Porosity and Permeability Evolution with Deviatoric Stress of Reservoir Sandstone: Insights from Triaxial Compression Tests and In Situ Compression CT

Abstract

Porosity and permeability are the two most important characteristics of underground gas storage in sandstone reservoirs. Injection of gas into reservoir rocks will cause rock deformation. The deformation will influence the porosity and permeability properties of the rocks. We investigate the evolution of these two properties of storage sandstone by triaxial compression tests and a uniaxial in situ compression CT test. As the deviatoric stress increases, the sandstone is compressed firstly (porosity reduction) and then dilates (porosity enhancement). With the increase in confining stress, the occurrence of volumetric dilation will be delayed. Trapped porosity of this sandstone at different deviatoric stresses is very small (0.122%-0.115%) which indicates that nearly all pores are connected. During the compression stage, the decrease in permeability is related to compression of pores and microcracks. During the volumetric dilation stage, it is related to increase in tortuosity. This interpretation can be confirmed by observations of in situ compression CT. The permeability evolution estimated by pore network modeling is consistent with macroscopic testing results.

Cyclic confining pressure and rock permeability: Mechanical compaction or fines migration

“…[ 37 ], according to which the interconnected pores and microcracks create the void space. During loading, microcracks are closed in which their volume is minor in comparison to the total volume of the void space [ 6 , 105 ]. and do not open when unloading.…”

“…The CT resolution was 30 μm, which is comparable to fairly large pores and is not enough to recognize changes even in large pores and pore throats during colloid migration thereby closing of micropores and microcracks under mechanical loading [ 100 , 101 , 105 ]. Due to limitations in CT resolution, micro changes that affect permeability can only be assessed by indirect methods.…”

Rock permeability evolution during cyclic loading and colloid migration after saturation and drying

“…Assessing the structure of rocks is a relatively simple task, even at low resolution. However, identifying subtle changes becomes challenging, even with high resolution and under significant loads, as in the example of work (Hu et al, 2020), where the confining pressure reaches 36 MPa, and changes in the structure are barely noticeable. It is important to note that the accuracy of comparative tomography is influenced by the presence of reference points and the precision of sample positioning during tomography.…”

Experimental and Numerical Assessment of the Influence of Bottomhole Pressure Drawdown on Terrigenous Reservoir Permeability and Well Productivity

“…A natural consequence of the decrease in the permeability of the reservoir rock is a decrease in the production wells' productivity. Such changes are typical for both granular [4][5][6] and fractured reservoirs [7][8][9]; most often the difference lies in the fact that the permeability of fracture systems is more sensitive to the reservoir pressure changes.…”

“…There are quite a lot of publications devoted to the study of the influence of effective stress changes on the permeability of rocks, and at the same time, in several studies, the authors do not pay much attention to the holding time of the samples at each step of stress increase [6,[10][11][12]. However, as shown in the works [13][14][15], the decrease in porosity and permeability of core samples can continue for quite a long time, which is associated with the manifestation of viscoplastic deformations within rocks.…”