Performance Evaluation of CO2 Huff-n-Puff in Shale Oil Reservoirs: Multiwell Huff-n-Puff and Parameter Optimization

Abstract CO2 huff-n-puff (HnP) is an established enhanced oil recovery (EOR) method for shale oil reservoirs. However, its field-scale implementation in multiwell fractured systems remains challenging because fracture-controlled interwell connectivity can cause operational interference and inefficient CO2 allocation. A field-scale numerical model of the HH shale oil reservoir, incorporating complex natural and hydraulic fracture networks, was constructed and history-matched against field production data, followed by a structured sequential one-factor-at-a-time analysis and descriptive Spearman rank-correlation screening of the associations between operational parameters and performance indicators. Results indicate that, for the calibrated HH reservoir realization, the intragroup synchronized and intergroup asynchronous strategy achieved the highest cumulative incremental oil production among the investigated strategies (9.59 × 104 t), together with a CO2 utilization efficiency of 0.266 t oil/t CO2. This performance was associated with the cross-group pressure contrast observed under the asynchronous operation of the two well groups. Rank-correlation screening indicated that cycle number and cyclic CO2 injection mass showed the strongest associations with oil recovery and CO2 utilization efficiency, respectively, within the investigated cases. Within the investigated ranges and under the experimentally constrained diffusion setting of the calibrated HH reservoir model, 45 days and 450 t/d were identified as the preferred soaking time and injection rate, respectively. Extending the soaking time from 45 to 55 days yielded only marginal improvements, whereas increasing the injection rate from 450 to 600 t/d slightly reduced both incremental oil production and CO2 utilization efficiency. Furthermore, compared with the uniform injection scheme, the differentiated injection scheme maintained a comparable OOIP-based recovery factor (15.97% versus 15.84%) while reducing the cumulative gross CO2 injection mass from 36.0 × 104 to 33.1 × 104 t, corresponding to an 8.1% reduction. The EOR-oriented CO2 utilization efficiency consequently increased from 0.274 to 0.305 t oil/t CO2. Therefore, its principal benefit was the more efficient use of the injected CO2 rather than a substantial increase in ultimate oil recovery. This work provides a field-scale numerical workflow for designing and evaluating coordinated multiwell CO2 HnP operations in fractured shale oil reservoirs.