Simulation study of sweep improvement in heavy oil CO2 floods

二氧化碳注入用于提高轻质油藏的采收率是一种常见的应用，因为二氧化碳与轻质油相对容易混相。然而，由于不利的流动性比，重油油藏中的二氧化碳驱通常不经济。油藏的非均质性进一步复杂化了这一过程，因为二氧化碳会通过高渗透层流动，导致提前突破。然而，通过选择合适的二氧化碳注入过程的修改，可以控制这一问题，实现更好的波及效率，使过程变得经济。本研究重点研究了两种这样的方法：水气交替注入（WAG）和通过在高渗透层中阻塞气体流动来改变油藏剖面。这些方法被研究用于油回收的物理机制、增加波及效率和减轻提前突破。使用一个类似重油（14°API）的油藏进行油藏模拟研究，该油藏有一个高渗透条带，其渗透率比相邻区域高出50倍。进行了详细的流体特性分析，以准确表示油藏流体。通过解释细管和岩心驱替模拟，了解了这种原油的油回收物理机制。使用阻塞剂进行剖面修改显示出非常令人鼓舞的结果。评估了不同的WAG比例，1:1的WAG比例 resulted in the highest oil recovery，这在岩心驱替模拟和油藏模拟中都是一致的。这与轻质油油藏中最高的WAG比例不同，通常看到的值为1:2。研究表明，通过仔细研究油藏地质和流体特性，应用这些方法可以显著提高重油驱的波及效率和油回收率。

CMG软件的应用情况

在本研究中，CMG软件被广泛应用于油藏模拟和流体特性分析。具体应用包括：
  1. 流体特性分析：使用WINPROP®软件进行流体特性分析，包括组分重组、相图匹配、PVT数据回归等，以建立准确的油藏流体模型。
  2. 油藏模拟：使用CMG-STARS®和CMG-GEM®软件进行油藏模拟，研究不同的提高采收率方法，如连续二氧化碳注入、水气交替注入（WAG）和剖面修改。通过模拟，评估了这些方法在提高波及效率和油回收率方面的效果。
  3. 敏感性分析：通过CMG软件进行敏感性分析，研究了不同参数（如WAG比例、阻塞剂注入半径等）对油回收率的影响。

作者单位

Venu Gopal Rao Nagineni，2006年毕业于印度丹巴德印度矿业学院，获得石油工程学士学位。2008年加入路易斯安那州立大学Craft and Hawkins石油工程系，2011年春季获得石油工程硕士学位。

Abstract

Enhanced oil recovery by CO2 injection is a common application used for light oil reservoirs since CO2 is relatively easily miscible with light oils. CO2 flooding in heavy oil reservoirs is often uneconomic due to unfavorable mobility ratios. Reservoir heterogeneity further complicates the process as CO2 channels through high permeability layers leading to premature breakthrough. However, this can be controlled by choosing a suitable modification to the CO2 injection process enabling better sweep efficiencies, and making the process economic. The current work focuses on two such methods; water-alternating-gas injection (WAG) and profile modification by blocking gas flow in the high permeability layer. These methods were studied for physical mechanisms of oil recovery, increasing sweep efficiency, and mitigating premature breakthrough.

Reservoir simulation studies of these methods were conducted using an analog heavy oil (14° API) field with a high permeability streak which had 50 times greater permeability than the adjacent zones. A detailed fluid characterization was performed to accurately represent the reservoir fluid. Slim tube and core flood simulations were interpreted to understand the physical mechanisms of oil recovery for this crude. Profile modification using a blocking agent showed very encouraging results. Different WAG ratios were also evaluated, and a WAG ratio of 1:1 resulted in the highest oil recovery which was consistent between both core flood simulations and field simulations. This is different from WAG ratios for highest recovery in light oil reservoirs where values of 1:2 are typically seen. It is shown that with careful study of the reservoir geology and fluid properties, application of these methods can significantly improve sweep efficiency and oil recovery in heavy oil floods.

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