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。拟议的NSERC DG研究计划的主要研究目标是追求对致密油地层CO2-EOR的全面基本理解，并产生新的技术知识。将深入研究以下四个重点研究课题：1。注入CO2与储层油、盐水、致密地层相互复杂的相互作用，并量化各自对CO2- eor、CO2诱导盐水结垢和储层完整性的影响；2. 轻质油-二氧化碳混相发展及主要的CO2-EOR机制，如co2致油膨胀效应、界面张力降低、轻质烃萃取、沥青质沉淀等；3. 连续水驱或混相CO2驱、水-交变CO2 （WAG）注入、CO2-交变水（GAW）注入、碳酸水注入等5种水-CO2基采油工艺在致密油地层中的应用与比较。每个过程一次只注入一个阶段（水/二氧化碳）。通过这种方法，可以确定水和CO2注入的最佳时机、最佳段塞尺寸和比例；和4。在致密裂缝性油层中，采用两相（水-二氧化碳）共注的水-二氧化碳基提高采收率新工艺探索。其主要目的是确定最合适的水气比（WGR）和总注入速率，同时控制注入水和CO2的流动性，显著提高致密和裂缝性油层的轻质油采收率。本研究项目的最终目标是从整体采收率、采收率、经济效益和环境效益等方面寻找最佳的水基和co2基EOR技术，以应用于致密油地层的CO2-EOR项目。目前，几乎所有Bakken地层的石油公司都计划在一次采油后采用各种水驱和二氧化碳驱工艺。这项新的NSERC DG研究计划将为开展更高效的二氧化碳eor项目和从大量富油致密地层中开采更多轻质原油提供直接、及时和强有力的技术支持。从长远来看，最佳的基于水和二氧化碳的EOR技术也将有助于保护加拿大丰富的淡水和清洁的空气。

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.