The biggest challenge was the differential pressure between the JST (tight gas formation) and JSK (naturally fractured carbonate) formations. In addition, an overbalanced condition was desired once inside the JSK formation due to the risk of H2S and to minimize the possibility of a gas influx from the JST formation.
High bottomhole temperature was another risk as it could modify mud rheology and fluid specific volume. Especially on deeper wells with a narrow mud weight window and high temperatures, the team knew that the difference between theoretical and actual values of drilling fluid properties couldn’t be ignored.
The actual well casing configuration and cement tops were used for modeling, with a surface temperature of 34°C and a bottomhole temperature of 181°C at 6,000 m (TVD) assumed. The temperature profile was then calculated for a near-balance condition at total depth.
For this project, MPD was used to allow the operator to dynamically manage the influxes coming from JST, thus reducing pressure-related events NPT. Improved well control was also anticipated with MPD because it provided flexibility to manipulate annular pressure.
The 6 ½-in. section was spudded with a 1.50 SG mud and gradually decreased to 1.46 SG as drilling depth approached the JSK formation. A wellhead pressure of 80 psi was kept while drilling or circulating. During connections, to compensate for the pressure provided by annular friction losses, the wellhead pressure was raised to match the circulating bottomhole pressure.
Trip gas was a constant factor while drilling the well due to the high bottomhole temperature. The use of MPD equipment allowed operations to continue during influx events, which saved the rig up to 30 hours in well control procedures.
The well was drilled down to 6,424 m with a 1.50 SG drilling mud. Only one trip was performed before reaching TD.
Overall, the team noted that one of the most remarkable benefits from using MPD in this project was the minimizing of NPT while drilling related to formation influxes during connections and tripping. Also, no differential sticking or losses were experienced in the transition zone from JST to JSK.
Details from this case study can be found in IADC/SPE 122200, “Successful Application of MPD Technique in a HP/HT Well Focused on Performance Drilling in Southern Mexico Deep Fractured Carbonate Reservoirs.”