Various laboratory and modeling activities have been performed to offset the rapid oil production decline in single wells to investigate enhanced oil recovery (EOR) in unconventional reservoirs. Several field pilots have also been conducted to test the EOR effects of different methods in major unconventional plays. Although high oil recovery was reported in laboratory and modeling work, the EOR results in field pilots were mixed. To fill the gap between theoretical work and field implementations, case studies were evaluated in this work to analyze the actual gas injection EOR field tests in the Bakken petroleum system (BPS). Based on these pilots, an EOR-monitoring workflow was developed to explore real-time visualization, forecasting, and control methods for improved reservoir surveillance during EOR processes.
Eleven EOR pilot studies that used rich gas, CO2, surfactant, water, or their combinations have been conducted in the BPS since 2008. Gas injection was involved in eight of these pilots with huff ‘n’ puff, flooding, and injectivity operations. Surveillance data, including daily production/injection rates, bottomhole injection pressure, gas composition, and tracer testing, were collected from these tests to generate time-series plots or analytics that can inform operators of downhole conditions. Predictive modeling based on reservoir simulation and machine learning was then conducted to rapidly forecast future performance for operators to compare against observed performance. The real-time comparison enables operators to take control actions to improve EOR outcome.
Case studies showed that pressure buildup, conformance issues, and timely gas breakthrough detection were some of the main challenges because of the interconnected fractures between injection and offset wells. The latest operation of coinjecting gas, water, and surfactant through the same injection well showed that these challenges could be mitigated by careful EOR design and continuous reservoir monitoring. A user interface was developed to integrate EOR-monitoring components and provide real-time visualization to enable operators to modify key EOR parameters, such as gas injection rate and pressure, and rapidly predict the subsequent EOR outcome. Results showed that monitoring gas composition could be more sensitive for detecting premature gas breakthroughs than other indicators.
Since only limited research has been reported to investigate actual field implementations and their surveillance, the findings in this study provided the necessary technical support to demonstrate how injecting gas into a Bakken reservoir can be used for EOR, thereby increasing ultimate oil recovery while reducing produced gas flaring and greenhouse gas emissions. With an increasing number of wells entering their late phase with low or uneconomical oil production rates, it is anticipated that the scientific understanding gained from field implementation and surveillance activities will lead to commercial deployment of gas injection EOR in the BPS and other unconventional plays within the next decade and perhaps sooner.
