Gas-driven subcritical crack propagation during the conversion of oil to gas

In this paper, we investigate subcritical propagation of an initially oil-filled, sub-horizontal microcrack driven by the excess fluid pressure associated with the conversion of oil to gas in a petroleum source rock under continuous burial. The crack propagation distance and propagation duration (the time required for the crack to propagate during conversion of all oil to gas), as well as the excess pressure inside the crack, are determined using a finite difference scheme that couples linear elastic fracture mechanics, oil–gas transformation kinetics and an equation of state for the gas. The effects of the source-rock temperature at the initial depth of the microcrack and fracture properties of the source rock are also considered. Our numerical results show that higher burial rates significantly reduce the crack propagation duration. However, the influence of the geothermal gradient on the propagation duration and distance is only marginal. Similar to the results for the oil-driven crack propagation during kerogen–oil conversion, the duration of gas-driven crack propagation is also governed by transformation kinetics because the subcritical crack propagation rate is much faster than the oil–gas conversion rate.