Matrix permeability could be a key factor controlling shale gas production from matrix to micro fractures and further to hydraulic fractures in Chinese shales due to the low porosity and permeability, affected by its unique geochemistry and geology settings including the Total Organic Carbon (TOC) content, mineral compositions, pore structure, and deposition environment. This paper aims to study the controlling factors of the matrix permeability in the continental shale formation, Ordos, China, through modified laboratory measurement mothod.
In this work, nine shale samples were collected from three wells in Chang 7 member, Yanchang continental formation, Ordos Basin, China, crushed at in-situ water saturation and sieved to certain size (20/40 mesh). Matrix permeability of these samples was measured with modified Pressure-decay method and compard with the results with Pulse-decay method. The reasons for the discrepancy of these results with different methods were analysed. Moreover, the effects of geochemistry and geology factors on matrix permeability were investigated by grouping these crushed samples according to the variation of TOC, mineral compositions and deposition depth. The relationships between shale matrix permeability and TOC as well as depth were established respectively. Furthermore, the effects of other factors such as mineralogical compositions and pore structure parameters were studied through the Scanning Electron Microscope (SEM) and X-ray diffraction (XRD) analysis.
The results show that the shale matrix permeability measured by the Pulse-decay method is generally up to two orders of magnitude higher than that by the Pressure-decay method due to the presence of the natural or artificial micro fractures. In addition, the geochemistry and geology parameters including the TOC, mineral compositions, pore structure and deposition environment have significant effects on the shale matrix permeability. Matrix permeability in Yanchang shale formation is strongly related to TOC and the mineral compositionand even at the similar depth, porosity and matrix permeability are different due to the variation of TOC as a result of geological heterogeanity. TOC in Yanchang formation varies considerably in the range of 1.8 wt % to over 11 wt %, resulting in significant changes in matrix permeability ranging from 0.02 nD to 10 nD, resulting from the influence of the organic matter and clay minerals on total pore volume based on the result of SEM and XRD analysis. The accurate measurement of matrix permeability is important for computer simulation modeling of long term shale gas production.