Measured Sorption Isotherms for Bakken Petroleum System Shales Using Carbon Dioxide and Produced Gas Hydrocarbons at 110 °C and Pressures up to 34.5 MPa

Abstract

Sorption isotherms were measured for organic-rich Lower Bakken Shale (LBS) samples from low-, mid-, and high-thermal maturity zones of the Bakken Petroleum System (BPS) using hydrocarbon gases and CO2 at reservoir temperature and pressures. Molar sorption was strongest for CO2 and propane (averaging 0.28 mmol/g gross rock for both gases at 34.5 MPa), followed by ethane (0.24 mmol/g), produced gas (0.20 mmol/g), and methane (0.17 mmol/g). Propane and ethane had the strongest sorption at low pressures (lowest Langmuir pressure, P L), followed by produced gas, CO2, and methane. All of the hydrocarbon gases showed good conformance with the Langmuir isotherm, but CO2 showed significantly lower sorption than the Langmuir isotherm at pressures above 15–20 MPa. Contrary to the expectations that the organic content would control sorption, all five gases showed the highest sorption on a total organic carbon (TOC) basis (mg or mmol gas/g TOC) for the LBS sample with the lowest TOC (4.5 wt %) and the lowest sorption on a TOC basis for the sample with the highest TOC (20.4 wt %). Sorption from a laboratory gas mixture representing field produced gas highly favored ethane and (especially) propane over methane and an average of 48.1 mol % of the sorbed gas was propane, 39.1 mol % was ethane, and only 12.8 mol % was methane, in contrast to the average molar composition of the produced gas mixture of 69.6 mol % (methane), 20.7 mol % (ethane), and only 9.7 mol % (propane). A comparison of previous reports of the five gases’ potential effectiveness for enhanced oil recovery based on minimum miscibility pressure, ability to swell and dissolve crude oil, and ability to recover oil hydrocarbons from rock samples against the isotherm results showed a trend that propane was usually the most effective gas, followed by ethane, CO2, and produced gas, with methane being the least effective gas at mobilizing crude oil as well as having the lowest molar sorption.