In a specific reservoir of an offshore field Abu Dhabi, several layers of the reservoir are producing subject to pressure depletion while other layers are partially or over pressurized due to water injection. Drilling through multi-layered reservoirs involves several challenges: faults and fractures, differentially stuck pipe and even total loss/kick scenarios due to pressure difference between the different layers. For reentry wells isolating the producing from injection layers is not possible due to the existing well schematic restrictions, completion requirements and target hole drainage size and location.
MPD was proposed to utilize a mud weight that minimizes the differential pressure between the producing and injecting layers within reservoir which mitigates the drilling problems related to high differential pressure. The game plan was to start drilling the depleted zone with the lowest possible mud weight and to enter the injecting zone (higher pressure) with this mud weight to calculate the exact pressure of the formation using the early kick detection and automatic control features of the fully automated MPD system, in this way reducing the amount of overbalance in the depleted layer if feasible.
Two wells were drilling using this method successfully. On both wells, a lower than conventional mud weight drilling fluids was used and the MPD system tested and verified the pore pressure of each of the reservoir layers. The calculated pore pressures were less than the wells prognosis. Swaying from the conventional mud weight consideration and the verification of pore pressure did minimize the differential pressure across the two layers which eventually eliminated the chances of drilling fluids losses and drill pipe differential stuck. The fully automated MPD system dealt safely with all the influxes during the pore pressure verifications tests. The new approach succeeded in solving the high differential pressure problem in the reservoir as drilling progressed shoe-to-shoe without interruption. The lower mud weight used had extra benefits in areas that historically required none-damaging weighting agent, this requirement was avoided by eliminating the need for this agent, FPWD was recorded across depleted reservoirs without pipe stuck events. Moreover, it was obvious the rate of penetration was higher on these two wells than offset wells in the same field when conventional mud was used.
For the first time in the UAE a closed-loop fully automated MPD system was utilized to lower the mud weight used when drilling across the reservoir, first time dynamic pore pressure tests were utilized to ascertain the wells prognosis and the first time FPWD was successfully recorded under substantially high differential pressure. In conclusion, the MPD was proven to be the right solution to overcome the uncertainty in pressure resulted from pressure maintenance program and reservoir depletion.