In a large carbonate oilfield located in the United Arab Emirates horizontal drilling has been used to enhance well productivity, injectivity and to reduce water coning. A key to the development of this field is an improved understanding of reservoir characterization to better appreciate the production impact of faulting, fracturing and secondary porosity.
In lieu of these objectives a new logging-while-drilling (LWD) tool has been deployed to acquire high resolution borehole images and multiple-depth of investigation (MDOI) laterolog resistivities. The use of a LWD tool is particularly relevant to lateral wellbores drilled in relatively thin layered reservoirs as the interpretation and deployment of traditional wireline-conveyed measurements is complex.
Traditional resistivity measurements acquired in horizontal wells can be difficult to interpret due to combined shoulder-bed, anisotropy and proximity effects, and as a result can lead to unrealistic estimates of water saturation (Sw). LWD laterolog measurements are mostly affected by invasion and relatively unaffected by resistivity anisotropy and shoulder-bed effects. This has a distinct advantage in horizontal wells as the resistivity measurements are negligibly influenced by nearby lateral beds or proximity effects, and if acquired while drilling also have minimal invasion effects.
In this large carbonate oilfield the new LWD tool was used along with conventional triple combo propagation resistivity, neutron porosity, and formation density measurements. Accurate formation saturations in this previously problematic environment were computed, and images similar to those from wireline micro-imaging tools were acquired for structural, fracture, and porosity textural analysis.
LWD acquisition parameters to optimize image quality were also investigated; most notably drilling rate of penetration (ROP), tool rotation speed (RPM) and formation/mud resistivity contrast (Rt/Rm).