Effects of gas generation on stress states during burial and implications for natural fracture development

• We bring burial process into the coupling study of gas generation and stress state.
• We make a procedure to model the mechanical effects of gas generation during burial.
• Gas generation can change differential stress depending on tectonic stress regime.
• Gas generation has more impacts on natural fracture development than undercompaction.

The complexity of fracture networks in shale gas reservoirs may result from the superimposition of several fracture sets forming in temporally modified stress states during burial. In addition to tectonic evolution, this modification can also be caused by the overpressure generated by gas generation. Compared with undercompaction, gas generation shows a different overpressuring process and may cause different effects on stress states. This paper addresses these effects during burial in terms of pore pressure stress coupling. A series of finite element simulations for this investigation are designed with different tectonic stress regimes using a simplified model with a homogeneous poroelastic medium. The results show that during burial, gas generation not only significantly decreases effective stresses but also changes differential stress by decreasing it in the normal faulting regime and increasing it in the thrust faulting regime compared with undercompaction. Furthermore, gas generation during burial may locally transform the given tectonic stress regime from normal faulting to strike slip or even thrust faulting regimes. The investigation implies that gas generation rather than undercompaction in burial history can significantly diversify the patterns of natural fractures by affecting the differential stress and even the local stress regime in addition to influencing their spatial distribution due to its spatial heterogeneity and its different effects on stress states in different stress regimes. As a result, the contribution of gas generation to overpressures should be differentiated from that of undercompaction before analysing the effects of overpressures on natural fracture development.