One the most common methods for well stimulation includes the use of mobile offshore drilling units (MODU) to pump fluids into the well with the main objective of restoring production or injection. A rig-based well stimulation is an expensive operation and in some cases a separate stimulation vessel is used to store and pump fluids. Economics to justify a stimulation operation may require a number of wells (depending on well productivity) to make the project economically viable. New industry developments in the last few years have made the use of special intervention techniques from vessels relatively popular. However, a riser based system remains the preferred method to intervene/stimulate deepwater wells. Riserless systems are becoming more and more popular but their applications vary from region to region based on water depths, sea conditions, equipment and technical personnel availability.
Since start of production from the Jubilee field, several oil producers had experienced gradual productivity decline with a significant reduction in flowing bottom hole pressure, despite an aggressive pressure maintenance program with water and gas injection. Following the success of previous acid stimulation work carried out off the rig in an attempt to restore productivity, a rigless acid stimulation treatment on the impaired Jubilee wells wasproposed. An initial feasibility study was conducted, whichindicated that a Rigless stimulation operations using a Multi Purpose Supply Vessel (MPSV) was considered to be a commercially and technically feasible operation. While the primary objective of the rigless stimulation campaign was to increase production, this work also demonstrated that the rigless intervention from the MPSV is a cost effective and technically feasible option for a long term campaign. The operation was successfully conducted using two vessels. Fluids where pumpd into the well using 4 × coiled tubing strings deployed in 1,600 m water depth. This paper explains the methodology used for the development of a rigless stimulation campaign in the Jubilee field, offshore Ghana.
