Effects of Fracture Characteristics on Spontaneous Imbibition in a Tight Reservoir

Abstract

Horizontal well fracturing technology is widely used in the development of shale gas resources. After the fracturing operations, natural and artificial fractures exist in the reservoir at the same time, which has significant effects on spontaneous imbibition to increase shale gas production. Based on nuclear magnetic resonance technology, this work studies the spontaneous imbibition property of fractured shale, including artificial and natural fractures, surface cemented minerals, boundary conditions, etc. The results show that the relaxation time spectra reflect the characteristics of water and gas migration in the rock pore space. The micropores and the small pores are the main storage space for water in shale. After water imbibition starts, the imbibition volume is roughly proportional to the square root of time. Through the comparative experiments on artificial fractures, natural fractures, and seamless cores, it is found that the existence of fractures can significantly promote water imbibition, and the imbibition speed of artificial fractured cores is faster than those of others. Because of the existence of artificial and natural fractures, the number of pores for gas discharge increases, and the imbibed volume is slightly more. When the cemented minerals such as quartzite and calcite are attached to the core surface, they have poor water sensitivity and block some of the pores, thus resulting in a slower imbibition speed. Different boundary conditions will not only affect the contact area between the core and water but also influence the types of imbibition, which changes the water flow path and imbibition speed. This study provides an important insight into spontaneous imbibition in gas shale after the fracturing operations.

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

“…Regarding the imbibition rates across forced imbibitions, they are not much different in these boundaries, which may be due to the approximate porosity, permeability, and total imbibition volume of these four parallel samples. 32,33 Moreover, the values of imbibition potential are such that TSO > ASO > HSO > OSO, illustrating the different contributions made by late imbibition stages to the whole forced imbibition process. However, these findings are contrary to the previous studies, 31,32,59 which can be attributed to the intrinsic differences of imbibition mechanisms between spontaneous imbibition and forced imbibitions coupled with the boundaries.…”

“…The results discussed above are consistent with the recognition of other scholars. 24,32,57 For ASO imbibition, a radial imbibition occurs on the cylindrical side, and a linear imbibition occurs on two ends. The radial imbibition on the cylinder side meets the linear flow on the two end faces and interferes with each other in the forced imbibition process.…”

“…Imbibition in AFO is predominantly by the capillary pressure with negligible diffusion, but imbibitions of TSO and OSO are predominantly by diffusion imbibition. Yang et al 32 demonstrated that there is not much difference in the imbibition rate between ASO and TSO boundaries, and they are both higher than that of OSO. The main reason behind these imbibition dynamic differences is that there are great differences in imbibition patterns of various boundary conditions between spontaneous imbibition and forced imbibition.…”

“…Umeobi et al 31 compared imbibition dynamics and flowback of fracturing fluids in sandstones by NMR imbibition experiments with AFO, TEC, and two-end-open (TEO) boundary conditions. Through the NMR imbibition experiments on fractured shales under three boundaries, Yang et al 32 found that boundary conditions mainly affect the imbibition by changing the movement relationship and contact area between the core and water. Wang et al 33 also conducted NMR experiments to study the imbibition laws of glutenite with ASO, OSO, TSO, one-side-closed, and TSC conditions.…”

Influencing Factors and Application of Spontaneous Imbibition of Fracturing Fluids in Lacustrine and Marine Shale Gas Reservoir

“…Illite can form various crystal structures, among which bundles and fibers can absorb water and expand, causing the most significant damage to pores. Swelling of the I/S interlayer will lead to the disintegration and migration of clay minerals, block the pores and throats, and worsen the reservoir’s physical properties. , Wang et al found that shale pore connectivity is significant for shale oil enrichment and development. Micropore connectivity is the largest, followed by mesopores and macropores.…”