The domestic shale gas production of China reached 10 billion cubic meters in 2018, but the gap between this and the production goal of 30 billion cubic meters in 2020 requires an acceleration in the progress. There are several shale gas operators in China, and the uncertainties introduced by different formation evaluation standards will lower the accuracy and efficiency in finding the sweet spot. In addition, the fast-paced operation needs a rapid and accurate production prediction method.
This paper presents a customized shale gas formation evaluation workflow that integrates core analyses and wireline logging data. Retort core measurement is successfully utilized to calibrate the porosity, permeability, and water saturation derived from a wireline petrophysical calculation. Ion-milled backscatter scanning electron microscope (BSE), digital rock analysis, and nuclear magnetic resonance (NMR) performed on both oil-saturated and brine-saturated cores give insight into the composition and distribution of organic pores and inorganic pores. The mercury injection capillary pressure test (MICP) and gas sorption tests reveal the pore size range of the reservoir. Finally, a robust methodology integrating wireline NMR, borehole images, and induced gamma spectroscopy shows great advantage in shale gas production prediction.
Case studies are presented from shale gas reservoirs of Huangjinba block in the Zhejiang oil field, which is located in the Zhaotong area of northern Yunnan and Guizhou provinces. The core experiments indicate that the shale pore size ranges from 2 to 30 nm, and the ratio of organic pores to inorganic pores is about 55:45. The core-validated petrophysical properties of lithology, porosity, permeability, total organic carbon content (TOC), gas content, and fluid saturation alleviate the uncertainties in determining the best zones. Careful study of transverse relaxation time (T2) cutoff was carried out on the wireline NMR to get the macropore porosity, which is related to high pore pressure due to good preservation conditions. On the other hand, fractures observed on borehole images also reveal the preservation conditions. More macroporosity from NMR and fewer fractures on the borehole image are the two dominant controlling factors for good preservation condition and thus better gas production.
This paper discusses a novel application of core-validated formation evaluation of shale gas reservoirs, which helps the operator understand the reservoirs and ascertain the potential for future development. The predicted gas production shows a good match to the operational results. The analysis of the controlling factors on production from a shale gas reservoir makes it possible to make quick decisions during lateral well drilling.