New insights into the volume and pressure changes during the thermal cracking of oil to gas in reservoirs: Implications for the in-situ accumulation of gas cracked from oils

Abstract

Previous pressure-volume calculations during oil cracking to gas, based on the conventional model that presupposes oil cracking to be completed by approximately 150C, underestimate the potential for gas accumulation in petroleum reservoirs. In this article, a compositional kinetic model of gas generation from oil cracking is suggested based on pyrolysis data using sealed gold tubes, and the pressure-volume changes are recalculated based on the new kinetic model under various geological conditions. The kinetic modeling of oil cracking confirms that crude oil begins cracking at about 160C for a heating rate of 2C/m.y., and that the oil-cracking process has two distinct stages with significant differences in gas composition. The first stage is characterized by dominant C2–5 wet gases, whereas the second is characterized by the recracking of C2–5 wet gases to methane and pyrobitumen, leading to a progressive increasing dryness of the gas. The pressure-volume-temperature simulations of oil cracking to gas show that initial oil saturation, temperature-pressure gradients, and openness of reservoirs are important geological factors that control gas accumulation in original petroleum reservoirs. For a reservoir that is geologically open and saturated with 100% oil, gas spills out of the trap at 196C. The gas loss at 240C is almost 50% of the total gas, far lower than the 75% based on the conventional model of oil destruction. With lower oil saturation, the gas loss decreases because the gas-water contact can shift downward, and the gas loss occurs mainly by solution. For effectively isolated reservoirs, oil cracking readily exceeds lithostatic pressure, leading to reservoir fracturing, which becomes more obvious when oil saturation decreases. The calculated fracturing temperatures for 100 and 50 vol.% oil saturations correspond to oil destructions of 95% and 86.4%, greatly exceeding the value of 1% as suggested by previous studies. A conceptual model of gas accumulation and loss in isolated and open geological conditions for a reservoir with 50% oil saturation is suggested. On the basis of this model, the Triassic carbonate gas pool in northeastern Sichuan Basin was discussed as a typical example for in-situ accumulation of gas cracked from reservoired oils. The present model infers that the reservoired oils were completely cracked into gas at 87.6 Ma, and that 75–85% of the gas has been preserved in the original reservoir rocks to form the in-situ gas pools with a huge amount of gas resources. We believe that gas accumulation from oil cracking in original petroleum reservoirs is much more prospective than previously thought, and that gas cracked from oil has great potential in other areas of the Sichuan Basin and the eastern Tarim Basin.