The development of Early Permian reservoirs in the Fairview Field is limited by highly depleted shallower Bandanna coal seams and naturally fractured formations that would not allow to reach the original geological goals. Offset wells in the area had been executed as three string well design due to long open hole section across depleted Bandanna and multiple loss zones. Multi-stage cementing was required to ensure there is proper zonal isolation. These resulted in significant incremental cost to achieve 5-1/2″ production casing. Through the depletion of upper coals, fracture pressure in these zones has decreased due to reduction in pore pressure.
Drilling in two string design carries higher execution risk due to long open hole section, depleted formations and differential sticking, highly fractured formations, narrow window between fracture gradient and pore pressure, and multiple loss zones which demands special techniques for treatment of loss zones and hole stabilisation while drilling. Considering the downhole conditions, which pore pressure ranging from 2 to 9 ppg in the different formations, the importance of zonal isolation, establishing barriers in aquifers and achieving overall cement coverage is also extremely important.
With introduction of various new LCM products, cement plugs, foam cementing, RCD for drilling with no returns (blind drilling), Santos has achieved major success developing Early Permian fields. This Paper describes how two string design was successfully implemented to drill Early Permian wells. This paper serves as an operational guide to ensure challenging depleted and highly fractured zones can be successfully drilled while minimising risks.