An analytical study was conducted to investigate the factors controlling hydrocarbon-generation microfracturing in organic-rich shales. The study considered both kerogen-to-bitumen and bitumen-to-oil conversions. Governing equations that describe a process of three main stages:
microfracturing; were derived.
Generally, Rock properties, density contrast and compressibility of the generated products are factors controlling pressure increase due to volume expansion. This study revealed that the level of organic richness, available void spaces and the existence of water are essentially key controlling factors. The role of organic richness and available void spaces become significant in the case of either lean or high-porosity shales during kerogen-to-bitumen conversion. For the role of water in volume expansion and pressure increase during bitumen-to-oil conversion, the solubility of water in bitumen gets into the governing equation. Pressure increase in the existence of water can reach as much as three times the pressure increase in the case of no water solubilized in bitumen.
Comparison of kerogen aspect ratios revealed that high aspect ratios (e.g. 4:1), which indicate thin flakes of kerogen, favor horizontal microfracturing while more circular shape-like kerogens (e.g. 1:1) favor vertical microfracturing. This suggests that the geometric shape of the kerogen controls hydrocarbon-generation microfracturing once the rock tensile strength is exceeded.
The effect of strength anisotropy and poroelastic behavior in microfracturing is manifested by the change of rock tensile strength with angle (with respect to bedding). The anisotropic poroelastic behavior depends on the anisotropy of the elastic moduli (Young modulus and Poisson's ratio). For kerogen aspect ratio, the difference between the principle vertical stress and the minimum horizontal stress should be four times greater than the tensile strength anisotropy in order to generate vertical microfractures. Otherwise, horizontal microfracturing is favored.
These observations lead to a logical sequence for hydrocarbon-generation microfracturing:
thermal maturation of organic-rich shale,
conversion of kerogen to bitumen,
reduction of kerogen volume and expansion of the generated bitumen volume,
increase in pressure,
solubilization of water in bitumen,
conversion of bitumen to oil,
significant expansion of the generated oil volume,
significant increase in pressure,
exceeding rock tensile strength and