NMR-Based Shale Core Imbibition Performance Study

Abstract

Shale gas reservoirs are unconventional resources with great potential to help meet energy demands. Horizontal drilling and hydraulic fracturing have been extensively used for the exploitation of these unconventional resources. According to engineering practice, some shale gas wells with low flowback rate of fracturing fluids may obtain high yield which is different from the case of conventional sandstone reservoirs, and fracturing fluid absorbed into formation by spontaneous imbibition is an important mechanism of gas production. This paper integrates NMR into imbibition experiment to examine the effects of fractures, fluid salinity, and surfactant concentration on imbibition recovery and performance of shale core samples with different pore-throat sizes acquired from the Longmaxi Formation in Luzhou area, the Sichuan Basin. The research shows that the right peak of T2 spectrum increases rapidly during the process of shale imbibition, the left peak increases rapidly at the initial stage and changes gently at the later stage, with the peak of the left peak shifting to the right. The result indicates that water first enters the fracture system quickly, then enters the small pores near the fracture wall due to the effect of the capillary force, and later gradually sucks into the deep and large pores. Both imbibition rate and capacity increase with increased fracture density, decreased solution salinity, and decreased surfactant concentration. After imbibition flowback, shale permeability generally increases by 8.70–17.88 times with the average of 13.83 times. There are also many microcracks occurring on the end face and surface of the core sample after water absorption, which may function as new flowing channels to further improve reservoir properties. This research demonstrates the imbibition characteristics of shale and several relevant affecting factors, providing crucial theory foundations for the development of shale gas reservoirs.

Mechanisms of Imbibition Diffusion and Recovery Enhancing of Fracturing Fluids in Tight Reservoirs

“…The viscosities for the petroleum product in blocks A, B, and C are 1.14, 3.92, as well as 6.73 mPa•s. The corresponding densities were 0.731, 0.763, and 0.803 g/cm 3 .…”

“…The computer images the oil droplet with a camera and calculates the interfacial tension through software. The interfacial tension for the crude oil had been calculated to be 3.92 mPa•s, with its density found to be 0.763 g/cm 3 .…”

“…(2) The rock skeleton and fluid compressibility are ignored. (3) The impact in stratification and pore structure is disregarded in tight reservoirs, which are homogeneous. (4) A twodimensional static imbibition mechanism in which gravity is disregarded (Figures 4 and 5).…”

Quantitative Analysis of Crude Oil Mobilization in Microscopic Pores during the Spontaneous Imbibition of CO2-Enhanced Fracturing Fluids in Shale Oil Reservoirs

“…Expanding on the research by Xiao et al, Zhang et al determined effective porosity initiation times for two tight sandstones, recorded at 1 and 2.8 ms, respectively. Gao et al classified shale pores into three types: inorganic intergranular pores, inorganic intragranular pores, and organic pores. − …”

Investigating the Influencing Factors of Imbibition of Fracturing Fluids in Tight Reservoirs

“…Tight oil is an unconventional oil and gas resource characterized by an accumulation origin and occurrence state that differs from conventional oil and gas. Following the commercial exploitation of marine tight oil in North America, tight oil has gained prominence as a focal point in unconventional global exploration and development [1][2][3][4][5]. However, due to the complex genesis and low porosity of reservoirs, the conventional production technology of tight reservoirs faces the challenges of low production of a single well, poor development efficiency, and large-scale application difficulty [6][7][8][9].…”