Experimental Investigation of Asphaltene Deposition and Its Impact on Oil Recovery in Eagle Ford Shale during Miscible and Immiscible CO<sub>2</sub> Huff-n-Puff Gas Injection
Mukhtar Elturki, Abdulmohsin Imqam
Abstract
One of the challenges in extracting oil from unconventional resources using hydraulic fracturing and horizontal drilling techniques is the low primary recovery rate, which is caused by the ultra-small permeability of these resources. Consequently, it is essential to investigate gas injection methods to produce the trapped oil in shale formations. However, the injection process can cause asphaltene depositions inside the reservoir, leading to plugging of pores and oil recovery (OR) reduction. There has been limited research on using gas injection techniques to improve oil production in tight/unconventional resources, although carbon dioxide (CO 2 ) and gas-enhanced oil recovery methods have been used in conventional resources. In order to determine whether or not the cyclic (huff-n-puff) CO 2 process improves OR and aggravates asphaltene precipitation, a rigorous experimental investigation was undertaken utilizing filter membranes and Eagle Ford shale cores. After the minimum miscibility pressure was calculated for CO 2, various injection pressures were selected to perform CO 2 huff-n-puff experiments. Investigations were carried out at 70 °C on injection pressure, cycle number, production time, and huff-and-puff mode injection. The results demonstrated that when the pore size structure of the membranes used was smaller and gas injection cycles increased, a higher asphaltene weight percent (wt %) was determined during the static experiments (i.e., employing filter paper membranes). Miscibility improved OR in dynamic testing (i.e., using shale cores), but a more oil-wet system was detected in wettability measurements taken following CO 2 huff-and-puff tests. The plugging impact of asphaltene particles on the pore structure was studied using optical microscopy and scanning electron microscopy imaging. Following the huff-and-puff tests, a mercury porosimeter revealed how severely the pores were plugged, and after the CO 2 tests, the pore size distribution reduced as a consequence of asphaltene deposition. This study examines the significance of CO 2 injection in OR under miscible/immiscible conditions to identify the critical parameters that could impact the effectiveness of CO 2 huff-n-puff operation in unconventional formations.