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Extend Surface Casing Deeper to Eliminate Borehole Instability in Oil Field Turnkey Project


 In one of the largest oil fields in Middle East, the typical well design is to set the conductor just 40 ft below the ground, the surface casing 100 ft inside next competent formation due to shallow total loss zone below the conductor and the intermediate casing in the first competent formation before entering the reservoirs interval. The intermediate section includes a combination of formations that have total losses zones and extremely unstable shales, especially when drilled with total losses.
 In most cases, encountering lost circulation results in formation collapse. The severity of the formation collapse risk in this section ranges from simple tights spots cleared with reaming up and down to stuck pipes, sidetracks and in several cases plug and abandonment of wells. To reduce the likelihood and severity of this risk, there was a need to review the casing seats to isolate unstable formations before entering the intermediate loss zones.
 A detailed study was carried out where diverse options were analyzed, including adding a casing string and changing casing seats of existing casing strings. The study concluded that keeping the same number of casing strings and deepening only the surface casing seat as close as possible to the intermediate loss zone, covering the entire unstable formation but not entering the intermediate loss zone, would be the most efficient solution. Experts supported this change based on the principle that dealing with an unstable formation when it is below the loss zone is easier than when the same formation is above the loss zone.
 The new casing seat choice has been successfully implemented at over 100 wells, with 100% success ending the borehole instability problems. Since then there has been no sidetrack or location plugged and abandoned for borehole instability in intermediate section.
 This paper encompasses a comprehensive analysis of the mechanism of unstable formations reaction to the loss circulation, resultant stuck pipe mechanisms, unsuccessful attempts to cure losses in this zone, improving fly mixed mud inhibition, impacts of modifications in well design on zonal isolation and finally reaching the safest, time and cost-effective solution.