Imbibition inducing tensile fractures and its influence on in-situ stress analyses: A case study of shale gas drilling

• Zoback's stress polygon model displays limitations when applied on shale formations.
• A new mechanism was introduced for the occurrence of DIFs in the organic-rich section.
• Water imbibition leads to effective internal stresses and intensifies the occurrence of DIFs.
• The organic matter provides displacement space for tensile fractures under reservoir conditions.

For conventional oil and gas reservoirs, the orientation and magnitude of in-situ stress can be estimated by analyzing wellbore failures such as borehole breakouts and drilling induced fractures (DIF) using Zoback's stress polygon model. According to this model, DIFs are most likely induced by the high mud weight, wellbore cooling, high stress difference, or pressure surge. A large number of DIFs have been identified by imaging logging in shallow well sections of shale gas well QY1 in the Sichuan Basin of China. However, all possible explanations of standard DIFs do not work well, suggesting the limitations of this model when applied to shale formations. Other mechanisms influence the formation of DIFs in this shale gas well that are not covered in this model. In this paper, the orientation and magnitude of the in-situ stress in the study area are constrained following the general procedure outlined by Zoback's stress polygon model. The DIF distribution in the QY1 wellbore and its relationship with organic matter are then analyzed. A new mechanism was introduced to explain the DIFs in the organic-rich section. The results show that the maximum horizontal stress (SHmax) in the study area trends N95°E±9°. The calculated in-situ stress magnitudes are consistent with a strike/reverse faulting stress regime (SHmax > Sv ≈ Shmin). In addition, the DIFs are concentrated in the organic-rich interval. For organic-rich shale, strong water imbibition is common, which is related to super-dry gas shale due to the water displacement of kerogen-transformed hydrocarbons, high capillary pressure due to micro-nano meter pore diameters, and the water-sensitive clay mineralogy. This spontaneous water intake generates the effective internal stress around the wellbore, producing DIFs in the shale gas well. The introduction of this new mechanism allows the Zoback's stress polygon model to constrain the in-situ stress in the study area. Therefore, this mechanism should be considered during estimation of the in-situ stress in gas shale formation when using Zoback's stress polygon model.