70-95% CO2 with methane, hydrogen sulfide and helium comprising the balance of the gas. Water content is near or at residual saturation. Naturally-occurring carbon dioxide, helium and methane are currently produced from the 1000+ foot thick Mississippian-age Madison Limestone at depths up to 18,000 feet. The carbon dioxide is under supercritical conditions, a direct analog to proposed carbon sequestration. The presence of several % H2S in the gas phase offers an additional analog for sequestration of stack gases without removal of sulfur content.
Core and log data from inside and outside the gas accumulation provides real-world constraints on initial mineralogy, porosity and permeability and later alterations caused by gas emplacement. Geochemical reaction path and reactive transport modeling based on these constraints are presented. The initial models show that in-situ pH can reach as low as 3.5 at high CO2 content. The natural pH buffering by dissolution of the calcite and dolomite produces increases in water salinity with concurrent changes in porosity and permeability. Increases in sulfur content are trapped as anhydrite or elemental sulfur depending on pH. Precipitation of carbonates appears limited by potential cation supply from aluminosilicate mineral dissolution.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009