Publications

Drilling in a highly depleted formation is a challenging operation and requires a proper pre-assessment study that takes into account the subsurface, geomechanics and drilling practices. This paper presents the case of a geomechanical modelling helped in the success of drilling a highly depleted formation in P field, Natuna Sea, which is one of the prolific hydrocarbon-proven basins in Indonesia. There are already several production wells in the field targeting multiple reservoir zones. After several years of production these reservoir zones are depleted and this creates a challenge for futuredrilling in the area.

A geomechanical model was built using available data from four offset wells, P-1, PA-1, PA-2 and PA-3. Some of the key challenges in offset well P-1 include pack-off events and tight holes while POOH and then RIH casing, as well as observations of cavings and fill on bottom while RIH with the BHA. P-1 well penetrated deeper Gabus Formation which is known to be highly overpressured. The other three offset wells, PA-1, PA-2, PA-3, were drilled much shallower than P-1 and did not penetrate the highly overpressured zone, hence these wells only experienced minor issues.

The pore pressure model in each offset well was calibrated using direct pressure measurements, gas data from mud logging, reported drilling events, and data from FracPack. minimum horizontal stress was calibrated using LOT/FIT data available from each of the offset wells and, more specifically, fracture closure pressure (FCP) reported by FracPack. The rock mechanical properties were calculated using uncalibrated, but regionally appropriate, empirical relationships in the absence of rock mechanical testing data. There were not any image data to show the presence of stress-induced wellbore failure in any of the offset wells so the magnitude and azimuth of the maximum horizontal stress was constrained using regional information. The final geomechanical model was calibrated and verified by comparing the predicted breakouts with the events observed while drilling the offset wells.

The planned well, PA-4, was expected to encounter multiple depleted reservoir zones. A wellbore stability analysis was conducted for this well to gain understanding of the risks while drilling and the results were used as guidance for the drillers. The end result was that the planned PA-4 well was drilled and completed safely, after following the pre-drill mud weight recommendations, with only encountering few drilling problems.

A post-drill analysis was conducted for well PA-4, where all the geomechanical parameters were calculated using actual log data. The key focus was the minimum horizontal stress which was calibrated using actual FCP data from the FracPack job. Ultimately, there were only minor adjustments to the pre-drill geomechanical model and it is considered that the newly-constrained and calibrated geomechanical model can provide guidance for future drilling in the P Field.