--> Post-Drill Analysis Of Pore Pressure And Fracture Gradient From Well Logs And Drilling Events – An Integrated Case Study Of A High Pressure Exploratory Well From Panna East, Mumbai Offshore Basin, India

AAPG Asia Pacific Region GTW, Pore Pressure & Geomechanics: From Exploration to Abandonment

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Post-Drill Analysis Of Pore Pressure And Fracture Gradient From Well Logs And Drilling Events – An Integrated Case Study Of A High Pressure Exploratory Well From Panna East, Mumbai Offshore Basin, India

Abstract

Over-pressure, one of the important drilling hazards seen globally, is mainly associated with narrow operating windows resulting in severe well control incidents and lot of nonproductive times (NPT). Accurate knowledge of pore pressure is fundamental to any safe and economic well construction. Mumbai Offshore Basin, a pericratonic rift basin in the western continental shelf of India, covers about 148,000km2 from coast to 200m isobath, sedimentary fill ranges from 1100-5000m. It accounts for nearly two-thirds of the annual hydrocarbon production of India. The studied vertical exploratory well has been drilled in the Panna East area of Heera-Panna-Bassein tectonic block to explore the Panna formation (Paleocene-early Eocene). The well, drilled to the Cretaceous basaltic basement, has a TD of 4100m with 33m water depth. Predrill pore pressure analysis from seismic velocity and offset wells estimated a pressure ramp in the interval 2500-3000m with pore pressure increasing from 10.2-13.5ppg tentatively, followed by a consistent overpressured shaley section of Panna formation till basement with maximum pore pressure reaching 14.7ppg. Pore pressure against Early Miocene-Oligocene shales of Bombay and Mahim formations (1265-2050m) has been interpreted as 8.8-10.8ppg from log responses and drilled with 10.5-12.5ppg mud. Mid Eocene Bassein formation limestones (2121-2657m) show sub-hydrostatic pressure condition, as measured in MDT. Underlying early Eocene Panna shales showed deviation from Normal Compaction Trend on resistivity and sonic logs, resulting in an increase of pore pressure from 9-11ppg in the interval 2670-2850m, drilled with 10.3-11.3ppg mud. 9 5/8” shoe was set at 2848.5m and FIT recorded as 17ppg. Drilling 8 ½” section started with 11.5ppg mud. But at 2862-2864m increase in return flow have been observed, well killed with 12.5ppg mud. Operator started carrying out fingerprinting prior to every connection beyond this point to check influx trend and early kick detection. High background gas and connection gas continued in 2872-2919m and mud weight increased to 12.7ppg. At 3061m, gas increased to 30% and well started flowing, BOP was closed, SIDP 1230psi, SICP 1507psi, initiated well killing by drillers’ method using 14.2ppg mud weight. After two days of continuous endeavor (NPT), well was killed with 15.5ppg mud, average flow reduced to 0.3bbl/min. Further drilling continued with 15.5ppg mud till 3750m. This section recorded flow rate of 18-24bbl/hour at several depth points (3172, 3308, 3394, 3422, 3451, 3479, 3536, 3565, 3593, 3623, 3679, 3707 and 3736m). 3750m onwards, the Palaeocene shales showed clear indication of further increasing pore pressure trend on resistivity and sonic log responses, high background and connection gases, influx (recorded 7.5-12bbl/hour at 3850-3879, 3907, 3936, 3964, 3993, 4021, 4050, 4078m), cavings and multiple tight spots encountered while reaming, and these led to the increase of drilling mud weight from 15.5ppg to 16.8ppg. Upon reaching TD at 4100m, fingerprinting carried out and flow check in trip tank recorded a 7.5bbl/hour influx with 10-13% background gas and 23% connection gas. Well finally has been killed by drillers’ method with 18 ppg mud prior to final pullout to surface. In terms of pressure related issues, this well had been very critical because of the Panna shales. Top of overpressure marked at 2670m; pressure ramp in 2670-3061m interval with pore pressure increasing from 9ppg to 15.5ppg in Early Eocene shale, followed by a long continued Paleocene to Early Eocene overpressured shale dominated section till 4090m with pore pressure 15.5-17.75ppg, resulting a narrow mud window of 1.5ppg at TD. Final postdrill analysis will be presented (in extended abstract) with overburden, interpreted pore pressure, fracture gradient, LOTs (present and offset wells), wireline pressure points, detailed drilling events (influx, connection gas, tight spots etc.) and mud weight used.