HIGH CO2 GAS FIELDS: NATURAL ANALOGS TO CO2 STORAGE IN DEPLETED GAS FIELDS NATIONWIDE AND IN THE APPALACHIAN FORELAND

A specific set of chemical interactions arise from injecting CO2 into depleted hydrocarbon reservoirs, primarily related to the mixing of CO2 with the existing immiscible brine and hydrocarbon fluids. To increase both the understanding of depleted hydrocarbon reservoirs as sequestration targets and capability to predict the short- and long-term fate of the injected CO2, we have begun to examine hydrocarbon fields that contain significant volumes of naturally occurring CO2. These fields (sometimes called low-BTU gas fields) contain from 4-80% natural CO2 mixed with methane. Other gases might include H2S, SO2, N2, He, and longer-chain hydrocarbons (e.g. propane) in concentrations that range from 0-15%.

Hydrocarbon fields with high CO2 concentrations are both more wide-spread and abundant than commonly recognized. They serve as excellent natural analogs for CO2 storage and potential storage sites as they have already held significant CO2 volumes. Several important fields have significant variations of CO2 content within the field, differing by depth, pressure, and geological formation. Other fields occur within the same formation but collected gas at differing pressure-temperature settings. A number of low BTU gas fields occur in the Appalachian foreland, which may serve to clarify local storage options, especially in the Ohio River Valley. By studying the modern and ancient chemistry of the fluid-rock-gas interactions in detail, an enhanced understanding of how injected anthropogenic CO2 might change the reservoir chemistry through time as concentrations change may be gained.