Fundamental Studies of Carbon Dioxide Enhanced Oil Recovery (CO2-EOR) in Tight Oil Formations

Bakken地层拥有约2710亿至5030亿桶轻质油，最近已成为加拿大西部和美国邻近各州的重要石油资源。该油层非常致密，渗透率为0.01 ~ 1 mD，比典型轻质油油藏低约100倍。该地层的大多数油藏目前都处于一次采油过程中，由于压力迅速枯竭，石油产量急剧下降。据估计，Bakken区块的采收率只有1% ~ 3%。作为一种二次采油工艺，通常采用水驱，并且通常被证明是有效的，但水很难大量注入致密地层。水源需求量大和水处理费用高是两个主要问题。另外，二氧化碳提高采收率（CO2- eor）方法可以作为第三次采油工艺，因为二氧化碳的粘性要小得多，因此比水更容易注入。然而，CO2- eor能否成功应用仍是未知数，因为CO2比水更容易突破致密油地层。此外，还需要确定何时以及如何注入水和/或二氧化碳，以优化Bakken地层的轻质油采收率。

The Bakken formation has recently become a vital oil resource in Western Canada and the adjacent US states as it has approximately 271 to 503 billion barrels of light oil in place. This oil formation is extremely tight with permeabilities of 0.01 to 1 mD, about 100 times lower than those of typical light oil reservoirs. Most oil reservoirs in this formation are currently under the primary oil recovery process, and the oil production rates are being reduced drastically due to quick pressure depletion. The Bakken’s oil recovery factors are estimated to be as low as 1 to 3%. As a secondary oil recovery process, waterflooding is usually adopted and normally proven to be effective but water is very difficult to inject in large volumes into tight formations. The large water source requirement and high water treatment costs are two major concerns. Alternatively, carbon dioxide-enhanced oil recovery (CO2-EOR) methods can be used as tertiary oil recovery processes because CO2 is much less viscous and, thus, much easier to inject than water. It remains unknown, nevertheless, whether CO2-EOR can be successfully applied as CO2 may breakthrough a tight oil formation much more readily than water. In addition, it needs to be determined when and how water and/or CO2 should be injected so that the light oil recovery in the Bakken formation can be optimized. The primary research objective of the proposed NSERC DG research program is to pursue a comprehensive fundamental understanding and generate new technical knowledge of CO2-EOR in tight oil formations. The following four key research topics will be studied in depth: 1. The mutual and complex interactions between the injected CO2 and the reservoir oil, brine, and tight formation and quantification of their respective effects on CO2-EOR, CO2-induced brine scaling, and reservoir integrity; 2. The light oil-CO2 miscibility development and the major CO2-EOR mechanisms, such as CO2-induced oil-swelling effect, interfacial tension reduction, light-hydrocarbons extraction, and asphaltene precipitation; 3. Application and comparison of the following five water- and CO2-based oil recovery processes in a tight oil formation: continuous waterflooding or miscible CO2 flooding, water-alternating-CO2 (WAG) injection, CO2-alternating-water (GAW) injection, and carbonated water injection. Each process involves one-phase (water/CO2) injection at a time. In this way, the best timing, the optimum slug size and ratio for water and CO2 injections will be identified; and 4. Exploration of a new water- and CO2-based EOR process in a tight and fractured oil formation by co-injecting water and CO2, which is a two-phase (water-CO2) injection. The major purpose is to determine the most appropriate water-gas ratio (WGR) and total injection rate, both of which help to simultaneously control the mobilities of the injected water and CO2 and significantly enhance the light oil recovery in the tight and fractured oil formation. The ultimate goal of this research program is to find the optimum water- and CO2-based EOR technique for applying CO2-EOR projects in tight oil formations in terms of the overall oil recovery rate and factor and the economic and environmental benefits. At present, almost all the oil companies in the Bakken formation are planning to apply various water- and CO2-based flooding processes after the primary oil recovery process. This new NSERC DG research program will provide direct, timely, and strong technical support for conducting more efficient CO2-EOR projects and recovering more light crude oil from the vast oil-rich tight formations. In the long term, the optimum water- and CO2-based EOR technique will also help to preserve Canada’s abundant fresh water and clean air.