Improvement to Gravity Drainage Recovery by Repressurization as a Criterion to Screen and Rank Naturally Fractured Reservoirs for Gas Injection

IF 2.1 4区 工程技术 Q3 ENERGY & FUELS
N. Alimohammadi, M. Pooladi-Darvish, B. Rostami, M. Khosravi
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引用次数: 0

Abstract

Many of the naturally fractured carbonate reservoirs of the Middle East exhibit low natural-depletion recoveries. The reason is that most of their oil reserves are stored in the low-permeability host rocks and are left behind by the fast advancing gas/oil contact (GOC) and water/oil contact (WOC) in fractures. Producing the remaining oil in the large gas-invaded zone of these reservoirs has been a crucial reservoir management issue. We show in this study using experimental observations, analytical calculations, and numerical investigations that repressurizing naturally fractured reservoirs (NFRs) by crestal immiscible gas injection has the potential to produce a large portion of this remaining oil by improving gravity drainage (GD) through two main mechanisms. One is that at higher pressures, the gas-oil interfacial tension (IFT) and hence the capillary forces that control recovery by GD are lessened, allowing additional recovery. This mechanism is aided by the other one, which is swelling of the oil at higher pressures. In this way, repressurization is thought to be not only a means for pressure maintenance but also a methodology for enhanced-oil recovery (EOR). This is confirmed by both laboratory studies and field performance of large-scale gas injection projects. Despite the desire for implementation of projects of repressurization, gas availability and cost of these projects are important concerns, requiring a cost-benefit analysis. Screening and ranking methodologies have been previously presented for some EOR techniques but not for repressurization by gas in NFRs. Evaluating the performance of gas injection in NFRs is often done using methodologies such as numerical simulations, which are in-depth, costly, and tedious. The methodology developed here is simple, requiring spreadsheet calculations. To develop the methodology, we first obtain simple relations to calculate additional GD recovery by considering the interplay of capillary and gravity forces in a matrix block subjected to pressurization by equilibrium gas injection and then use experimental data from literature to show that these relations can predict primary and secondary GD recoveries to a good approximation. We also show by mechanistic studies using a history-matched numerical model that IFT reduction and oil swelling are the main mechanisms contributing to additional oil recovery. Then, we propose a methodology to screen and rank candidate NFRs for gas injection that uses commonly available reservoir data and is based upon two criteria, these being additional oil recovered from a matrix block by pressurization and required volume of gas to produce an additional barrel of oil. We then implement this methodology to more than 20 Iranian NFRs and identify six reservoirs with potential for additional recovery of more than 20%. By quantifying and including the uncertainties associated with the reservoir data, we illustrate that for the reservoirs under study, capillary pressure parameters along with matrix-block height are the main parameters affecting GD recovery and should be characterized more accurately.
以加压提高重力排水采收率为标准对天然裂缝性储层进行注气筛选和分级
中东许多天然裂缝型碳酸盐岩储层表现出较低的自然衰竭采收率。原因是这些地区的大部分石油储量都储存在低渗透的储集岩中,而裂缝中快速推进的油气界面(GOC)和水/油界面(WOC)留下了石油储量。在这些储层的大气侵带开采剩余油已成为一个重要的油藏管理问题。本研究通过实验观察、分析计算和数值研究表明,通过顶部非混相气体注入对天然裂缝性储层(NFRs)进行再压,有可能通过两种主要机制改善重力泄油(GD),从而产生大部分剩余油。一是在较高的压力下,气-油界面张力(IFT)和控制GD采收率的毛细力减小,从而允许额外的采收率。这一机制还得到另一机制的辅助,即油在较高压力下的膨胀。因此,加压不仅是维持压力的一种手段,也是提高采收率的一种方法。实验室研究和大型注气项目的现场表现证实了这一点。尽管希望实施再增压项目,但这些项目的天然气供应和成本是重要的问题,需要进行成本效益分析。筛选和排序方法之前已经针对一些EOR技术提出过,但还没有针对NFRs中的气体再增压提出过。对NFRs注气性能的评估通常采用数值模拟等方法,这是一种深入、昂贵且繁琐的方法。这里开发的方法很简单,需要电子表格计算。为了开发该方法,我们首先通过考虑平衡气体注入加压的基质块中毛细管力和重力的相互作用,获得计算额外GD采收率的简单关系,然后使用文献中的实验数据表明,这些关系可以很好地预测初级和次级GD采收率。我们还使用历史匹配的数值模型进行了机理研究,表明IFT降低和油膨胀是促进额外采收率的主要机制。然后,我们提出了一种方法来筛选和排名候选的NFRs注气,该方法使用常用的油藏数据,并基于两个标准,这两个标准是通过加压从基质块中回收的额外石油和生产额外一桶石油所需的天然气体积。然后,我们将该方法应用于20多个伊朗NFRs,并确定了6个额外采收率超过20%的储层。通过对储层数据的不确定性进行量化和计入,研究表明,对于所研究的储层,毛管压力参数和基质块高度是影响GD采收率的主要参数,应更准确地表征这些参数。
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来源期刊
CiteScore
5.30
自引率
0.00%
发文量
68
审稿时长
12 months
期刊介绍: Covers the application of a wide range of topics, including reservoir characterization, geology and geophysics, core analysis, well logging, well testing, reservoir management, enhanced oil recovery, fluid mechanics, performance prediction, reservoir simulation, digital energy, uncertainty/risk assessment, information management, resource and reserve evaluation, portfolio/asset management, project valuation, and petroleum economics.
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