Long-Term Conductivity Evolution of Propped Fractures in Interbedded Highly Plastic Shale Reservoirs: Effects of Lithology and Proppant Parameters

Interbedded shale reservoirs commonly contain multiple lithologies, such as shale and limestone, resulting in heterogeneous long-term fracture conductivity under identical proppant conditions. This lithological heterogeneity introduces uncertainty into proppant design and may lead to uneven stimulation effectiveness. This study investigates the effects of lithology, proppant size, and proppant areal concentration on the evolution of propped fracture conductivity and develops lithology-dependent empirical correlations to quantitatively describe long-term conductivity evolution within the tested parameter ranges. Conductivity experiments were performed using shale and limestone rock plates under simulated reservoir conditions of 80 °C and 40 MPa closure stress, with a long-term loading duration of 50 h. The results indicate that propped fracture conductivity exhibits strong stress sensitivity and time-dependent degradation. Continuous compaction of the proppant pack and reduction in effective flow channels are considered possible mechanisms contributing to conductivity loss during long-term loading. Under identical proppant parameters, limestone fractures generally exhibit higher long-term conductivity, while the difference in conductivity retention depends on proppant conditions rather than shale fractures, with the lithological difference becoming more pronounced under low-proppant-concentration and small-proppant-size conditions. Within the directly comparable conditions of 5 and 7.5 kg/m2, increasing proppant size and areal concentration generally reduced the conductivity difference observed between shale and limestone fractures. For shale fractures, the experiments covering 5, 7.5, and 10 kg/m2 further showed that the effect of areal concentration depended on proppant size. Based on the experimental data, lithology-dependent empirical correlations were established for shale and limestone fractures and evaluated using leave-one-condition-out cross-validation. The correlations provide a quantitative description of conductivity evolution within the tested parameter ranges. This study identifies lithology-dependent conductivity responses within the investigated proppant conditions and provides experimental evidence for adapting proppant design to heterogeneous interbedded shale reservoirs.