Improved Formation Evaluation of Organic-Rich Shale Formations by Integrating Digital Rock Analysis with Core Data and Well Logs
Authors: He Yong , Jiang Liwei , Chi Lu , Wang Xin , Chen Qiang , Roth Sven , Dong Hu 
Published: October 2018

To reliably evaluate the petrophysical, geochemical, and geomechanical properties of an organic-rich shale formation in China, we integrated digital rock analysis (DRA) with conventional core data and well log interpretation. The objectives of this paper included (a) to create a complete and accurate formation evaluation model for Wufeng-Longmaxi shale gas formation by combining pore-scale (digital rock), core-scale, and log-scale data; (b) to accurately characterize the micro-scale pore space, rock matrix, and organic matters in this formation, and create 3D pore network models from core samples; and (c) to identify the geological and engineering sweet-spot along vertical wellbore.

For well log interpretation, we obtained Gamma Ray (GR), spectral GR, neutron, density, resistivity, sonic logs, and elemental spectroscopy logs in the wells. For core measurements, we performed static and dynamic geomechanical experiments on core samples. For DRA, we obtained multi-scale images of the organic-rich shale samples, using three-dimensional (3D) micro-Computed Tomography (CT), 3D Focused-Ion-Beam Scanning Electron Microscope (FIB-SEM), and high-resolution Back-scattered Electron (BSE) imaging. Mineralogical and elemental analysis was also obtained by QEMSCAN. We then quantified various petrophysical properties from the digital rocks, including organic/inorganic porosity, Total Organic Carbon (TOC), elemental concentration and mineralogy. Most of the obtained properties were cross-validated with log data. Furthermore, we extracted pore network models from the digital rocks to quantify pore connectivity, pore throat size distribution, organic pore radius distribution, … etc, to provide more micro-scale information within the rock. Next, we determined the origin of quartz and the cause of high natural gamma-ray sections in the formation, based on point-by-point elemental analysis on SEM images and geochemical analysis. At last, we investigated various geomechanical properties using digital rock, core and log data. We compared geomechanical properties from core experiments and logs, then performed sensitivity study by DRA.

Two vertical wells in Wufeng-Longmaxi shale formation were studied by the introduced workflow. The DRA, core, and log data were mostly in good agreement, confirming the reliability of these methods. When multiple logs showed discrepancies in TOC, DRA provided additional key information for judgment. Based on the obtained petrophysical, geochemical, and geomechanical properties, we accurately characterized the Wufeng-Longmaxi formation, predicted the shale gas sweet-spot along the wellbore, and provided suggestions for future operations of horizontal drilling and fracking in this formation.

The exploration and development of shale gas formations in China attracted extensive interests among Chinese national oil companies and international operators. However, it was extremely challenging due to the complex geological features of organic-rich shale formations in China. Furthermore, conventional methods of core analysis and well log interpretation were usually unreliable in these complex formations, and unable to illustrate micro-scale information in shale. The integration of DRA with conventional core and log analysis significantly improved formation evaluation in organic-rich shale formations in China, and can provide basis for future development decisions.