Determination of shale osmotic pressure using spontaneous potential log

Wellbore instability in shale results in annual expenditure for petroleum industry especially for shale gas/oil development. Osmotic pressure is one of the most significant effects, which affect wellbore stability during drilling in shale formation using water-based mud. Until now, methods to calculate osmotic were only undertaken in the pre-drilling or post-drilling phase. This paper presents a new developed method to estimate shale osmotic pressure using an spontaneous potential (SP) log, which makes it possible to calculate osmotic pressure during drilling. The relationship between shale osmotic pressure and SP log was investigated. It was found that osmotic pressure and SP value both depend on the shale cation exchange capacity, which was used to bridge the SP log and osmotic pressure. An empirical equation relating the SP value and osmotic pressure was developed, which can compute osmotic pressure using the SP value. Moreover, the depth-dependent earth temperature was considered in the osmotic pressure calculation. A case study was undertaken using the SP log of a wellbore section located in Shengli Oil Field, China.