Effect of a Pore Throat Microstructure on Miscible CO₂ Soaking Alternating Gas Flooding of Tight Sandstone Reservoirs

Abstract

Miscible CO2-SAG flooding is an improved version of CO2 flooding, which compensates for the insufficient interaction of CO2 and crude oil in the reservoir by adding a CO2 soaking process after the CO2 breakthrough (BT). The transmission of CO2 in the reservoir during the soaking process is controlled by the pore-throat structure of the formation, which in turn affects the displacement efficiency of the subsequent secondary CO2 flooding. In this work, CO2-SAG flooding experiments at reservoir conditions (up to 70℃, 18 MPa) have been carried out on four samples with very similar permeabilities, but significantly different pore size distributions and pore-throat structures. The results have been compared with the results of CO2 flooding on the same samples. It was found that the oil recovery factors (RFs) when using CO2-SAG flooding are higher than when using CO2 flooding by 8-14%. In addition, we find greater improvements in RF for rocks with greater heterogeneity of their porethroat microstructure compared with CO2 flooding. The CO2 soaking process compensates effectively for the insufficient interaction between CO2 and crude oil due to premature CO2 BT in heterogeneous cores. Moreover, rocks with a more homogeneous pore-throat microstructure exhibit a higher pressure decay rate in the CO2 soaking process. The initial rapid pressure decay stage lasts 80-135 minutes (in our experimental cores), accounting for over 80% of the total decay pressure. Rocks with the larger and more homogeneous pore-throat microstructure exhibit smaller permeability decreases due to asphaltene precipitation after CO2-SAG flooding, possibly because the permeability of rocks with more heterogeneous and smaller pore-throat microstructure is more susceptible to damage from asphaltene precipitation. However, the overall permeability decline is 0.6-3.6% higher than that of normal CO2 flooding due to the increased time for asphaltene precipitation. Nevertheless, the corresponding permeability average decline per 1% oil RF is 0.11-0.34%, which is lower than that for CO2 flooding, making the process worthwhile. We have shown that CO2-SAG flooding has the potential to improve oil RFs with relatively little damage to cores, especially for cores with small and heterogeneous porethroat microstructures, but for which severe wettability changes due to the CO2 soaking process can become significant.

Prediction of Minimum Miscibility Pressure for CO2 Flooding Based on Microscopic Pore-Throat Structure

“…This paper focuses on the effect of the throat's size and distribution on CO 2 -oil miscible flooding. The mainstream throat contributes 95% to the permeability of core samples, and its radius can be calculated by the following equation (Meng and Liu, 2019;Wang et al, 2020b;Zhong et al, 2021).…”

A Minireview of the Influence of CO2 Injection on the Pore Structure of Reservoir Rocks: Advances and Outlook

“…Lee and Lee 94 demonstrated that asphaltene precipitation during CO 2 huff and puff in shale reservoirs would reduce porosity and permeability, and shifts wettability toward the oilwet state. Wang et al 103 proposed that after CO 2 soakingalternating-gas (SAG) flooding, asphaltene precipitation leads to permeability reduction, and the damage degree is positively correlated with the heterogeneity of pore microstructure. Compared with CO 2 flooding, it has less damage to the core, especially for the core with poor pore-throat structure.…”

Laboratory Study on the Oil Displacement Process in Low-Permeability Cores with Different Injection Fluids

“…The miscibility process of CO 2 and crude oil is a process in which CO 2 continuously extracts the components of crude oil from light to heavy, and CO 2 continuously dissolves in crude oil . At reservoir temperature, the minimum miscible pressure (MMP) of CO 2 and crude oil is key to determine the miscible flooding effect. − In addition, in tight reservoirs, matrix is the main reservoir space of crude oil. However, due to low permeability and small pore diameter of the matrix, an effective displacement system cannot be established in the reservoirs.…”