东营凹陷低渗透砂岩油藏地层水电阻率对饱和度计算的影响

地层水电阻率R_w取值方法主要有典型水层反算法和地层水分析法。针对东营凹陷南斜坡沙三段、沙四段低渗透率砂岩油藏,当单井油、水层R_w取相同值往往饱和度计算结果与密闭取心分析存在矛盾。使用油层水分析测定的R_w计算的典型水层含水饱和度低于80%;使用典型水层反算的R_w油层含水饱和度计算偏高,说明即使在同一套油水系统中,油层和水层R_w也可能不同。从Archie公式进行推导并结合实际数据发现,油层中的R_w小于水层的R_w。从成藏角度分析认为,油气成藏后油层水主要为伴随烃运聚的高矿化度水赋存于岩石微孔隙中。含水量较低且与水层或围岩隔离,不易扩散或交换,得以继续保持成藏流体高矿化度特征;水层中大都保留了原始沉积的低矿化度水,且当中的盐分由于扩散或与围岩交换,随时间推移,矿化度会逐步降低,从而导致了油、水层不同的矿化度。这一认识对提高东营凹陷低渗透油藏饱和度解释精度具有重要的意义。

Effect of Formation Water Resistivity of Low Permeability Sandstone Reservoir in Dongying Sag on Oil Saturation Interpretation

Formation water resistivity (Rw) is a key parameter for oil saturation interpretation based on Archie equation.There are two methods to get this parameter,the standard water layer data back calculation or formation water analyzation.The water layer and oil layer Rw are always considered equally.But for Dongying Sag south slope of ES3 and ES4 low-permeability sandstone reservoirs,if making the oil layer and water layer Rw equal,there often appear same contradictions in the process of practical application.When using the Rw which is calculated by analyzing data of water in oil layer,the standard water layer Sw is less than 80%.But if the Rw was calculated by analyzing data of water in standard water layer,the Sw of oil layer is on the high side.It shows that even in the same oil-water system,the Rw of oil and water layer can be different.Firstly from Archie equation and the practical log data,we found that the Rw of oil reservoir is less than water layer.And then,by analyzing the reason from the angle of accumulation found that,after hydrocarbon accumulation the water of oil reservoir is mainly high salinity which is accumulated with hydrocarbon and saved in micro pores.This kind of water content is low and isolated from water layer and adjacent formations,so it is difficult to diffuse or exchange.Therefore,the water has the characteristics of high salinity.In the water layer,the formation water keeps low salinity,and the water salinity will reduce as time goes.So,these reasons lead to the different water salinities in oil and water layer.This knowledge has a significance to improve the accuracy of low permeability sandstone reservoir saturation and calculation of reserves.