Fluid Transport Mechanisms in Unconventional Tight Rocks Based on NMR Imaging

Unconventional resource development has been enabled by advanced completion methods such as horizontal drilling and multistage hydraulic fracturing. However, unconventional reservoir production rates decline more rapidly relative to conventional reservoirs, and recovery factors remain relatively low. Enhancing hydrocarbon recovery can be improved through a fundamental understanding of transport processes inside unconventional rocks. This work provides unique insights into fluid transport mechanisms in unconventional rocks during miscible displacement using a direct and noninvasive method to quantitatively detect hydrocarbons and their distribution inside rock samples by proton NMR imaging coupled with theoretical analysis of experimental results. This study shows the importance of unconventional rock morphology and pore structure to better understanding fluid transport and overall mass transfer rates. Different transport mechanisms in representative homogeneous and heterogeneous unconventional tight rocks are demonstrated. For homogeneous tight rocks, viscous or hydraulic flow is the dominant fluid transport mechanism in the direction of an applied pressure differential. However, a more complex behavior is observed in heterogeneous tight rocks. Despite smaller average pore sizes, the heterogeneous tight rocks can possess macroscopic permeabilities comparable to larger-pore rocks due to microfracture(s) in the sample that remain open at field relevant net confining stresses. Such microfractures enable a dual fluid transport mechanism inside the heterogeneous tight rock: hydraulic fluid transport along microfracture(s) and diffusion dominated fluid exchange across microfracture(s) inside the rock matrix. Overall mass transfer rates through heterogeneous tight rocks depends on the interplay of these two transport mechanisms, relative rates and travel distances. Experiments with lab cores showed that for fluid exchange of an injected fluid with the in situ fluid, diffusion in the matrix is a rate limiting step. More study is required to upscale our observations to a field relevant scale.