Full Length ArticleTheoretical study on the injected gas override in condensate gas reservoirs
Introduction
As one of the unconventional gas reservoirs, condensate gas reservoir has very important economic value. During the development of condensate gas reservoirs, the pressure drop leads to the occurrence of retrograde condensate phenomenon, the formed condensate oil would block the gas seepage channels and hamper the gas production process [1], [2]. Therefore, condensate gas reservoirs are usually developed by the cyclic gas injection technique [3], [4]. The main function of the procedure is to maintain the formation pressure and prevent the occurrence of condensate oil. However, in the process of gas injection, gravity overburden of different degrees are likely to occur [5], during which the injected gas breaks through from the upper part of the reservoir and forms a secondary “gas cap”, greatly decreases the recovery efficiency. However, research on the gas override phenomenon are rarely found in literatures.
Experimentally, Zhang [6] carried out tests in a visual PVT cylinder and studied the phase behavior of condensate gas during the dry gas injection process. The results exhibited a lighter dry gas phase at the upper part, an intermediate gas phase (mixture of dry gas and condensate gas) in the middle, and in some cases, a heavier condensate oil phase (the oil phase was prone to emerge when the testing condition lay in the vicinity of the critic areas) in the lower part. Besides, due to the mass transfer process, the boundaries of the three pseudo phases would maintain a certain amount of time before they became blurred and vanished. Zhang’s research implied that the gravity differentiation of hydrocarbon components occurs after dry gas injection in condensate gas reservoirs.
Many other studies are focused on the gas override phenomenon taking place during the steam flooding process and the gas – liquid displacements. In the course of steam flooding, premature breakthrough of steam results in unfavorable sweeping areas and poor displacement efficiencies [7], [8]. Nzekwu [9] studied the effect of density difference on the override of injected agent. They observed the gravity overburden in liquid – liquid displacement with various density differences. Besides, they also reported the override and underride with different density combinations of injected fluid and the in – place liquid during simultaneous injection of gas and liquid. There were also studies that took the advantage of gas's own density to displace the bottom oil and achieved good results, especially in the dipping reservoirs, which exhibited significant effects [10].
Rossen [11] and Shi [12] proposed the gravity number to measure the degree of gravity overburden. The gravity number is related to the pressure difference between the gaseous and liquid phases. Rossen argued that the larger mobility would induce a more obvious gravity overburden, and in continuous injection mode, a larger injectivity would result in a higher speed and pressure, and the consequently smaller gravity number can mitigate the influence of gravity overburden.
Based on their study, Boeije [13] studied the influence of injection speeds on gravity overburden, and Boeije suggested different injection strategies for the injected foam with different mobilities to reduce the influence of overburden. Shi [14] confirmed with numerical simulation that the attainable maximum injection pressure at the maximum injection rate brought the most favorable result in the continuous foam injection process, and at the same time, led to the least development of the gravity overburden.
Some studies argue that the employment of surfactant or the surfactant – alternating – gas strategy can overcome gravity override [9], [15]. Shan [10] believed that the impact of gravity overburden can be mitigated by injecting a surfactant slug before the gas slug and maintaining the injection pressure below the maximum fracture pressure of the formation at the same time
Taking into consideration of the viscosity-gravity ratio, Stone [16] and Jenkins [17] studied the starting point where gravity overburden took place for gas-water mixing zones in rectangular and radial reservoirs. They divided the formation into gas phase zone, gas – water mixing zone and water phase zone, and they established a theoretical model to describe the gas – water mixed – phase displacement at steady state. Their studies indicate that the formation height and the absolute horizontal permeability of the reservoir only affect the injection pressure and would not affect the location where the gravity differentiation occurs. They noted that the increase of the horizontal pressure gradient would be an effective way to extend the flowing distance of the gas – water two – phase zone before the gravity overburden occurred. But their model did not consider the diffusion process and failed to describe the development of overburden with time.
Using the fractional-flow theory, Rossen [18] studied the effects of injecting positions on gas overburden during gas-water coinjection process. In their study, the water and gas were injected at the upper and lower part section of formation separately. They pointed out that during the injection process, the injection time interval would only affect the injection pressure and phase boundary shape, and would not affect the position where gravity overburden happened. Mehran [19] verified Rossen’s theory using a physical model filled with glass beads and confirmed the efficiency of the water – above – gas injection technique. However, their model have very strict requirements: Newtonian fluid, the instantaneously reached steady state, and without consideration of the diffusion process.
Generally, most research regarding the override phenomenon were concentrated on gas – liquid phases, and rare have they focused on the override in the gas phase alone. The major differences lies in that the diffusion process is of great importance for the emergence of gas override in condensate gas reservoirs.
In this paper, the injector – producer model is established according to the actual well spacing. A mathematical model is established on the basis of Darcy flow theory and concentrated matter transfer theory to predict the formation and development of the injected gas override. Besides, the effects of different injection strategies and reservoir properties on the development of gravity overburden were also studied. The results show that the gravity overburden is mainly affected by the density difference between the injected gas and the condensate gas, and the dry gas demonstrates the greatest degree of overburden, and the carbon dioxide can greatly mitigate the unfavorable effects caused by overburden. In the end, optimum development strategies are given according to the importance of different factors.
Section snippets
Mathematical formulation
This study aims to solve the problem of injected gas override during the cyclic gas injection process in condensate gas reservoirs. To achieve that purpose, a two dimensional cross section porous medium was constructed with width b and height h. The porous medium was considered to be saturated with condensate gas in the first place.
Generally, the mass transfer easily takes place during the development of condensate gas reservoirs. Besides, considering the density difference between the injected
Model validation
A 2D physical sandpack model is designed to monitor the overburden evolvement of the injected gas. The dimension of the model’s actual sand cell are 0.7 m × 0.7 m × 0.025 m. There are 16 sampling ports distributed on the surface to test the gas composition at different parts of the model. Interpolation method is used to generate the contour plots with the data obtained from the 16 sampling ports. Moreover, the inlets and outlets located on the left side and right side can be arranged in
Conclusions
This paper carried out theoretical studies concerning the gas override in condensate gas reservoirs developed by the cyclic injection technique. A mathematical model was established to describe the gas override behavior in condensate gas reservoirs. The mathematical model adopted the invented override term and the Maxwell-Stefan diffusion equation. The model reveals the flowing behavior of the injected gas and the original condensate gas under the influence of gravity, meanwhile, exhibits the
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
This work was supported by the National Natural Science Foundation of China. Grant No. 51974013.
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