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Some of the oil and gas fields in the in Middle East are prone to a shallow sand subsidence while drilling conductor and surface sections, which may result in the in cellar and surrounding area to subside. In the worst case scenario, it may lead to a well abandonment. This issue is known locally as a cellar subsidence or sinkhole and carry high severity risk. Considering the out of proportion risk, drilling engineering team of project formulated an out of the box solution with enough contingencies to cope with this risk and drilled subsequent wells successfully.The task of sinkhole prevention and mitigation plan was divided in three phases. In first phase, its risk level was identified by generating a risk identification map of the field after analyzing almost all the wells (more than 500 wells) drilled. After identifying with help of this map that sinkhole risk exists mainly in specific areas, temporary plan was not to drill in this high-risk area of +/- 2 km2. However, it left this area of the reservoir undrained which was not acceptable to reservoir development.As neither leaving the area of the field undrained nor abandoning the wells due to ground subsidence was sustainable, the task was taken to the next phase to formulate a plan to make the drilling possible in this area or similar areas in this field or other fields. As such based on investigative analysis of all the previous incidents, the engineering team proposed a structural approach for an innovative and vigorous plan along with risk assessment and complete scope of contingency measures. This included pilot hole drilling, changing drilling fluid system, drilling parameters, identification of early signs of the risk, changing casing seats, utilization of casing while drilling and a pre-defined strategy to optionally cure losses based on depth of losses. Following the identification of the structure of the plan, a detailed procedure with specifics on fluid properties, expected rock behavior during drilling, detailed contingencies and flow charts were developed.With this plan, subsequent wells were drilled with 100% success, in high risk zone of sinkhole where the plan was further updated with lessons learnt on well by well basis.This unique technique which included more than one measures targeting each specific aspect of cellar subsidence mechanism, is adopted for the first time. Considering its success rate, it can be adopted in any other project, where similar situation is faced.This paper provides of details including mechanism of sinkhole, severity of risk, conventionally attempted unsuccessfully prevention attempts, newly developed strategy followed by its successful applications and lessons learnt. |
