Multi-heating-rate pyrolysis study for modeling hydrocarbon production from in-situ conversion of Chang 7 shale

The in situ conversion process (ICP) is one of the most developed technologies to recover oil from fined grained, low thermally matured shale. However, mathematical models capable of predicting the production profile of oil and gas under the constraints of heating rate and temperature remain underdeveloped. Therefore, pyrolysis experiments with different heating rates (2 °C /d, 5 °C /d, 10 °C /d and 20 °C /d) were performed on shales from the seventh member of the Triassic Yanchang Formation (Chang 7 shale), a representative target for ICP implementation. The effects of heating rate and temperature on the production yields of oil, hydrocarbon gas, and hydrogen sulfide (H₂S) were investigated, and mathematical prediction models to predict their production profiles were established. The results show that the influence of heating rate on oil production yield at fixed pyrolysis temperature exhibits stage-dependent characteristics across different temperature regimes: below 380 °C, the cumulative production yield decreases with increasing heating rate. However, above 380 °C, the cumulative yield increases with higher heating rates. In contrast to oil production, hydrocarbon gas production shows consistent inverse correlation with heating rate across all temperatures, and H₂S production shows positive correlation with heating rates. The mathematical models predict that when reaching the same thermal maturation of EasyR_o = 2.3% (before complete oil cracking into gas), the maximum oil production yields reach 40.67 mg/g rock at 1 °C/d and 37.74 mg/g rock at 0.5 °C/d. Corresponding peak hydrocarbon gas production yields are 10.61 ml/g rock and 9.43 ml/g rock, respectively. Due to the formation mechanisms of H₂S, the predictive model is only applicable within heating rates of 5-20 °C/d. At rates below 2 °C/d, its production may become negligible. When designing optimized temperature program for ICP, the competing factors of oil production, hydrocarbon gas production and H₂S yield should be carefully balanced.