13th International Conference on Fracture June 16–21, 2013, Beijing, China -1Subcritical Crack Propagation and Coalescence Induced by the Oil-Gas Transformation Zhi-Qiang Fan1, Zhi-He Jin1* , Scott. E. Johnson2 1Department of Mechanical Engineering, University of Maine, Orono, ME 04469, USA 2School of Earth and Climate Sciences, University of Maine, Orono, ME 04469, USA * Corresponding author: zhihe.jin@maine.edu ________________________________________________________________________________ Abstract The present work develops a multi-physics model to investigate subcritical propagation of initially oil-filled, sub-horizontal collinear microcracks driven by the excess pressure induced during the conversion of oil to gas in a petroleum source rock under continuous burial. The crack propagation distance, propagation duration, crack coalescence and overpressure in 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 numerical results for a source rock with typical properties show that when the crack spacing is greater than b/a0 = 3 (where a0 is the half crack length and b the half distance between crack centers) the cracks do not coalesce and the duration of gas-driven crack propagation is governed by the transformation kinetics because the subcritical crack propagation rate is much faster than the oil-gas conversion rate. The collinear cracks coalesce for smaller crack spacing and the crack propagation duration may be reduced significantly due to crack interactions. The multi-physics model developed in this work together with our previous model for crack propagation during conversion of solid kerogen to oil indicates that microcracks resulting from buildup of excess fluid pressure during hydrocarbon generation may serve as an effective pathway for primary petroleum migration. Keywords: Collinear cracks, Migration, Oil-gas conversion, Overpressure, Subcritical growth ________________________________________________________________________________ 1. Introduction Overpressures widely observed in the deep part of sedimentary basins coincident with primary zones of gas generation are commonly attributed to thermal cracking of kerogen to oil/gas and oil to gas. Microfractures are a common feature of petroleum source rocks in these basins interpreted to serve as migration conduits for oil and gas [1]. Understanding the dominant mechanisms responsible for the microfracture initiation and development in petroleum source rocks at great depth is crucial for hydrocarbon exploration and safe drilling. Fine grained source rocks like shales rich in kerogen undergo progressive burial, which leads to an increase in bulk density and loss of porosity. With increasing depth of burial, there is also a marked decrease in permeability. As temperatures and pressures increases, kerogen breaks down to release oil when it becomes mature. Conversion of kerogen to oil results in significant volume increase due to the density difference between the precursors and the products. Meanwhile, part of the overburden load will be transferred to the newly generated oil. As a natural result, overpressure is generated [2, 3]. Clay-sized minerals within source rocks functioning as effective seals enable local overpressure build-up [1]. When the overpressure exceeds the mechanical strength of the source rocks, microfractures around kerogen particles are initiated, thus creating a migration pathway for oil [4, 5]. As burial proceeds, temperatures continue to increase. When the gas window is reached, oil retained in the microfractures will be subjected to thermal cracking to form gas, which consists predominantly of methane [2-3]. The volume increase associated with transformation of oil to gas is more appreciable. According to Barker [6], less than two percent of oil conversion to gas would
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