Drilling in Area of Unstable Shallower Formations Prone to Rig Pad Subsidence: Turning the Impossible to Possible

Abstract

In a major Oil Field in Middle East, eight wells are abandoned and ten had narrow escape, while drilling surface hole due to shallow sand collapse resulting in cellar and surrounding area to subside, locally known as Cellar Collapse 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 cellar collapse prevention and mitigation plan was divided in three phases. In first phase, cellar collapse risk level was identified by generating a risk identification map of the field after analyzing almost all the wells (more than 500 wells) drilled in this field. After identifying with help of this map that cellar collapse risk exists mainly in a specific area of the field, temporary plan was not to drill in this high-risk area of +/- 2 km2 of field. 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 cellar collapse 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 cellar collapse 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 collapse mechanism, is adopted for the first time the field. Considering its success rate, it can be adopted in anywhere since most fields in middle east have incompetent shallow formations. This paper provides of details including mechanism of cellar collapse, severity of risk, conventionally attempted unsuccessfully prevention attempts, newly developed strategy followed by its successful applications and lessons learnt.