Douzhong Zhang’s research while affiliated with Yangtze University and other places

The in-situ stress and formation pressure are important parameters in shale gas development. They directly affect the well wall stability, the direction of horizontal well drilling, and the fracturing effect during the shale gas development process. There are abundant shale gas resources in the southeastern Chongqing-Sichuan area, but the structure in the area is complex, and it is difficult to predict the in-situ stress and formation pressure. Therefore, in this paper, a finite element simulation model was established based on a large number of seismic, logging, and experimental rock mechanics data and the prediction accuracy of the stress field simulation was effectively improved. The construction of the stress field was based on the combined spring model, as well as the data related to the measured in-situ stress and the formation pressure obtained during drilling. The coupling relationship between the in-situ stress, the formation strain, and the formation pressure were derived to carry out the prediction of the distributions of the formation pressure and the formation pressure coefficient. The prediction results showed that the present-day maximum principal stress direction in the study area was about NE65°–110°, and the present-day maximum principal stress was 56.12–93.79 MPa. The present-day minimum principal stress direction was about NE335°–20°, and the present-day minimum principal stress was 48.06–71.67 MPa. The formation pressure was 2.8–88.25 MPa, and the formation pressure coefficient was 0.74–1.55. The formation pressure distribution was greatly affected by fault, tectonic location, in-situ stress and rock petrophysical properties, and the overpressure areas of the formation were distributed in the synclines and the deeply buried areas. This study shows that the finite element based formation pressure prediction method is effective.

Shale is a low-porosity and low-permeability reservoir, and structural fractures are the main controlling factor for the migration and accumulation of shale gas. Moreover, tectonic fractures are controlled by the paleo-tectonic stress field. In this paper, taking the Longmaxi Formation of the Lintanchang area as an example, the finite element numerical simulation technology is used to analyze the distribution law of the paleo-tectonic stress field, and further, the fracture development areas under the superposition of two periods of tectonic stress are predicted using seismic, rock mechanics, and field data. The results show that the tectonic fractures developed in the Lintanchang area are mainly EW- and NNW-striking conjugate shear fractures formed in the Mid-Yanshanian period, followed by the NWW- and SWW-striking conjugate shear fractures formed in the late Yanshanian period. The distribution of tectonic fractures is affected by faults, folds, rock physical parameters and tectonic stresses. It is found that the comprehensive fracture coefficients of the anticline core and fault areas are both greater than 1.1, which are the areas with the most developed structural fractures, and these areas have poor shale gas preservation conditions. However, the comprehensive fracture coefficients of the western flanks of the anticline and the eastern and western dipping ends are between 1.0 and 1.1, which are areas with better shale gas preservation conditions. In addition, the development degree of tectonic fractures in the east and northwest areas of the Lintanchang anticline is lower than that in other areas. The comprehensive fracture coefficients of shale in these areas are between 0.9 and 1.0. The shale is in a state of “breaking without cracking”, and shale gas can be well preserved.

Lower Paleozoic dark shale is developed in the western Middle Yangtze Block, which lays a material foundation for the enrichment and accumulation of marine shale gas. In order to ascertain the control action of geological structures on the differential preservation of shale gas and reveal the key factors in shale gas preservation, this paper firstly analyzed the structure characteristics of this area, carried out structure pattern recognition and structural belt division, and studied structural deformation mode and intensity. Based on this, the relationships between different structure styles and shale gas preservation conditions were analyzed. Finally, combined with the structural deformation and the lithofacies paleogeographic characteristics of marine shale, the favorable exploration zones of shale gas were proposed. And the following research results were obtained. First, the western Middle Yangtze Block can be divided into four structural deformation belts, and three types of piggyback structural patterns have been identified, including restricted type, weakly reformed type and strongly reformed type. Second, the restricted type is located in the northwestern part of Hunan and Hubei Provinces. In this pattern, piggyback structure is incomplete and thrust belt and Compressive fold belt are developed. Third, the weakly and strongly reformed types are located in the western parts of Hunan and Hubei, and Wulingshan area, respectively. They both have complete piggyback structures, but the former has lower deformation intensity and has never undergone the late superimposed reformation. Fourth, there are three structural transfer belts in the western Middle Yangtze Block, i.e. the structural transfer belt between the East Sichuan fault–fold belt and West Hunan–Hubei fault–fold belt, the structural transfer belt between West Hunan–Hubei fault–fold belt and Wulingshan fault–fold belt, and the structural transfer belt between the outcrop and the hinterland of Middle Yangtze Block. The first one is structurally transformed at the Qiyueshan fault. The East Sichuan fault–fold belt on the west is an ejective fold with low fault density and formation denudation intensity, where shale gas is enriched in anticlines and slopes; while the West Hunan–Hubei fault–fold belt on the east is a trough-like fold with strong faulting and high formation denudation intensity, where shale gas is enriched in residual synclines. In conclusion, shale gas preservation conditions of Upper Ordovician Wufeng Formation–Lower Silurian Longmaxi Formation in this area are the best in Zigui syncline, thrust–detachment zone and western margin of Qiyueshan fault. The favorable exploration areas of shale gas of Lower Cambrian Niutitang Formation are distributed in the western flank of Yichang Slope, Kaixian thrust zone, compressive fold zone and thrust–detachment zone.