Production potential of Niobrara and Codell: integrating reservoir simulation with 4D seismic and microseismic interpretation

Abstract

The objective of this research was to build a compositional model to evaluate current and future production performance of Niobrara and Codell, which are the targeted zones in the Wattenberg Field located in Colorado’s Denver-Julesburg Basin (DJ Basin). The study area is called Wishbone section. It is a square-mile section that includes eleven horizontal wells. The production began in September 2013. The seismic Baseline, Monitor 1, and Monitor 2 surveys were conducted before the eleven horizontal wells were drilled, after hydraulic fracture stimulation, and after two years of production. One thing that needs attention is the complexity of the study area: the tight and highly heterogeneous reservoir, the existence of faults, and the light hydrocarbon content. To build a realistic simulation model, an integrated workflow was developed, which includes data from geology, geophysics, petrophysics, completion and production. The geologic input data were obtained from seismic and well log interpretations. The reservoir model hydrocarbon fluid data were prepared from several comprehensive oil and gas PVT reports. These data were complemented with information from daily well production and pressure records. The hydraulic fracturing interpretation results from GOHFER were imported into the flow simulation model to describe well performance. After history matching was done, four methods were used to validate the model. First, rate transient analysis was conducted based on both actual production data and simulated results. It was found that the reservoir permeability calculated from both cases are similar, which verifies the simulation model. Second, the producing gas-oil ratio (GOR) is increasing for wells from east to west. The simulated gas saturation distribution in the fracture system is consistent with GOR performance, which also adds credibility to this simulation model. Third, microseismic clusters suggest increased fracture density, which correlate with higher gas saturation zones. Similarly, the zones that have less frequency of microseismic events are consistent with the lower gas saturation areas. Fourth, to integrate the reservoir model with seismic responses, the most significant observation was the free gas evolved in the fracture system. The P wave impedance difference between Monitor 2 and Baseline surveys show negative value in the western side, which is consistent with the higher gas saturation observed from the flow simulation model. Thus, the reservoir model credibility was substantiated by the agreement among actual gas production, modeled gas saturation, microseismic events and 4D seismic responses. After the model was validated, a portion of the produced rich gas was injected into a Niobrara well and a Codell well separately to evaluate the enhanced oil recovery (EOR) potential. The results were confirmed by the tracer data analyses, which indicate strong inter-well and inter-formation communication. For instance, when gas was injected into a Niobrara well, the oil production of a nearby Codell well was more enhanced than its neighboring Niobrara wells. The modeled gas injection EOR yielded 2% - 4% incremental oil during a production period of 15 years. This indicates a strong likelihood of EOR potential in unconventional fields by wet gas injection. Considering the geologic complexity of the study area, the modeling study is a powerful tool for understanding the flow mechanism in shale formations of the DJ Basin. Furthermore, the modeling study provides a great insight to the stake holders involved in assessing the EOR potential in the DJ Basin unconventional reservoirs.