Methodology for quantitative prediction of fracture sealing with a case study of the lower Cambrian Niutitang Formation in the Cen'gong block in South China

Abstract

Industrial levels of shale gas have been produced from the Niutitang Formation in the lower Cambrian of the Cen'gong block, South China. The practice of exploration and development of shale gas in the Cen'gong block shows that the abundance of gas in different layers and wells is closely related to the degree of development of fractures. In this study, data obtained from outcrops, cores, and thin sections from the Cen'gong block were used to determine the development characteristics of such natural fractures. Combined with the analysis of burial history and fluid inclusions, the stress fields during three periods (the Yanshanian period, Himalayan period and neotectonic period) are suspected to play a critical role in fracture sealing. The paleotectonic stress tensor is determined using fault activity analysis, acoustic emission tests, and burial history. The magnitude and direction of current in situ stresses are determined by drilling-induced fractures, deviated well information, and cross multipole array acoustic (XMAC) logging. Under the constraints of XMAC logging interpretation and mechanical experiments, the spatial distribution of mechanical parameters is obtained by seismic inversion, and 3D heterogeneous geomechanical models are established using a finite element method (FEM). Then, the fracture density, reactivity, and aperture are identified. The results show that the magnitudes of the maximum horizontal principal stress during the Yanshanian period, the Himalayan period, and the neotectonic period are 150–170 MPa, 110–130 MPa, and 35–45 MPa, respectively. Using the fracture sealing calculation model, the fracture sealing that developed in the Himalayan period is the worst, the fracture sealing that developed in the Yanshanian period is intermediate, and the fracture sealing that developed in the neotectonic period is the best. Based on three-dimensional heterogeneous geomechanical modeling and tectonic stress evolution estimates, the fracture sealing calculation model proposed in this paper can be widely used in the evaluation of the preservation conditions of shale reservoirs, tight sandstone reservoirs and low-permeability reservoirs in complex tectonic areas.