基于三维流-固耦合模型的油井出砂细观机制研究

Research on micromechanism of sand production in oil well based on 3 dimensional coupled fluid-solid model

油井出砂机制的研究是提高油藏产能和石油开采成本减小的关键课题,而常规的宏观力学理论和方法不能全面反映油藏开采过程中油井出砂的发生和发展。鉴于砂岩储层的物理性质和射孔试验特征,从岩土力学的角度建立基于柱坐标系的三维颗粒流数值模型,与理论分析成果进行比较,以说明该细观数值模型可行性,有效地模拟出砂过程中的渗流及流-固耦合效应。在该基础上,综合考虑流体压力梯度力和拖曳力,基于PFC3D模型模拟流体不同运动时的砂岩性态。数值分析得到的模型宏观应力图形说明流体运动对砂岩力学特性的影响不可忽略,且在相同条件下,流量越大,砂岩的塑性区越大,形成砂岩破坏出砂的几率也越大。同时,不同工况的砂岩黏结分布和颗粒转动图形表明,相同条件下流量越大,颗粒间平行黏结破坏越多,颗粒转动越大,失去黏结约束的颗粒也越多,出砂量就越大,可见两种细观特征图形与宏观应力图形变化规律一致,该模型可用于油井出砂机制的研究,可为出砂量预测及出砂控制提供新的研究思路。

Research on mechanism of sand production has been the key topic to improve the oil production and reduce the oil extraction cost; however, current investigations are mainly based on rock macromechanical theory and methods; and these achievements cannot comprehensively reflect initiation and propagation of the sand production. Considering physical property of the sand reservoir and real characteristics of the perforation test, 3-Dimensional numerical model based on PFC^3D under cylindrical coordinate system from a geomechanics prospective was established to research the fluid-solid coupling effect and the micromechanism of sand production. The comparison with theoretical results shows the feasibility of the forward 3-Dimensional micro numerical model; and the model could simulate the fluid flow and fluid-solid coupling effect in the process of sand production. Based on the above numerical model, simultaneously considering pressure gradient force and drag force, research on initiation and propagation of the sand production was carried out based on PFC3D considering different fluid flowing conditions. The figures about simulated stress distribution indicate that fluid flow affecting the sand mechanical characteristics should not be neglected during oil extracting; and under the same conditions, the greater the flow rate is, the more the bonds between the particles are; and the larger the sandstone plastic zone is, and also the greater the probability of the sandstone broke and sand production is. In addition, there were research achievements about the initiation and propagation of the broken parallel bond and magnitude of the particles rotation considering different cases. The figures show that under the same conditions, the larger the flow rate is, the greater the range of the plastic zone and the particle rotation was to result in more particles losing restraints; and also the more volume of the sand production. The above numerical results explained the same developing trend comparing the two kinds of figures about micro characteristics of the particles between the figures about macro stress distribution, and the results indicated that the forward micro numerical model could simulate the sanding mechanism and also provide a new thought to predict the sand volume and control the sand production.