Abstract

Deep Gas exploration wells in the Fahud Salt Basin located in North Oman were traditionally drilled with high mud weights. Due to this conventional drilling practice in low permeability reservoirs and inherent petrophysical measurement and sampling limitations, a high level of uncertainty remained as to the presence of producible (or commercial) gas. This resulted in an increased risk when the decision to complete, hydraulically fracture and production test was taken – or conversely, the well was abandoned. Often, even test results proved inconclusive, with a strong suspicion of impaired reservoir obscuring the true production potential. The well discussed here was the 12th well that penetrated a tight Paleozoic reservoir in the Fahud Salt Basin and stepped away from this work flow due to the high cost and low success rate for the previous 11 wells. Of the previous 11 wells, one well tested economic gas flow, four were marginally economic and 6 ultimately deemed inconclusive. In this well, the primary objective of the vertical pilot hole was to unambiguously identify mobile hydrocarbons. UBD was selected as the key technology to meet this goal. Once the objective was achieved in the pilot hole, the budget for drilling a lateral hole, with the aim to optimize production, could be more confidently released.

To drill this well underbalanced, by reducing the mud weight from 16 kPa/m to 10.1 kPa/m well site water, PDO management had to change their well control mindset and take on the risk of major borehole breakouts and stuck pipe. The result of confronting these two major risks was manageable well control and a largely in-gauge well bore, allowing high quality wireline logs in the pilot hole and LWD logs in the lateral section. Most importantly, the flow measured while drilling and during a short UBD production and build-up test in this well was the most direct and unequivocal reservoir evaluation available in any of the tight gas wells of North Oman. The technical success of this exploration project resulted in the planning of eight more underbalanced exploration wells on neighbouring prospects, some of them with even deeper reservoirs and more pressure uncertainties.

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