Acoustical methods to evaluate pore fluid saturant through casing



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This dissertation reports the results of the development of an acoustical and interpretation method to measure in-situ pore fluid saturant in casedholes. The purpose of this study is to determine the feasibility of ultrasonic pulse-echo and non-specular oblique incidence measurement techniques to detect gas, oil, and water interfaces through-casing. There are many factors that aff ect ultrasonic reflectivity and transmissivity in a cased-hole environment. Casing and cement-layer thickness, borehole shape, size and irregularities, as well as borehole fluids, are some of the cased-hole environmental factors that influence the cased-hole log response. In this dissertation, we propose to investigate the eff ects of the casing, cement-layer, and rock saturation condition on the reflection and transmission of broadband and tone-burst pulses at normal and oblique incidence. A one to one scale cased-hole laboratory model was not practical to assemble, coat, and fully saturate the rock samples studied. The cased-hole was modeled with a steel-plate cemented to consolidated sedimentary rock samples. Ray acoustic, short wavelength analysis, and experimental studies were used to justify and simplify the size of our experimental planar layered model without sacrifi cing the goal of this study. Three types of well-known sedimentary rocks studied in laboratory testing of hydrocarbon reservoirs were selected: (1) Berea sandstone, (2) Leuders limestone, and (3) Jamison shale. Four sample assemblies (steel-plate/cement-layer/rock-sample/acrylicplate) were fabricated and coated with epoxy, and immersed in an oil-fi lled ultrasonic testing tank. Experiments were performed to measure the reflection and transmission of ultrasonic waves in the sample assemblies under dry- and wet-conditions. The non-specular oblique incidence measurements technique produced better results. In general, stronger reflectivity from the dry-samples was observed. Under wet-conditions, the frequency, thickness and angle of incidence-dependent transmission loss is higher in the oil-saturated sample assemblies than in the water-saturated sample assemblies. Theoretical analysis supported by experimental results, reveal that the steel-plate or casing and the degree of saturation of the porous cement-layer and rock samples behind steel-plate govern the transmission and reflection of ultrasonic energy. At laboratory conditions, the cement-layer behaves as a fi ne-texture-porous rock that adds attenuation to the ultrasonic wave. It turns out that the cement-layer and Berea sandstone acoustic impedance are practically the same under dry- and wet-conditions. Consequently, in most water and oil reservoirs poor borehole-reflectivity will be detected due to the acoustic energy coupling to the unbounded rock formation.