Numerical simulation of water production process and spontaneous imbibition in a fractured gas reservoir – A case study on homa gas field
Introduction
Fractured reservoirs contain considerable proportion of hydrocarbons in the world especially in the Middle East. Because of the particular structure, these reservoirs have special flow behavior and consequently producing from them is more challenging than from the conventional reservoirs (Lai and Pao, 2013). Fractures are discontinuities in reservoir caused by consecutive breaks in reservoir rock which result in capillary pressure and permeability difference between fracture and matrix (Civan and Rasmussen, 2012). This is the main reason of recovery factor difference between fractured and conventional reservoirs. In this regard, identification of effective mechanisms of oil and gas production from matrix and improvement of recovery factor of fractured reservoirs is very important.
Production mechanisms of fractured reservoirs differs from conventional reservoirs. As mentioned before, this is due to the large difference between capillary pressure of fracture and matrix. The major mechanisms governing production from fractured reservoirs are gravity force and spontaneous imbibition (Abbasi et al., 2017).
Gravity drainage mechanism was first introduced by Cardwell and Parsons as an important method of production from fractured reservoirs (Cardwell and Parsons, 1949). When the gravity force dominates other forces such as capillary force or viscous force, gravity drainage is the mechanism of gas-oil, gas-water or oil-water displacement in production from fractured gas or oil reservoirs. The oil-saturated matrix of which the surrounding fracture network is gas invaded, is capable of production by gravity force if this force overcomes capillary forces (Abbasi et al., 2018a,b). Gas-oil density difference and matrix block height are the factors determine to what extent this mechanism influences reservoir production. As the matrix block height increases, gravity-drainage becomes more important. Capillary continuity between matrix blocks can also increase the effective matrix height and the importance of this mechanism.
The process of imbibition via capillary force or spontaneous imbibition is one of the main mechanisms of production from fractured reservoirs. In general, displacement of non-wetting phase by wetting phase is called imbibition process. This process is divided to three different categories that are dynamic spontaneous imbibition, pseudo-quasi spontaneous imbibition and forced imbibition (Abbasi et al., 2018a,b) (T. Babadagli, Al-Bemani, and Boukadi, 1999). In this study, dynamic spontaneous imbibition or briefly, spontaneous imbibition in which wetting phase is pulled into the porous media by capillary force is investigated. Based on the flow directions of the two phases towards each other, spontaneous imbibition is classified to two general flow types consist of opposite flow (counter-current spontaneous imbibition) and agreeing flow (co-current spontaneous imbibition). According to the boundary conditions, either of these two cases may exist in fractured gas or oil reservoirs. When the two phases flow in the same directions, co-current spontaneous imbibition occurs, otherwise, counter-current spontaneous imbibition happens in the reservoir. Many researches has been conducted on this mechanism ranging from molecular to field scales; some of which include effect of wetting phase saturation (Yazzan et al., 2013), spontaneous imbibition modeling (Li et al., 2003), effect of wettability alteration (Tayfun Babadagli, 2002), and also effect of matrix shape and size, gravity, fluids viscosities and the rock type (Tayfun Babadagli and Hatiboglu, 2007) (Mirzaei-Paiaman and Saboorian-Jooybari, 2016). Despite the large number of researches in oil reservoirs, noticeable studies on gas production from gas reservoirs through this phenomenon have not been reported. This phenomenon becomes important in reservoirs with strong aquifer exposed to water level rise.
Nevertheless, water production is usually observed in fractured reservoirs. This phenomenon occurs because of coning arising from high pressure gradient. This gradient causes gas-water contact to rise and eventually water breakthrough towards production well. Excess water production is a key factor which causes:
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Restriction of economic production life of a gas well
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Early arrival at the end of reservoir production life
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Increase the costs of water separation and transportation
Furthermore, if the well production capacity is not sufficient to produce the water arrived at well, the applied pressure drop may make the water to remain at the bottom of the wellbore and causes reduction of production flow rate and eventually the well is killed. According to the research of Beattie and Roberts, (1996), water breakthrough time decreases proportional to the following conditions:
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more production rate
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Narrower well completion interval
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Less distance between gas-water contact and well completion interval
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Thicker and stronger aquifer
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Less horizontal permeability
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More vertical permeability
Unlike oil reservoirs, in the past years, fewer studies about coning in gas reservoirs have been done. Considering the previous studies, the purpose of this work is to investigate water rise in production wells of Homa field and the behavior of aquifer front movement.
Section snippets
Methodology
According of the importance of accurate identification of fluids flow around gas producing wells, a three-dimensional, fine-grid cylindrical model is defined in this study. This is because of that the fluids flow around production and/or injection wells is cylindrical in which by approaching flow to the wellbore, fluids velocity and fluids mixing increases. In this situation, a solution of flow equation in cylindrical coordinate is required. In addition, according to the high pressure-drop near
Results and discussion
In the following, the results of natural production from gas well in different conservative production scenarios are evaluated.
Conclusions
In this study, different simulations with the purpose of investigating gas-water contact rise in a conceptual model of one of the gas fields in south of Iran was conducted using cylindrical model. The fracture network model of this field was defined clearly in horizontal and vertical directions. Applied simulations with different scenarios of production, evaluated the essential controlling parameters of water production process. The results of these simulations showed that the water production
CRediT authorship contribution statement
Mohammad Ghasem Akbarifard: Writing - original draft, Investigation, Resources. Amin Azdarpour: Methodology, Validation, Resources, Data curation, Supervision, Project administration, Funding acquisition, Writing - review & editing. Zahra Arab Aboosadi: Methodology, Funding acquisition, Writing - review & editing. Bizhan Honarvar: Methodology, Funding acquisition, Writing - review & editing. Moein Nabipour: Resources, Funding acquisition, Writing - review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The authors would like to gratefully acknowledge and appreciate the Department of Chemical Engineering, Faculty of Engineering, Marvdasht Islamic Azad University, Marvdasht, Iran, for the provision of the laboratory facilities necessary for completing this work.
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