Experimental and Modeling Investigation for the Effect of Effective Stress on Seawater Permeability of Clay-Free Limestones, Dolomites, and Clay-Rich Sandstones
Authors: Lei Gang , Zhou Xianmin , Al-Abdrabalnabi Ridha , Elsayed Mahmoud , Kamal Muhammad Shahzad 
Published: February 2022

The reservoir fluid pressure decline (natural energy consumption) and associated pore compaction is a major oil displacement mechanism during primary oil production. Seawater injection is a common technique to maintain the reservoir pressure and recover oil from such reservoirs as secondary oil recovery for carbonate and sandstone reservoirs, especially for carbonate reservoirs in the Middle East. Either in the primary or secondary oil production stage, the change of reservoir rock permeability occurs with the reservoir fluid pressure (pore pressure) and overlying pressure (overburden pressure) change, affecting oil production performance. This study examined the significance of liquid permeability sensitivity to the net pressure change (difference between overburden pressure/triaxial stress and pore pressure/axial stress) and the effect of pressurized histories of the axial stress on liquid permeability of clay-free and rich rocks.

Several experiments were conducted on three types of rocks, Indiana limestone, dolomite, and sandstone, to investigate the triaxial and axial stress-dependent seawater permeability of such rocks during the process of injecting seawater into the core samples. The experiments of seawater injection included: 1) Optimization of the injection flow rate of seawater; 2) The axial stress loading and unloading process at constant overburden pressure/triaxial stress; and 3) The triaxial unloading stress at constant fluid pressure. A novel modeling technique was used to match experimental data of unloading and loading processes. In addition to the experiments mentioned above, several additional experiments have been completed to explain and understand that the effect of effective stress on the liquid permeability of rocks. To explain experimental results, additional experiments were included as below: 1) Mineral composition analysis of rocks; 2) Pore size distribution of rock before and after pressure unloading and loading process by NMR, and 3) Acoustic velocity experiments to explain experimental results.

Based on the experimental and modeling results, we found that with the increase of effective stress due to the change of liquid pressure, the liquid permeability decreases for clay-free limestones and clay-rich sandstones under pressure unloading or loading process. However, compared with clay-rich cores, under the constant liquid pressure condition, the decrease of overburden pressure/triaxial stress slightly affects the liquid permeability. For dolomite rocks, the compaction caused by increasing overburden pressure plays a decisive role in reducing the liquid permeability.

This paper comprehensively studied the effective stress on liquid permeability of various rocks, including the selection of injection flow rate, the analysis of effective stress caused by changing fluid pressure or overburden pressure which provides valuable information for the formation protection of different types of reservoirs.