Measuring Gas-Water Relative Permeability in Tight Sandstones at High-Temperature High-Pressure Conditions: Experimental Design and Correction
Authors: Yang Zhixing , Li Yangfan , Chen Zili , Zhang Xiaolong , Lu Kefeng , Du Xiaolei , Guo Ping 
Published: May 2017

Gas-water relative permeability curve is critical in reservoir simulation and development for tight reservoirs. It is generally obtained from unsteady-state flow experiment under lab condition (i.e., room temperature and normal pressure). The measured results often significantly differ from the curves measured under high temperature and high pressure condition. To provide accurate experimental data for correcting core data, we develop a new experimental workflow by using self-manufactured equipment (up to 200 °C and 200 MPa), and establish the correlation method for the gas-water relative permeability curve between room condition and reservoir condition.

The measurement of gas-water relative permeability curve under room condition follows the Buckley-Leverett equation and gas equation of state: unsteady-state experiment ignoring the effect of capillary pressure and gravity. The measurement of gas-water relative permeability curve under reservoir condition uses Full-diameter Seepage Flow Equipment. The changes of water volume, gas volume and the gas dissolution due to temperature and pressure difference are considered. The correction table for the core analysis data from room condition to reservoir condition is established using the measured water and gas production data.

Comparing with the conventional experiment under room condition, we observed that the gas/water displacement experiment under high temperature and high pressure has the following features: 1. Larger two-phase seepage zone; 2. Lower immobile water saturation; 3. Relatively higher gas relative permeability under the same gas saturation. These observations suggest that the two-phase flow capability of both gas and water is higher at reservoir condition.

We observed that the water/gas displacement experiment under high temperature and high pressure condition has the following features: 1. Lower immobile water saturation and little difference among several cores; 2. Gas relative permeability declines faster and the curve moves to the lower left corner; 3. Higher water relative permeability in the residual gas saturation; 4. Higher residual gas saturation and larger difference in residual gas saturation among several cores; 5. Significant decline in equal-permeability point value and smaller two-phase region. These observations indicate that lower interfacial tension and lower viscosity ratio under reservoir condition result in higher residual gas saturation. Furthermore, the reservoir heterogeneity also squeezes two-phase flow region. The water/gas displacement efficiency is significantly reduced.

This study developed a new experimental process to measure gas-water relative permeability in tight sandstones by using self-manufactured Full-diameter Seepage Flow Equipment. We established the correlation between the gas-water relative permeability curves measured at room condition and reservoir condition. The experimental data presented in this paper can provide a good foundation for theoretical analysis and a good reference for other experiments.