碳酸盐岩油藏酸化施工最优注入条件影响因素分析

Analysis of influencing factors on the optimum stimulation conditions of the acidizing treatment in carbonate reservoirs

碳酸盐岩油藏酸化效果取决于酸蚀岩石生成的溶蚀模式,对于给定的油藏及选定的注入酸来说,溶蚀模式与酸注入地层的速度有关.在最优注入条件下,酸溶蚀岩石形成一些窄且长的蚓孔,这些蚓孔可以穿过井筒周围的污染带,大大地改善甚至是提高近井地带的渗透率.由于技术条件限制,最优注入条件只能通过岩芯尺度的实验或数值模拟得到,而将岩芯尺度的最优注入条件应用到油藏酸化施工中时,需要明确岩芯形状、尺寸比例等参数的影响.本文基于梯形流动单元,使用非结构网格对求解区域进行离散、有限体积方法对控制方程进行离散,数值求解碳酸盐岩酸化反应流模型.定义形状因子和比例因子分别表示岩芯的形状变化和尺寸比例,并研究其对酸化溶蚀模式、最优注入条件等参数的影响.结果表明:以流量表示注入条件时,岩芯形状对最优注入条件没有影响;以最优注入流量将酸液注入不同形状的岩芯得到的蚓孔结构具有相同的分形维数,说明岩芯形状对溶蚀模式没有影响;当比例因子小于1时,最优注入流量随比例因子的增加而增加,当比例因子大于1时,最优注入流量保持不变;径向流对应的比例因子处于最优注入流量保持不变的范围,因此,酸化施工优化设计时的岩芯实验,只需将酸线性地注入岩芯,而不需进行难度更大、成本更高的径向流酸化实验,但实验用到的岩芯截面直径应大于其长度,且将岩芯实验的结果应用于油藏酸化施工时,应使用流量表示最优注入条件.

The acidizing result of carbonate reservoirs depends on the resultant dissolution patterns. For a given reservoir and a selected type of acid, the dissolution patterns are related to the injection rate of the acid. At the optimum stimulation conditions, the injected acid dissolves some of the minerals, forming some narrow and long channels, known as wormholes. These wormholes can bypass the near-wellbore damaged region and hence recovers or increases the permeability near the wellbore. Because of the limitation of the technology, the optimum injection conditions can only be obtained through the core scale experiment or numerical simulation.Consequently, it is necessary to understand how the geometry of the core used in the experiment can affect the wormholing dynamic and the acidizing results when extrapolate the optimum injection condition from pore scale to reservoir scale. In this paper, the wormhole formation during carbonate acidizing is simulated based on the trapezoidal domain shape by solving the two-scale reactive flow model. The finite-volume method is used to discretize the governing equations and the computational domain is discretized by using Delaunay triangulation. Two parameters, shape factor and aspect ratio factor are defined to represent the geometry and aspect ratio of the rock, respectively and the effects of them on the wormhole fractal dimension, the dissolution structure and the optimum injection conditions are investigated. It is found that there is no effect of domain shapes on the optimum injection condition when the optimum injection condition is expressed in terms of injection rate. The wormhole fractal dimension is found to be same when acid is injected into cores with different shapes at the same injection rate, which indicates that the domain shape has no effect on the dissolution structure. Besides, the optimum injection rate increases with the increase of the aspect ratio factor when its value is less than 1, and remains unchanged when the aspect ratio factor is greater than 1. From this study, it can be concluded that the performance of carbonate acidizing can be characterized by the results obtained from linear cores acidizing experiments. So, there is no need to conduct radial experiments which are difficult and expensive. Moreover, it should be noted that the core diameter used in the experiment should be greater than its length. The optimum injection conditions should be expressed in terms of injection rate when it is extrapolated from linear core experiment to reservoir acidizing treatment.