缝洞型碳酸盐岩油藏流固耦合数值模拟

Numerical modeling for coupled hydro-mechanical processes in fractured-vuggy carbonate reservoirs

关键词:  缝洞型油藏  渗流-自由流耦合  流固耦合  离散缝洞模型  数值模拟

Key words:  fractured-vuggy carbonate reservoirs  coupling porous-free-flow  hydro-mechanical coupling  discrete fracture-vug model  numerical simulation

摘要:

缝洞型碳酸盐岩油藏分布广、储量大,但储集空间类型多样并可跨越多个尺度,宏观上表现为渗流-自由流耦合特征;且埋藏较深,超过5300 m,裂缝和溶蚀孔洞在开发过程中易发生变形,具有强应力敏感性。考虑储层介质的弹性变形,建立离散缝洞模型的流固耦合数学模型及其数值模拟方法。其中多孔介质渗流区域采用Biot方程,在渗流场和应力场中均对裂缝进行降维处理,建立离散裂缝的Biot流固耦合模型;溶洞为自由流区域,采用Navier-Stokes方程;两个区域间通过扩展的Beavers-Joseph-Saffman条件进行耦合。应用混合有限元方法对该流固耦合模型进行数值求解,其中渗流区域采用经典的Galerkin有限元方法,自由流区域采用Taylor-Hood混合元方法,通过数值算例验证模型和方法的正确性。结果表明:溶洞中的压力传播速度较快,相对于渗流区域可视为一流动等势体;在降压生产过程中,裂缝尖端和溶洞附近区域易发生较大面积的破坏,而较高的流体压力对于溶洞和裂缝壁面具有一定的支撑作用,因此在此类油气藏的开发中应适当采取保压措施,以避免溶洞坍塌和裂缝闭合。

Abstract:

Many fractured-vuggy carbonate oil reservoirs have been found around the world, which have significant contributions to the oil and gas reserves. The storage and transport spaces of such reservoirs mainly consist of pores, fractures and cavities, with dimensions ranging from micrometers to meters, which can result in coupling of porous flow and free fluid flow in the reservoir. The depths of theses reservoirs in Northwest China are over 5300 m. Therefore, the geo-mechanical effects need to be considered. In this study, a novel hydro-mechanical model based on the discrete fracture-vug model was developed. The hydro-mechanical coupling process was described by using Biot 's equations within the porous matrix and Stokes equations within the vugs, and an extended Beavers-Joseph-Saffman boundary condition on the porous-fluid interface was applied. The corresponding numerical scheme based on a finite element method was proposed. The classical Galerkin finite element method was used for discretizing the Biot equations, and the Taylor-Hood mixed element method was applied to the free fluid flow region. Several numerical examples were designed to validate the proposed model and to demonstrate the computational procedure for evaluation of the hydro-mechanical behaviors of the fractured-vuggy carbonate reservoirs. The simulation results show that a cavity can be considered as an equal-potential flow region due to a faster propagation of the pressure within the cavity than that in the porous flow region. During the depletion production, mechanical failure of rock matrix can occur around the factures tips and cavities boundary area, and high fluid pressure is beneficial to avoid the collapse of the cavities and the closure of the fractures. Therefore, effective pressure maintenance during production of the fractured-vuggy carbonate reservoirs is recommended.