Characteristics of Reservoir Rock and Field Tests of EOR Technology in a Tight Oil Reservoir

Although several pilot tests have been conducted in the tight oil reservoirs of the Lucaogou Formation to enhance crude oil recovery, a detailed analysis of the oilfield test results has not yet been carried out, and further in-depth research is required on the geological reservoir reasons within the outcomes. This article, based on geological research on reservoir lithology, properties, and sensitivity, combines the results of oil reservoir physical indoor experiments with a thorough analysis of the field production results. It explores the fundamental reasons behind water Huff & Puff and Hydraulic Refracturing failure. Simultaneously, this study explicitly identifies the potential role of CO2 Huff & Puff in enhancing oil recovery. First, a thorough assessment of the reservoir, encompassing factors such as mineral composition, lithology, physical properties, sensitivity, and pore structure. The results of reservoir sensitivity experiments reveal that terrigenous clastic rocks and carbonate rocks both demonstrate weak water sensitivity, weak acid sensitivity, and moderately weak to moderate speed sensitivity. The primary geological factors influencing reservoir quality include lithology, physical properties, and pore structure parameters. Second, this study provides a detailed analysis of the injection and production parameters of water Huff & Puff and Hydraulic Refracturing in well W02. In conjunction with the results of reservoir sensitivity experiments, it is evident that the “water blocking” effect is the primary cause of the decline in production from 8t/d to 0.6t/d–2t/d. Therefore, there is a plan to shift the recovery medium from a liquid system to a gas system, with the early-stage fracturing fluid backflow rate identified as a key indicator for selecting wells in experiments aimed at enhancing recovery rates. Finally, PVT experiments were conducted with oil and gas in two stages, utilizing CO2 and CH4 as the primary gas mediums. Under reservoir conditions, both the gas-oil dissolution ratio and the oil expansion factor of CO2 surpass those of CH4. Additionally, when dissolved in crude oil, CO2 exhibits a more significant reduction in viscosity. Through CO2 Huff & Puff test in well W01, it was observed that a high return rate of fracturing fluid could lead to a “water block” effect hindering oil reservoir production efficiency. CO2 Huff & Puff effectively mitigates this issue by mobilizing deep-seated crude oil, resulting in an increase in oil production. Specifically, the single-well recovery rate rises from 8.53% to 13.67%.