Investigation on water-drive performance of a fault-karst carbonate reservoir under different well patterns and injection-production modes based on 2D visualized physical models
Introduction
It was reported that approximately 60% of global oil and 40% of global gas, respectively, resided in carbonate reservoirs, and the total hydrocarbon output accounted for about 60% worldwide (Alzayer and Sohrabi, 2018; Schlumberger, 2007; Wei et al., 2017; Yousufi et al., 2019). The research institute of CNPC also counted that 47.5% of the world's remaining recoverable oil and gas reserves (approximately 2000 × 108 t) came from carbonate rocks (CNPC RIPED, 2017). However, due to complex depositional environments and pervasive diagenetic history, the characteristics of carbonate reservoirs are vastly different from those conventional siliciclastic sand reservoirs (Anselmetti and Eberli, 1993; Durrani et al., 2021; Lucia, 1995), which usually consist of numerous dissolution pores, vugs, fractures, and caves with different scales (Camacho-Velázquez et al., 2005; Sun et al., 2021; Tian et al., 2019a; Tian et al., 2019b; Yefei et al.).
Recently, a new type of carbonate reservoir has attracted significant attention in China; it consists of many large caves interconnected by high-angle tectonic fractures or small vugs and develops various irregular fractured-vuggy units along with the main faults (Wang et al., 2021; Yingtao et al., 2019). This specific category of carbonate reservoirs is named the fault-karst carbonate reservoir (Cao et al., 2021; Lu et al., 2015). Up to now, many representative fault-karst carbonate reservoirs have been discovered in the Tarim Basin of the northwest of China, such as the Shunbei oilfield, Tahe oilfield, Halahatang oilfield, and Fuman oilfield (Deng et al., 2022; Ding et al., 2020; Xuewen et al., 2022; Zhiwen et al., 2020). The fault-karst carbonate reservoirs are also different from those general porous, fractured, or fractured-vuggy carbonate reservoirs (Wang and Sun, 2019; Zhang et al., 2017). During geological processes of large-scale multiphase tectonic evolution, multiple-stage karstification, and hydrothermal reforming, the atmospheric freshwater or hydrothermal fluid can flow along these faults and associated fracture systems under nonexposed conditions and consequently, the fault-controlled karst fractured-vuggy reservoirs are formed along the strike-slip fault zones (Bertotti et al., 2020; Mu et al., 2021; Su et al., 2021; Wang et al., 2020; Wu et al., 2016; Zhao et al., 2014). Although many hydrocarbon resources exist in the fault-karst carbonate reservoirs, it is still challenging to develop this kind of reservoir effectively and economically due to the strong heterogeneity of the formation media (Bagni et al., 2022; Tian et al., 2016, 2017).
Lu et al. (2017) pointed out that the faults are not only karstification and hydrocarbon migration pathways, but also the hydrocarbon accumulation traps in the fault-karst carbonate reservoirs. They classified the fault-karst carbonate reservoirs into three different types according to distinct segments along the strike-slip faults: dendritic, sandwich, and slab reservoirs. Meanwhile, they found that the dendritic fault-karst reservoir is the most hydrocarbon-rich. Li et al. (2019) summarized three-component structural characteristics and three architectural patterns for the fault-controlled karst reservoirs. The three-component structure including fault core, damage zone, and host rock in the zone of fault activity can be formed in modern karst, in which the caverns are usually developed in the fault core and the fracture-vugs are also fully developed. Among the three architectural patterns of fault-controlled cavern complex, fault-controlled cavern, and fault-controlled vugs, the highest production always appears in the fault-controlled cavern complex. Zhang et al. (2021) had divided the fault-controlled karst reservoir into four levels of architecture elements: the strike-slip fault impacting zone, the fault-controlled karst reservoir, the fracture-cave zone, and the fillings inside caves. The fracture-cave zone can be further subdivided into the dissolution cave, dissolution pore and vug, and fracture zones. On the whole, all geological studies have proven that the inner structures of fault-karst carbonate reservoirs have strong heterogeneity, which definitely increases the risk of oilfield development.
