Probable reasons of Rock Sample Apparent Permeability Loss over Time in Long-term Measurements

In the last few years, tight oil production has increased significantly. This is why the study of the filtration properties of low-permeable rocks (permeability near 1 mD) has acquired particular significance. Such rocks can be subject to considerable compaction during development, which manifests itself in the non-linear permeability loss in time at constant net confining stress. In the laboratory, the compaction (or creep) of porous rock samples under constant stress conditions was observed by many researchers for experiment durations ranging from several hours to several weeks. Conducting such lengthy flow experiments accompanies by a number of challenges. First of all, it is necessary to exclude factors not related to the deformation of the sample. In this study we analyzed possible causes of time trends observed under constant net confining stresses in long-term measurements of low-permeable sample permeabilities. Experimental study of flow in a limestone core sample was conducted. During the experiment with duration of 40 days, the fluid pumping was carried out in several stages with different constant values of the confining pressure and the pore pressure gradient. As a result, the permeability of the sample decreased by 10 times. It was shown that such significant decrease in the permeability in time can be caused by clogging of the sample pore space. The additional experiment with sequential pumping of single-phase gas and liquid through the sample showed that when pumping gas, the sample permeability remained almost constant most of the time. We propose that gas bubbles contained in the flow of liquid can act as a dispersed phase that clogs pores. The estimations show that even very low particle concentrations at large time periods lead to significant decrease in the permeability. The possibility of clogging of the core sample pore space must be considered when conducting the long-term experiments on study of the permeability by the steady-state method.