DLH油田低渗砂岩孔隙分形定量表征方法研究

A Study on Fractal Quantitative Characterization Method of Low Permeability Sandstone Pore in DLH Oilfield

针对常规表征方法难以精确表征低渗砂岩储层孔隙空间分布复杂性和不规则性的问题,提出了适用于低渗砂岩储层的分形维数计算方法,实现了低渗砂岩储层孔隙特征的定量表征。基于不同分形维数计算方法差异性的分析,优选采用MIFA方法求解低渗砂岩储层的分形维数(在2.042~2.324),相关性最佳;确定了排驱压力、平均孔喉半径、变异系数以及均值系数作为储层孔喉分布复杂程度和非均质程度的综合表征参数;基于恒速压汞分形维数的求解,发现低渗砂岩储层非均质程度呈中小孔喉大于微小孔喉,喉道分布大于孔隙分布的特点;低渗砂岩储层的分形维数与启动压力梯度和应力敏感性损害率的实验结果均存在一定的相互关系,分形维数越大,孔喉分布的非均质性越强,启动压力梯度越大且应力敏感性的损害程度也将加剧。低渗砂岩储层分形维数的计算可用于室内实验结果的定性预测和判断,也可作为油藏工程中应用相渗曲线时的重要判别标准。

In view of the difficulty of conventional characterization methods to accurately characterize the complexity and irregularity of pore space distribution in low-permeability sandstone reservoirs,a fractal dimension calculation method is proposed to realize the quantitative characterization of pore characteristics of low-permeability sandstone reservoirs.Based on the analysis of the difference between different fractal dimension calculation methods,the MIFA method is preferably used to solve the fractal dimension of low permeability sandstone reservoirs(between 2.042 and 2.324),with the best correlation.The displacement pressure,average pore throat radius,coefficient of variation,and mean coefficient are determined as the comprehensive characterization parameters of the complexity and heterogeneity of reservoir pore throat distribution.Based on the solution of the fractal dimension of mercury injection at a constant rate,it is found that the heterogeneity of low-permeability sandstone reservoirs presents the following characteristics:small and medium pore throats are larger than small pore throats,and throat distribution is larger than pore distribution.There is certain correlation between the fractal dimension of low-permeability sandstone reservoirs and the experimental results of starting pressure gradient and stress sensitivity damage rate.The larger the fractal dimension,the stronger the heterogeneity of the pore throat distribution and the larger the starting pressure gradient is needed and the damage to the stress sensitivity will increase.The calculation of the fractal dimension of low-permeability sandstone reservoirs can be used for qualitative prediction and judgment of laboratory experimental results,and also as an important criterion for the application of relative permeability curves in reservoir engineering.