Differences in Microstructures Between Marine and Continental Shales and Its Influences on Shale Reservoir Fracbility

doi:10.7657/XJPG20220104

Abstract

Abstract:

Taking the shales of the Longmaxi formation in the Sichuan basin and the Yanchang formation in the Ordos basin as examples, scanning electron microscopy, nuclear magnetic resonance, CT scanning, rock mechanics test and hydraulic fracturing experiments were carried out to investigate the differences in the shale microstructures between marine and continental facies, and then the influence of the differences on shale fracbility was analyzed, and the two kinds of shale were compared by considering fractal geometry and the theory of rock mechanics. According to the research results, the following findings are obtained: a.The continental shale (Yanchang formation) has a high content of clay minerals, accounting for about 45.3%, while the marine shale (Longmaxi formation) is dominated by brittle minerals such as quartz and feldspar, accounting for about 67.9%. b.The porosity of the marine shale is about twice that of the continental shale. c. The proportion of macropores in the marine shale is higher and the pore size distribution is wider, while mesopores and small pores in the continental shale occupy larger space, and natural fractures and beddings are developed. d.The average initial pressure of the continental shale is about 22.52% lower than that of the marine shale, mainly due to low mechanical strength caused by the high clay mineral content in the continental shale, but less effect from pore pressure. e.The major hydraulic fractures induced in the marine shale are transverse cracks that are basically symmetrical along the wellbore, while the natural fractures and beddings in the continental shale can easily capture hydraulic fractures, resulting in shear fractures perpendicular to the major fractures, and the open and connected beddings and weak surfaces are conducive to form a more complex fracture network in the continental shale. f.The three brittleness indicators of the continental shale are all lower than those of the marine shale, but the continental shale has advantages of lower initial pressure, better fracture network and better fracturing potential, so it is inaccurate to evaluate the compressibility of the continental shale by using a brittleness indicator. g.The low porosity, low permeability and high water sensitivity are not conducive to hydraulic fracturing stimulation to the continental shale reservoir.

Key words: marine shale, continental shale, microstructure, fracbility, fractal dimension, hydraulic fracturing, fracture

CLC Number:

TE122.2

WANG Yinggang, SHEN Feng, WU Jinqiao, SUN Xiao, MU Jingfu, TANG Jiren. Differences in Microstructures Between Marine and Continental Shales and Its Influences on Shale Reservoir Fracbility[J]. Xinjiang Petroleum Geology, 2022, 43(1): 26-33.

Figures/Tables 7

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Table 1

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