Genesis of the abnormal formation pressure in the Wufeng-Longmaxi Formation, Sichuan Basin and a generalized Poisson’s ratio prediction method: A case study of the Changning area

doi:10.13745/j.esf.sf.2020.12.2

Abstract

Abstract:

We compared the measured pressure coefficient of the Wufeng-Longmaxi Formation, Sichuan Basin with the current tectonic division and the eastward movement velocity of the Qinghai-Tibet Plateau, and found that the formation pressure of the Wufeng-Longmaxi Formation might be affected by the multi-stage tectonic movement since the Yanshan Movement. The overpressure zone is located in the central areas of the high steep tectonic belt in eastern Sichuan and low steep tectonic belt in southern Sichuan. During the Yanshan-Xishan period, the east side of the Huayingshan fault zone was strongly extruded to form a long strip-shaped high steep anticline and an open syncline. The eastward movement of various blocks of the Qinghai-Tibet Plateau might also have influenced the pressure distribution in the Wufeng-Longmaxi Formation. Drilling results showed that after the deposition of the Wufeng-Longmaxi Formation in the Sichuan Basin, organic matters in the formation were transformed into crude oil and then cracked into natural gas under the action of abnormal geothermal gradient. Therefore, hydrocarbon cracking led to fluid expansion, which was the second causative factor of abnormal formation pressure in the Wufeng-Longmaxi Formation. Meanwhile, local abnormal formation pressure may reflect the shale gas preservation condition. Based on the analysis of the formation pressure anomaly in the Wufeng-Longmaxi Formation, the abnormal formation pressure was attributed to horizontal and vertical forces, and the Poisson’s ratio formation pressure prediction method was developed based on Poisson’s ratio calculation using seismic parameters. It is proved that this prediction method is suitable for predicting the shale gas formation pressure in the Wufeng-Longmaxi Formation. After years of testing and improvements, this prediction method has been widely applied in the exploration and development of shale gas fields in the Weiyuan-Changning area, bringing huge economic and social benefits. By 2019, the cumulative production of the Changning shale gas field has reached 13 billion cubic meters. The generalized Poisson’s ratio formation pressure prediction method is one of the key technologies propelling the Changning shale gas field to become the No.1 shale gas field in China.

Key words: Changning area, Sichuan Basin, Wufeng-Longmaxi Formation, shale gas formation pressure analysis, generalized Poisson’s ratio prediction;

CLC Number:

JING Cui, HAO Long, ZHANG Jing, DENG Xia, YU Wenhui. Genesis of the abnormal formation pressure in the Wufeng-Longmaxi Formation, Sichuan Basin and a generalized Poisson’s ratio prediction method: A case study of the Changning area[J]. Earth Science Frontiers, 2021, 28(1): 402-410.

Figures/Tables 10

Fig.1 A schematic diagram showing the macro-distribution of the formation pressure coefficients and the current tectonic division in the Wufeng-Longmaxi Formation, Sichuan Basin and the distribution of sedimentary facies belts in the region. Modified from [7,8,9,10].

Fig.2 Log analysis of the abnormal formation pressure of Wells A and B in the Wufeng-Longmaxi Formation, Changning area, Sichuan Basin

Fig.3 Relationship between formation pressure and burial depth of the Wufeng-Longmaxi Formation, Changning area

Fig.5 Results of digital petrophysical simulation on the equivalent digital cores (shown in Fig.4) under different confining pressures. (1, 2) Variation ofPoisson’s ratio with pore pressure for 4% or 6% porosity; (3, 4) P- and S-wave velocity change with pore pressure for 4% or 6% porosity.

Fig.4 Composite illustration for the Wufeng-Longmaxi Formation. (1) Electron microscope images of shale rocks; (2) An equivalent digital core model built by the process method; (3) Equivalent digital cores with 4% (bottom) or 6% (top) porosity.

Fig.6 Scatter plots of formation pressure coefficient vs. seismic parameter measured in Wells A, B and E in the Wufeng-Longmaxi Formation, showing the variation trends

Fig.7 Results of drilling porosity, gas content and pressure coefficient analyses for wells A, B, and E in the Changning area. (1) Correlation between total gas content and porosity; (2-4) Plot of total gas content difference (left panel) or pressure coefficient difference (right panel) between indicated well pairs at different well layer

Fig.8 Flow chart for calculating formation pressure coefficients of the Changning area

Fig.9 Application of the Poisson’s ratio formation pressure prediction method in the Changning area (1)~(3) Calculated formation pressure coefficient contour maps showing three local formation pressure distributions in the Changing area.

Table 1 Comparison of Poisson’s ratio formation pressure prediction results and actual drilling results (Changning area)

项目	A	B	后验井C	后验井D
实钻结果	1.35	2.15	1.50	1.65
泊松比地层压力预测成果	1.38	2.17	1.51	1.63

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