The drilling of wells offshore West Madura, East Java, can be challenging. The geological structure of the area often requires drilling at high deviations with large step-outs, through formations consisting of sands, shales and carbonates. As a result, wellbore stability issues are frequently encountered. In order to lower the associated risks, a pre-drill multidisciplinary study was performed to provide a robust set of mud weight windows for the drilling of development wells in this very challenging environment.

Two development wells have been drilled to date in this development. The pre-drill multidisciplinary study enabled the wells to be drilled faster and cheaper than planned. Drill-in caliper data showed mostly an in-gauge profile, suggesting that adequate mud weights had been used for the drilling process.

However, when tripping out, time-lapse borehole enlargements were observed from caliper data in some of the shale formations. The hole enlargements complicated the hole cleaning process, leading to several tight hole events.

All relevant data from the two development wells were thoroughly investigated. These data included, among others: daily drilling, geological and mud reports, time-lapse caliper log data, logging while drilling data, caving reports, mud programs, SCE reports, well surveys and lithology logs.

Though there exist several possibilities for the root cause of the borehole enlargements, the data analysis identified mud pressure penetration into shale formations as being the most likely mechanism. For instance, hole enlargements were not observed in the sand and carbonate formations, but shale sections demonstrated time-lapse borehole enlargements even when drilled with high mud weights. Splintery shale cavings were sometimes reported, which could be an indication of supercharged shales. The inhibited water-based muds which were used for drilling of the development wells were potentially susceptible to mud pressure penetration.

This paper summarizes pre-drill mud weight recommendations resulting from the pre-drill multidisciplinary study, along with the post drill evaluation. The results of the investigation into the time-lapse borehole enlargements are provided, along with recommendations in order to minimize the risk of such enlargements.

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