Integrating time-lapse gravity, production, and geologic structure data in a gas reservoir study

Abstract

Time-lapse gravity is most commonly used to monitor fluid movement and is especially useful when monitoring water encroachment in a gas reservoir. Although time-lapse gravity data are directly sensitive to density changes in reservoirs, it is still necessary to integrate multiple types of data with complementary information to enhance the time-lapse gravity interpretation. When monitoring water influx in a reservoir, the changes in water yield in production wells may directly indicate saturation changes with time and provide such complementary information about the areas of fluid movement. We have developed a workflow to invert a time-lapse gravity data set and production data to help monitor the edge-water encroachment through a case study at the Sebei gas field in western China. Three time-lapse gravity surveys were acquired between 2011 and 2013, resulting in three sets of time-lapse gravity difference data. Production data were also collected from 286 wells during the same period of time. We integrate the two data sets and the structural information in the reservoir through a framework of constrained time-lapse gravity inversion. In this workflow, we incorporate information from the production data into the inversion by converting the gas and water yield into a reference model. We also incorporate geologic structural information through spatially varying bound constraints. Through this approach, we construct a set of time-lapse density contrast models that are consistent with the time-lapse gravity data, production data, and structural information. The resultant density contrast models better delineate the regions of reservoir with increased water influx and also enable improved porosity estimation in the reservoir.