Unveil the Significance of the Thin Laminated Carbonate Layer in Unconventional Reservoirs

Abstract Many unconventional resources are featured with thin laminated layers in the reservoir. Though this feature is often considered the cause of reservoir heterogeneity, the actual influence from the laminated layers can be more significant. In a comprehensive study from Sichuan shale gas, we have found that the thin laminated shell limestone layer played a very important role for the well performance. Sichuan shale gas is well known for its geological and geomechanical complexity. One of the formation sub-units, Wufeng, did not perform well enough according to its reservoir properties. Various evidence, including formation micro-image, core sample XRD and geomechanical analysis, shows that one laminated thin carbonate layer on the upper part of the Wufeng unit, often referred to as Guanyinqiao carbonate, could be the main reason for the under-performance of Wufeng unit. Due to the limited thickness, the lithology and geomechanical property are not fully well-captured or comprehended with traditional loggings and workflow. However, our new geomechanical results indicate that a much stronger stress barrier condition is in place for Wufeng, which provides a more realistic controlling mechanism of the hydraulic fractures. The following fracture modelling and reservoir numerical simulation indicates the impact from this lamina to the production is significant. The direct conclusion of this study is that the existence of the thin lamina of shell limestone can play a vital role in Sichuan shale gas reservoirs. And as a recommended practice in the same region, the geomechanical measurement need to be calibrated with high resolution formation image log, based on which both horizontal lateral landing and multistage completion strategy can be optimized. This observation may also help the understanding of other unconventional reservoirs where similar laminations of carbonate are encountered, so that the proper geo-engineering approach may apply accordingly to justify the preferred exploration and appraisal practices.