Abstract
Computer-assisted tomography (CAT) is used to obtain cross-sectional images of Berea sandstone cores during oil displacement experiments. Local oil saturation averaged over an area of about 0.03 × 0.03 in. [0.8 × 0.8 mm] square is computed as a function of spatial position and time. A series of CAT scan images displaying the time evolution of the fluid distribution at one cross section are shown to illustrate the formation of viscous ringers.
Introduction
CAT 1–2 is a method that uses computerized mathematical algorithms to reconstruct tomographic image of an object. The image reconstruction is based on multiple X-ray measurements made around the object's periphery. This technique has been used in the present research to obtain oil saturation distribution information during immiscible oil displacement in Berea sandstone cores. The objective is to investigate various problems involved in oil recovery processes, including,heterogeneity of the porous structure,surface interactions between oil. the displacing fluid, and the reservoir rock formation, andthe viscosity ratio between the two fluids.
The flow phenomenon is very complex. and previous experimental methods have offered insufficient information for the understanding of oil recovery processes. The CAT scan image acquisition is rapid: thus, it yields directly local oil saturations over a cross section as a function of spatial position and time. Dynamic fluid distribution profiles then can be used to analyze the effectiveness of various oil recovery strategies.
Experiment
An unmodified second-generation CAT scan apparatus (DeltaScan-50 CT scanner by Ohio-Nuclear) is used to obtain oil distribution histories during immiscible oil displacement experiments in oil-bearing Berea sandstone cores. In spite of the dense silica materials. CAT scan has been used successfully to observe fluid flow in sandstone cores. The porous-media models used in this laboratory are cylindrical Berea sandstone cores (5 cm in diameter and 25 cm in length). The permeability of the core is 300 aid, and the porosity is about 20%. The core initially was evacuated and filled completely with oil. A displacing fluid of 1 M KI solution was injected into the core at a rate of 10 mL/hr [10 cm3/h] (superficial velocity is approximately 24 in./D 160 cm/d]). This core was placed in the CAT scanner, and cross-sectional images were taken at different axial locations and different times during the displacement experiment. The computer unit computes local X-ray attenuation coefficients over the scanning, cross section for picture elements of 0.8 by 0.8 mm [0.03 × 0.03 in.] per square. The thickness of the element is approximately equal to the width of the X-ray beam, which is about 1 cm [0.4 in.]. These average X-ray attenuation coefficients result from linear combinations of the silica rock formation and the oil/KI solution mixtures that occupy the pore spaces. Therefore, the oil saturation distribution over a cross section can be computed from local X-ray attenuation data for each CAT scan image.
Results and Discussion
A typical CAT scan image showing the oil saturation distribution at a certain axial position is illustrated in Fig. 1. Darker regions indicate water-rich area where most of the oil has been displaced, and lighter regions indicate oil-rich area. From Fig. 1, the spatial distribution of oil and water can be observed. The ordered fluid saturation changes seem to indicate sonic periodicities coincident with the presence of bedding planes existing in Berea sandstone cores. If sequential scannings are taken at different axial positions at a given time, the structure of the water "fingers" can be reconstructed. Figs. 2A, 2B, and 2C plot the oil saturation distributions at 5 cm [2 in.] from the injection point after 3, 4.5, and 15 PV of the displacing fluid (1 M KI) have been injected into the sandstone core. The top of the diagram indicates 100% oil, and the bottom of the diagram indicates 100% water (1 M KI). These CAT scan images have dramatically represented the invasion of water into the oil region and the displacement of oil from a specific cross section as a function of time. Time derivatives of oil saturations also are computed from these image data, which yield rate of changes of local oil saturations
SPEJ
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GEODYNAMICS