Gamma-Camera Imaging of Oil Displacement in Thin Slabs of Porous Media

Abstract

Summary Experiments are reported in which a gamma camera is used to observed fluid displacement in thin slabs of porous media. Technetium-99m (half-life of 6.5 hours) is used as tracer for the water phase and gamma ray activity measured over a 64x64 picture field in which each pixel is 4x4 mm. A method for image enhancement is applied. The gamma camera gives a quantitative determination of local fluid saturations over the area of observation. Introduction The literature contains many references for use of X-rays to measure the local oil saturation during coreflooding experiments. Slobod and Caudle used potassium iodide dissolved in the aqueous phase to increase its X-ray attenuation. The aqueous phase was injected into the edge surface of an oil-saturated thin slab of sandstone. The displacement was followed by irradiating the area of the slab with X-rays and exposing a film plate that was located under the slab. An X-ray shadowgraph of the oil domain was the result. However, the process was slow and only a qualitative image was obtained. More recently, Wang and Wang et al. showed that computed tomography could be used to obtain a quantitative and spatially specific image of the oil saturation during coreflooding. The literature also contains many references to other imaging methods that, with varying degrees of success, have attempted to measure oil saturation during a displacement experiment. Microwave methods, Conductivity measurements, refractive-index methods, and colorimetric methods have been reported. With a Procedure that completely destroys the core, Chatzis et al. used a monomer as the hydrocarbon phase and ultimately polymerized it at the residual state to determine the distribution of hydrocarbon that had just been immobilized. In this paper, we describe a new quantitative gamma-camera method for imaging oil displacement in thin slabs of porous media. The gamma camera is a quantitative imaging system that can measure the two-dimensional spatial distribution of a gamma emitting radionuclide. For many engineering applications, this apparatus has been shown to he excellent for observation of flow and mixing in reactor vessels, for measurement of liquid film thickness on the surface of a rotating disk, and for characterization of crossflow in multichannel mixing assemblies. Experimental Method Specifications of the Gamma Camera. The gamma-camera-imaging facility used in this study is a Searle PHO/GAMMA Low Energy Mobile Scintillation Camera interfaced to a PDP I 1/10 computer. The camera is shown schematically in Fig. 1. The detector is made of a disk of sodium iodide crystal 1.27 cm [0.5 in.] thick and 25.7 cm [ 10.1 in.] in diameter. Above it is a hexagonal array of photomultiplier tubes; below it is a parallel-hole lead collimator. The gamma rays emitted from a radionuclide source that are perpendicular to the plane of the collimator pass through it, strike the crystal, and initiate a scintillation. The photomultiplier tubes locate the x-y position of this event and record it. The size of a picture element (pixel) depends on the type of collimator. in this case, each pixel is 4 × 4 mrn, and the total image is represented by a 64 × 64 matrix. The quality of the image depends on the total number of counts at each pixel. Because the counting statistics error is proportional to the square root of the total number of counts and because in this work we sought an error of less than 5 % in the regions of high oil saturation (low counting rate), the frame time was kept around 90 seconds for all the images reported herein. The frame time, the uncertainty in counting, and the specific activity of the gamma ray source are interrelated; these factors must be optimized for a given imaging situation. More specific details are given in Ref. 13.