Improved description of shallow shelf-carbonates with resistivity-based imaging while drilling, Abu Dhabi, U.A.E.

This paper presents the experience gained with resistivity-based imaging-while-drilling (IWD) in a recent three-well horizontal drilling campaign in a giant onshore reservoir of the United Arab Emirates. The campaign highlighted that optimal drilling parameters were required to produce optimum results from the IWD technology, namely: careful bit selection and bottom-hole assembly, constant weight-on-bit, use of mud additives and real time monitoring for a continuous adjustment and optimization of drilling parameters. Experience from this campaign confirmed that good quality image logs could be acquired while drilling, providing time and cost savings without sacrificing detailed reservoir characterization. As with any new application, acquisition parameters and tool set-up had to be fine-tuned for the specific downhole environment The 5000ft-long horizontal IWD in well W-480 shows laterally continuous clusters of distinctive Lithocodium/Bacinella bioherms in the economically important A3 reservoir layer in Field-W. Bioherms are distinct in reservoir character with high porosity and a connected macro pore system. They are typically 10-30 ft in width (with a range of 5 to 100 feet) with an interbioherm space of typically 1 to 3 feet. This lateral continuity of the ASFB lithofacies with its distinctively higher average permeability compared to the other reservoir facies (SPWP and FSW) found in the Thamama A reservoir, further supports the current Zone-A FDP strategy of initially restricting water injection to this subzone only. A significant number of faults and fractures could be identified in all three wells. The more extensive presence of fractures in Zone-A supports recent observations from cores that fracturing is probably more pervasive in this field than previously assumed. At least one fault clearly identified from IWD logging was conclusively correlated with the 3D seismic interpretation. Reservoir pressure fall-off tests and water flow log analysis from water injector w-480 indicated that more than half of the horizontal wellbore length could be considered "effective" for fluid injection. Furthermore, the large storativity ratio indicated that despite the extensive fracturing, a dual porosity system linked to the matrix properties of the biohermal ASFB facies is present in subzone A3. Results of two pressure fall-off (PFO) tests acquired in this water injector were repeatable and supported the presence of a dual porosity system, the relatively high proportion of a secondary porosity system being consistent with a vuggy algal boundstone facies as opposed to a fracture system. The impact of this finding on reservoir performance was being evaluated at the time of writing.