Simplifying the Complex: Saturation Modelling in a Challenging Carbonate Reservoir with a Tilted Contact & Significant Imbibition in the Sultanate of Oman

Abstract

The fluid fill concept of a complex carbonate reservoir of the Shuaiba Formation has been re-evaluated after 45 years of production. Structural changes post primary drainage resulted in a tilted contact towards the north east and water imbibition into the oil zone in a considerable volume of the reservoir. Rock facies change substantially vertically and laterally as observed from the 25+ cored wells and have camouflaged the fluid fill impact. Reservoir heterogeneity increases from the relatively homogeneous mud-dominated rock type in the central south-east to more heterogeneous grainstones and grain-dominated packstones further away. This paper demonstrates the value of using simplified and structured approaches to analyse saturation profiles to deduce possible fluid fill concept(s) and water saturation distribution. The saturation profile with core permeability and core facies interpretation were used to identify imbibition, map the water table (imbibition) surface and recognize the trend of structural tilting post-charge. The central south-east area has negligible tilting and imbibition with a saturation model that could be simplified to primary drainage. Above the water table, the mud-dominated rock saturation model was the reference point. Due to scarcity of wells unaffected by imbibition or tilt, a benchmark primary-drainage saturation model of analogue carbonate reservoirs was used to create a first-pass saturation model of the mud-dominated rock and identify entry height. The paleo-reference (original FWL) was reconstructed from the OWC picked from 72 vertical wells and smoothed by the general trend of tilt from the shallower seismic horizons. The workflow relied on using the mud-dominated rock saturation model to flag other rock types in uncored wells and possibly reducing the uncertainty in facies distribution. With the complexity at hand, difficulty in populating the facies between wells and time constraints, the water saturation below the water table was interpolated from all vertical wells. The paleo-reference reconstruction indicates substantial tilting of the structure post-charge. One-third of the hydrocarbon volumes are in the imbibed region. The reservoir has been charged close to the irreducible water saturation and the majority of logged wells display paleo imbibed transition zones so it was not possible to utilize saturation-depth signature to identify a large number of distinctive rock types. Due to the aforementioned, the saturation model for all of the reservoir rocks could be simplified by a single saturation function above the paleo-reference and a current water table depth.