It has been previously shown that Nernst’s equation is not reliable for the interpretation of spontaneous potential (SP) measurements acquired in hydrocarbon-bearing rocks. We have examined whether the difference between borehole SP measurements and Nernst-equation predictions could be used to estimate in situ hydrocarbon saturation of porous rocks. For this purpose, a new petrophysical model and a mechanistic finite-difference algorithm for simulating SP borehole measurements in the presence of mud-filtrate invasion are used to establish the limits of detectability of hydrocarbon saturation using only SP logs. We find that the optimal conditions for the detection of hydrocarbon saturation from SP borehole measurements are when (1) capillary forces dominate the process of mud-filtrate invasion, (2) the matrix-pore interface region, known as the electrical double layer, has a relevant impact in the diffusion of counterions, and (3) the electrolyte concentration of drilling mud is greater than that of formation water. We also determine why high values of the endpoint of the water relative permeability favor the detection of hydrocarbon-saturated rocks with the SP log. Using measurements acquired in three key wells within a mature and active hydrocarbon field, three blind tests find that our petrophysical model together with the mechanistic SP simulation algorithm enable the estimation of hydrocarbon saturation from SP borehole measurements without the need for resistivity logs or porosity calculations. The estimation is reliable when the volumetric concentration of shale is negligible, the pore network structure is constant throughout the reservoir, and radial invasion profiles are similar to those observed in the calibration key wells.

中文翻译：

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从钻孔自发势测量估算多孔岩原位烃饱和度

先前已经证明，能斯特方程对于解释含烃岩石中获得的自发势能（SP）测量值是不可靠的。我们已经研究了钻孔SP测量值与能斯特方程预测之间的差异是否可用于估算多孔岩石的原位烃饱和度。为此，使用了一种新的岩石物理模型和一种机械有限差分算法，用于在存在泥浆滤液侵入的情况下模拟SP井眼测量，以仅使用SP测井来确定油气饱和度的可探测性极限。我们发现，从SP井眼测量中检测烃饱和度的最佳条件是（1）毛细作用力主导泥浆滤液侵入过程，（2）基质-孔隙界面区域，所谓的双电层，对平衡离子的扩散有重要影响，（3）钻井液的电解质浓度大于地层水的浓度。我们还确定了为什么水相对渗透率终点的高值有利于使用SP测井法检测烃饱和岩石。使用在成熟且活跃的油气田中的三个关键井中获得的测量结果，三个盲测发现，我们的岩石物理模型与机械SP模拟算法一起，可以通过SP钻孔测量估算烃饱和度，而无需进行电阻率测井或孔隙度计算。当页岩的体积浓度可忽略不计，整个储层的孔隙网络结构恒定时，估算是可靠的，