Up to now, there are just a few conducted studies on improved oil recovery (IOR) of fault-karst carbonate reservoirs in the literature. Song et al. (2020) had listed some main physical models used in previous studies on general fractured-vuggy reservoirs, but none were designed for the fault-karst systems. They fabricated a two-dimensional (2D) physical model of the karst fault system based on a specific geological model and discussed the fluid flow and production performance during primary gas flooding. They found that a remarkable increment of oil recovery could be achieved in the karst fault system by primary gas flooding, and injecting gas near the top of the pay was recommended to fully take advantage of bottom water drive during gas-assisted gravity flooding. Liang and Hou (2021) had designed 2D visualized physical models of a fault-karst fractured-vuggy carbonate reservoir to investigate the flow characteristics and phase behaviors of nitrogen gas and foam assisted nitrogen gas floodings. They pointed out that since foam-assisted nitrogen gas drive could effectively inhibit the occurrence of gas channeling and improve swept volume, this technique could be employed in oil recovery enhancement in the fault-karst systems efficiently. In fact, these physical models mentioned above are all designed and constructed to represent specific reservoir settings in actual fields, rendering the generalization of the associated experimental results to other fault-karst reservoirs difficult.
Besides, Hou et al. (2016) had established a three-dimensional physical model to simulate different waterflooding methods (continuous, intermittent, and pulsed injection of water) in a fractured-vuggy reservoir based on a geological unit in the Tahe oilfield. Wang, 2018, Wang, 2020, Wang et al., 2012, Wang et al., 2014 had investigated the fluid flow characteristics, well pattern optimizations, inter-well interferences and remaining oil distributions of fractured-vuggy reservoirs based on different water-drive physical models. In summary, although these studies had focused on waterflooding characteristics, the corresponding physical models were all fabricated according to the general fractured-vuggy reservoirs, but not the fault-karst carbonate reservoirs.
Overall, the complex geological structure and the strong heterogeneity make it difficult to develop fault karst carbonate reservoirs by water-drive effectively and economically. Until now, there is no systematic study on well pattern optimization during waterflooding for the typical fault karst carbonate reservoir. In this paper, two 2D visualized physical models were designed and fabricated firstly based on the typical geological structure of the fault-karst systems in the vertical and horizontal directions respectively. Then several groups of control waterflooding experiments had been laid out and performed to investigate water-drive performance of fault-karst carbonate reservoirs under different well patterns and injection-production modes. Finally, some main experimental results of water-drive performance were discussed in detail and a series of valuable recommendations related to the optimization of well patterns and injection-production modes were obtained. The authors believe that this study can provide instructive references for well pattern optimizations and in designing waterflooding operational parameters in actual fault-karst carbonate reservoirs.
Section snippets
Design and fabrication of 2D visualized physical models
The fault-karst carbonate reservoirs are composed of different types of storage and flow spaces – the complicated fault-karst systems are composed of faults, caves, vugs, fractures, and pores with different scales. Typically, fault-karst traps with relatively high production of hydrocarbon can be detected via their funnel or flower shape in seismic profile and irregular distributions of branches or band shape in the plane view (Deng et al., 2022; Lu et al., 2017; Zhiwen et al., 2020). Fig. 1
Experimental section
Based on the purpose-made 2D visualized physical models, several groups of control waterflooding experiments had been laid out and performed to investigate water-drive performance of fault-karst carbonate reservoirs under different well patterns and injection-production modes. All details related to the experimental investigation are introduced in this section, including the used fluids, established setups, conducted procedures, and designed scenarios.
Results and discussion
As designed experimental scenario above, lab investigations on the water-drive performance of the typical fault-karst carbonate reservoir had been performed favorably based on the fabricated 2D visualized physical models. Experimental results of the two physical models were calculated and summarized as shown in Table 4, Table 5, both of which have listed the injected volume at water breakthrough, total injected water volume, total oil recovery, cumulative water cut, and cumulative water-oil
Summary and conclusions
In this study, based on the typical geological structures of the fault-karst carbonate reservoirs in the vertical and horizontal directions, two 2D visualized physical models were designed and fabricated respectively to perform water-drive experiments and investigate production performance under different well patterns and different injection-production modes. Accordingly, the following significant summary and conclusions can be obtained:
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Average oil recovery can reach 62.96% for the vertical
Author statement
Ke Sun: Conceptualization, Methodology, Investigation, Writing – original draft, Writing – review & editing; Huiqing Liu: Conceptualization, Supervision, Methodology; Juliana Y. Leung: Writing – review & editing; Min Yang: Resources, Investigation; Jing Wang: Writing – original draft, Investigation; Xiang Li: Investigation, Writing – review & editing; Zhijiang Kang: Investigation; Yun Zhang: Figure drawing.
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.
Acknowledgments
This work was supported by the Joint Fund of NSFC for Enterprise Innovation and Development (Grant No. U19B6003-02-06) and the National Natural Science Foundation of China (Grant No. 51974331 & Grant No. 52004303). The authors would like to sincerely acknowledge these funding programs for their financial support. Particularly, the support provided by the China Scholarship Council (CSC) during a visit of Ke Sun (File No. 202106440065) to the University of Alberta is also sincerely acknowledged.
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