This work presents the carbon dioxide (CO2) storage capacity of the Wolfcamp formation in West Texas. The potential of CO2 for enhanced oil recovery from unconventional liquid reservoirs have been under investigation in recent years. Quantifying the rock ability to store CO2 is necessary for the economic assessment of CO2 EOR, and for the evaluation of carbon sequestration capacity. We measured the porosity, the rock compressibility and the CO2 sorption in sidewall core plugs taken from two wells drilled in the Wolfcamp shale. Computed tomography scanning technology was used to image the samples and investigate the relation between rock density and porosity, and rock density and CO2 sorption.
A gas expansion pycnometer was used for the experiments. To accommodate the challenges associated with extremely tight rocks the equipment was provided with accurate temperature control, and a high resolution gauge with the capability of recording pressure as a function of time. Porosity and compressibility were measured simultaneously using helium. We used different treatments for the rock compressibility and explored its behavior at low pressure. A Langmuir isotherm was used to account for sorption of CO2 on the organic matter of the shale. The volume available for free gas was corrected by incorporating the rock compressibility and the volume of the layer of sorbed CO2 in the analysis of the sorption data.
This study adds to the understanding of the storage capacity of the Wolfcamp shale. We found the porosity to be between 5.94 and 10.30%, the Langmuir volume between 38.77 and 154.51 scf/ton, and the Langmuir pressure from 512.59 to 1384.52 psig. Using CT number as a proxy for total organic content, we observed a linear relation between CT number and sorption capacity. Porosity also showed a linear relation with CT number. Free CO2 storage is the main storage mechanism, at pressures above 2500 psig (for all samples) accounting for more than 80 %. Sorption is a relevant storage mechanism at low reservoir pressure. Neglecting the rock compressibility or the volume of the sorbed layer can lead to wrong estimates of sorption capacity.
The techniques developed in this work provide for an economic and accurate mean of measuring porosity, compressibility, and sorption in tight unconventional core plugs. The use of a non-destructive approach is necessary to characterize core plugs that must remain intact for further testing such as the evaluation of enhanced recovery techniques in unconventional reservoirs. This research provides relevant data for the economic assessment of CO2 EOR in the Wolfcamp and the potential for carbon sequestration. The linear relations we observed for both, porosity and sorption capacity, with CT numbers, are of extreme interest as they can be exploited for faster petrophysical evaluation of unconventional tight rocks.