Geophysical characterization of complex geopressure systems in unconventional reservoirs: A case study of the Jurassic reservoir in the Fuxing area

Abstract

Objective The prediction of abnormal formation pressure is a challenging issue in oil and gas development. With the continuous development of unconventional reservoirs such as shale gas and tight sandstone gas, the complexity of geopressure systems has increased. The coexistence of overpressure and underpressure in the same area often occurs, making it important to accurately characterize complex pressure systems. Methods In this work, two sets of overpressure systems and one set of underpressure systems formed in the Jurassic shale gas reservoir and tight sandstone reservoir in the Fuxing area were analysed. The geophysical responses and sensitive elastic parameters of overpressure and underpressure were analysed. The bulk modulus of the mudstone formation was strongly correlated with the formation pressure over the underpressure-overpressure interval. Based on this, an integrated prediction method for overpressure and underpressure based on the bulk modulus of mudstone was proposed and applied in this area by using well log and seismic data. The key of this method is to use the mudstone of overpressure and underpressure formations for integrated pressure prediction, reducing the influence of lithological changes. Second, the bulk modulus, which is directly related to the physical mechanism of abnormal pressure, was selected as the input parameter to improve the accuracy. Results According to the results of pressure prediction in the study area, the preservation conditions of the strong overpressure system in the Dongyuemiao Member are good, the pressure structure is stable, and the maximum pressure coefficient is approximately 2.0. The preservation conditions of the overpressure system in the Lianggaoshan Formation are relatively poor, with large lateral differences in pressure structure characteristics, and the maximum pressure coefficient is approximately 1.65; the pressure coefficient of the underpressure system in the Lianggaoshan Formation is approximately 0.5 to 1.0, and it has a certain coupling effect with the underlying overpressure system. Conclusion The predictions of overpressure and underpressure using well logs and seismic data in the study area both achieved high agreement with the measured data.

Keywords

• unconventional reservoir
• geopressure prediction
• fuxing area
• shale gas
• tight sand
• complex geopressure system
• geophysics