Impact of clay minerals on oil displacement efficiency in tight oil reservoirs

Although water and gas drives are still the most commonly used methods for most reservoirs, the high clay content adversely affects production efficiency. This study investigates three development methods (water, CO
2
, and N
2
) in high-clay reservoirs. It aims to identify factors affecting productivity and explain the underlying mechanisms. The mineral composition of natural cores was analyzed using X-ray fluorescence (XRF) and X-ray diffraction (XRD) techniques. We studied water, CO
2
, and N
2
drives in tight oil reservoirs through both core experiments and molecular simulations. The simulations focused on quartz and clay minerals (kaolinite and montmorillonite). Due to the limitation of reservoir fracturing pressure, the recovery rates of the natural cores were found to be as follows: water drive > CO₂ drive > nitrogen drive. Molecular simulations showed that all three replacement media performed best on quartz surfaces, with water drives being most effective on kaolinite surfaces and CO₂ drives being most effective on montmorillonite surfaces. The negatively charged surface of kaolinite, affected by the adsorption capacity of minerals, leads to weak CO₂ adsorption, resulting in poor recovery. Increasing the pressure to 35 MPa increased the CO₂ drive efficiency to 96.3%, confirming that the pressure increase contributes to the formation of a mixed phase. In kaolinite reservoirs, nitrogen drive outperformed CO₂ thanks to its non-polar properties and gas expansion effect, resulting in better penetration into micropores and less interference from clay minerals. Finally, the water drive performed poorly on both clay surfaces due to hydration and swelling of the clay caused by low salinity. The higher ion exchange capacity of montmorillonite, meanwhile, exacerbated pore plugging.