Nuclear Magnetic Resonance Investigation of Forced Imbibitions in Longmaxi Shales: Consideration of Different Boundary Conditions

The forced imbibition in shale reservoirs plays a significant role in achieving high-performance recovery of shale gas. Forced imbibition is the main reason for bringing low flowback efficiency (<30%) of fracturing fluid and some reservoir damages in shale reservoirs. When a large amount of fracturing fluid is injected into the reservoir to enhance the production of gas, the effect of the imbibed liquid on the fracturing fluid leak-off remains poorly understood. In this study, we conducted systematic forced imbibition experiments combined with 1D and 2D nuclear magnetic resonance (NMR) to investigate imbibition behaviors in Longmaxi shales with four boundary conditions: all-side-open (ASO), two-side-open (TSO), one-side-open (OSO), and half-side-open (HSO). The NMR imbibition experimental results (T ₂ spectra, fluid saturation profiles, and spatial T ₂ images) showed that imbibition dynamics of OSO and TSO are more complicated than those of HSO and ASO, which illustrates that the shale bedding structure is the key to determine different imbibition behaviors and imbibition patterns of four boundaries. For four boundaries, ASO imbibition is composed of the dominant imbibition parallel to beddings and the affiliated imbibition against beddings, and HSO is a combination of the imbibition parallel to beddings (the open part) and the imbibition perpendicular to beddings (the sealed part). In contrast, OSO imbibition and TSO imbibition are unidirectional cross-layer imbibition and bidirectional cross-layer imbibition, respectively. Six imbibition parameters including water saturation, gas recovery, imbibition capacity index, diffusion capacity index, imbibition rate, and imbibition potential are adopted to quantificationally analyze the different forced imbibition dynamics. The understanding of how these boundary conditions affect imbibition dynamics in shale gas reservoirs can be applied to interpret fracturing liquid retention and to optimize the hydraulic fracturing design.