Quantifying Water Saturation in Steel Pipes using X-rays

An experimental apparatus has been built to measure the relative permeability of water and steam in a two phase flow through porous rock for a range of flow configurations and conditions. In the experiment, the pressure gradient will be measured and consequently the relative permeabilities of the two phases will be assessed. We would like to determine the relative permeability as a function of the volume fraction of the liquid (water saturation) in the two phase flow. It is therefore important to make direct measurements of the water saturation at various locations in the experimental apparatus. In this paper, X-ray imaging equipment (moveable C-beam) was used to quantify the liquid fraction in steel pipes filled with different materials. The experiments were done using steel pipes of different diameter and wall thickness. Our preliminary results showed that useful X-ray images could not be obtained with wall thickness greater than 8 mm with the X-ray equipment at hand, most likely because of limited energy of the beam (max of 106 keV). X-ray penetration was sufficient when we limited pipe diameter to 100 mm and thickness to 8 mm. This method was used on a pipe with diameter 76 mm and wall thickness 4.5 mm and filled with angular quartz fragments in effort to simulate a geothermal reservoir. The pipe was subsequently filled gradually with water while taking X-ray images every step of the way. Using image software to evaluate the images from our experiments we observed that as we added water into the pipe the grayscale of the image changed gradually. To obtain an absolute scale for the grayscale of the images we used a step wedge made of steel. This experiment shows that we can use X-ray imaging, along with absorption reference for comparison (step wedge) to quantify water saturation in steel pipes. INTRODUCTION X-rays have been used for medical purposes for over 100 years, but geoscientists have also been using Xrays, mostly CT, to ascertain the internal structure of core samples, figure out flow in fractures, porosity of rocks and consequently their permeability. Numerous studies have been made for geothermal purposes where experiments were performed to quantify the steam-water relative permeability in two phase flow of water and steam (W. J. Ambusso, 1996; Dastan, 2006; G. F. Mahiya, 1999; Satik, 1998). Repeated studies on this matter have had difficulty in producing consistent result or repeatability because of the difficulty of determining the individual steam and water flow rates. The traditional Darcy equation, used to describe the mass flow for a horizontal one phase and one dimensional flow is shown in Eq. (1). k dp m A v dx   (1) Where k is the intrinsic permeability of the surrounding porous media,  is the fluids kinematics viscosity and dp/dx is the pressure gradient of the flow. For two phase flow the intrinsic permeability is not alone sufficient to account for the fluid permeability. A new factor is introduced into Eq. (1), called the relative permeability, krw and krs for water and steam respectively. Eqs (2) and (3) show the mass flow for the two phases in water and steam flow where the subscript w accounts for the water portion and s for the steam portion.