INVESTIGATION OF IMBIBITION SATURATION EXPONENT UNDER MIXED SALINITY ENVIRONMENT

摘要

An important challenge in resistivity well logging is the interpretation of electrical data to derive the water/oil saturation of an oil reservoir during sea water injection for instance. The sea water resistivity is then commonly used to obtain the water saturation from Archie’s law,regardless of the potential mixing between formation brine and sea water;this is certainly valid close to the injection wellbore. However low frequency resistivity tools can have high enough depth of investigation to be affected by this mixed salinity environment. Cross-well electromagnetic(EM)resistivity method for instance,recently used to provide fluid distribution mapping at inter-well scale,can be significantly affected by the change in water resistivity during water flooding. A core analysis program was initiated to understand the change of resistivity in mixed salinity environment using the porous plate resistivity method in imbibition cycle. Despite the known existence of hysteresis between drainage and imbibition cycles,the cementation factor m and saturation exponent n,obtained from Archie’s law [1],are generally measured in the laboratory respectively before and during a primary drainage porous plate experiment with resistivity measurements. Imbibition tests are rarely performed,and when they are,no mixing effect is observed since the same formation brine is injected into the sample. In this paper,the brine mixing effect on rock electrical properties during a porous plate imbibition test is investigated and a simple salinity model is proposed to derive the equivalent resistivity of the brine mixture for appropriate water saturation calculation.