Abstract

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Drilling through shale formation can be challenging and sometimes results in wellbore instability problems due to the reaction between hydrophilic shale and drilling fluids. The study of wellbore stability in shale is quite important because 75% of all formation drilled worldwide are shale formations and 90% of all wellbore instability problems occur in shale formations costing the industry more than $1 billion USD/year. This study was carried in selected fields (FIELD A and FIELD B) in Niger Delta, Nigeria to evaluate the properties of the shales and its effects in wellbore instability during drilling operations. The properties evaluated are the shale's permeability, cation exchange capacity, and mineralogy composition. The X-Ray diffraction method was used to ascertain the mineral content and distribution of the selected shale samples across the wells in the fields A and B. Results obtained showed that the shale samples had typically very low permeability between 0.1353md to 0.2110md. The cation exchange capacity of the shales was observed to be low also, between (2.5-10.5)Meq/100g. The mineralogy of the shale results obtained showed that several clay minerals were identified, including palygorskite, nacrite, kaolinite, chlorites among others. Smectite group of clay minerals was observed as mixed layers in the form of sodium montmorillonite and ChloriteMontmorillonite.Also test results indicated that samples in field A contained 55% clay minerals and 45% nonclay minerals, while samples in field B contained 58% clay minerals and 42% non- clay minerals. This is an indication that swelling tendencies of shale arising from drilling mud interaction with the shale even in the same well depends on the depth, shale composition mud type and composition.