Reservoir Engineering - Improvements in the X-Ray Saturation Technique of Studying Fluid Flow

Improvements in the X-ray method of measuring liquid saturation and saturation distribution are presented. Two identical direct current amplifiers have been added to measure continuously the intensities of the X-ray tube output and the beam which has passed through the core under test. The ratio of these intensities is automatically recorded on a modified Brown Electronik Recorder. Arrangements have been made to drive the core automatically and in synchronism with the recorder chart so as to give a continuous curve showing the variation of liquid saturation along the length of the core. By determining the ratio of the two X-ray beam intensities simultaneously the measurements are made essentially independent of small fluctuation in voltage and current supplied to the tube. This improved technique has been used for investigating the fluid distribution along the length of cores and core assemblies when one fluid is displaced by another. Typical results of these studies are shown. INTRODUCTION In a previous publication' a method was described for measuring the oil saturation in cores which was based on the absorption of X-rays by the fluid content of the core. While that technique is capable of yielding accurate results, the original apparatus was not especially convenient for determining the fluid distribution along the length of a core. This was primarily due to the slow response of the amplifier and galvanometer and the necessity of making a point by point survey of the material under investigation. It was, therefore, not only possible for sections of the core having a variable or unusual saturation distribution to escape observation entirely, but changes in output of the X-ray generator between readings on the unknown and the monitor introduced errors in the measurements that could be avoided only by further stabilization of the source or by taking a large number of readings. This paper describes a method and apparatus for recording continuously and automatically the intensity of the X-ray beam that is transmitted by a core. The variation of saturation with time at a given section of the core may thus be found, or by scanning the core by driving it in synchronism with the recorder the saturation distribution may readily be determined. As indicated in the previous paper, the X-ray method possesses an advantage over the weighing technique in relative permeability experimentation in that the core need not be removed from the holder for the purpose of making a saturation measurement. Furthermore, the actual distribution of fluid and the effects upon that distribution of inflow and outflow heads may be investigated. And errors due to gas bubbles or loss of liquid that are frequently sources of error in volumetric methods of saturation determination are avoided. PRINCIPLE OF THE METHOD The energy in an X-ray beam that has passed through an absorbing material is expressed by the well known formula: E = E0, e-cd (1) where E, is the energy in the incident beam, µ the mass absorption coefficient, c the density or concentration of absorbing material, and d is the length of path of the beam in the absorber. The coefficient µ varies with the wavelength of the radiation as well as with the absorbing material. The output of an X-ray tube varies in an approximately linear manner with the tube current, but to the second or higher power of the anode voltage. Moreover, the quality or wavelength distribution of the radiation varies with the voltage.