MECHANISTIC DESCRIPTION, SIMULATION, AND INTERPRETATION OF SPONTANEOUS POTENTIAL LOGS

摘要

Spontaneous Potential (SP) and Gamma Ray (GR) logsare often the only measurements available in many oldhydrocarbon fields. Reliable coupled petrophysical interpretationof GR and SP logs requires previous accuratequantitative calibration for the effects of shale laminations,water electrolyte concentration, temperature,bed thickness, borehole diameter, hydrocarbon saturation,and invasion, among others. Although the lattereffects on SP logs are well known, there is no publishednumerical algorithm that quantifies the relative contributionsof all geometrical, petrophysical, and fluid effectson SP logs.This paper develops a new mechanistic description ofSP logs for their numerical modeling and petrophysicalinterpretation in vertical wells penetrating horizontallayers. The main objective of the new SP modelingmethod is to estimate water saturation reliably in wellswhere only SP and GR logs are available but whereresistivity and porosity calibrations can be performed ina key well within the same field or can be inferred fromcore data.The mechanistic SP modeling algorithm is based on thefundamental concept of anion and cation migration inaqueous solutions with unequal electrolyte concentrationvia diffusion and membrane potentials. It explicitlyaccounts for specific borehole conditions, temperature,variable shale properties, finite bed thickness, shalelaminations, hydrocarbon saturation, and invasion (includingsalt concentration profiles) in the numericalmodeling of SP logs. Further, hydrocarbon saturationeffects on SP logs account for the rock’s effective throatradius. Sensitivity studies are carried out to quantifyselectively the effect of each environmental and geometricalvariable on measured SP logs. The modelingalgorithm is also successfully used to reproduce SP logsacquired in the presence of salt and water saturationfronts in invaded hydrocarbon-saturated rocks. Examplesof application include sandstone sequences thathave internal shale laminations.