Heavy and ultra heavy oils are commonly found in many parts of the world. From MacGregor (1996), and UNITAR (1998), they were reported that conventional oil reserve is only 30%, but heavy oil and ultra heavy oil is 15% and 55%, respectively1. Proper reservoir characterization for these reservoirs is not always an easy task, especially when they are associated with (1) variation of different permeability formations (2) reservoir fluid viscosity including, extremely high viscosity fluids, (3) unconsolidated sand environments, and (4) Low Resistivity and Low Contrast Reservoirs. This paper presents different challenges of obtaining reservoir fluid information from six different heavy oil fields from the South of Oman where reservoir fluid viscosity ranges from < 100cp to more than 5000cp (at downhole conditions). The formation permeability varies from good permeability (Darcy range) to tight formation (<1 mD). Reservoir fluid identification and quality fluid samples are not easy and sometimes seems to be impossible with associated challenges. The used of openhole logs alone cannot be used to conclusively identify reservoir fluid, and therefore, Wireline Formation Testers (FT) or Full Scale Testing is required to reduce the fluid typing uncertainty and enhance reservoir characterization. This paper discusses challenges of formation sampling for both openhole and casedhole conditions for both heavy and ultra heavy oils. Three openhole and three casedhole field examples will be discussed in this paper from pre-job planning to operation results. For openhole cases, the use of 3D Radial Probe application will be introduced with different types of displacement downhole pump to help successful sampling operation. For casedhole cases, the dual packer FT were used to obtain reservoir fluids within the perforated interval. Due to complexity of these jobs, proper job planning was required to ensure the job objective can be achieved. This paper discusses pre-job planning in the casedhole considering the following (1) well control issue, (2) perforation types and lengths, (3) operation steps, (4) pre-job simulations for the formation tester tool selection, (5) results and recommendations for future jobs. This paper also introduces the use of Pressure Transient to help identify reservoir fluids. Pressure Transient Analysis (PTA) simulation Work will be presented in the casedhole section. Reservoir sampling can be done successfully even with reservoir and operation challenges in these wells using the right FT tools, pump types, and conveyance methods. From our experience, it can be concluded that pre-job planning and real-time monitoring were two keys to ensuring successful acquisition of formation representative heavy oil samples. Results of this paper help asset teams to minimize cost and obtain the most accurate reservoir information using our workflow.

PDO has a large portfolio of mature Gharif fields with significant hydrocarbon potential to be unlocked by efficient well and reservoir management (WRM). In 2016, focussed surveillance on Closed-in Wells (CIW) in a mature PDO South Gharif field, highlighted significant bypassed oil opportunity in Low Resistivity Pay (LoRP) reservoir –"C". Presence of conductive minerals and different clays in the LoRP sands, led to a pessimistic assessment of resistivity based hydrocarbon saturation. As such, histrocially, these oil bearing sands were not perforated. LoRP was production tested in 2 CIW & 3 New Oil (NO) wells in 2015, doubling field's net oil production (Figure1). Insights from historical field production, pressure depletion, core and Nuclear Magnetic Resonance (NMR) data were integrated to update the static subsurface model which led to an increment of 21% in the field STOIIP. Current inventory of high water cut (>95%) and high sand producing oil producers are being converted to dedicated LoRP producers on opportunity basis. This has reduced the frequency of expensive well workovers. Quick screening of LoRP has been initiated for all PDO South fields. Initial results show that similar bypassed oil opportunities exist in other mature brown PDO South fields as well. Broadly speaking, such bypassed oil opportunities can be attributed to one of the following subsurface mechanisms at play i.e. presence of thin sand- shale intercalations which are not vertically resolved on wireline logs, presence of condutive minerals e.g. pyrites or clays and high irreducible water saturation in microporous rocks. With the current low oil prices, re-exploration of mature fields by interpreting and maturing low resistivity pay reservoirs, presents an attractive business proposition.