Abstract

A large amount of fracturing fluid in fracking is imbibed into the shale fracture/matrix system, which leads to a significant uncertainty in gas recovery evaluation. The mechanism of imbibition impact on the gas–water two-phase flow is not well understood. In this study, systematic comparative experiments are carried out to simulate imbibition in fractured shale samples obtained from the Wufeng-Longmaxi Formation in China and the imbibition effect in the fracture–matrix system is qualitatively and quantitatively investigated. Nine shale samples are collected to measure their porosity and permeability using a helium porosimeter and nitrogen pulse–decay tests. Gas/liquid single-phase flow experiments are then carried out on three dry and saturated fractured samples using methane and KCl solution, respectively. Subsequently, dynamic imbibition experiments are carried out on three samples in a visualization container. The gas–water interfacial tension, water imbibition amount, and displacement velocity are recorded. A single-phase gas/liquid flow test shows a high linear correlation between the fluid displacement velocity and pressure gradient in the fractured samples as the fracture is the main flow channel, dominantly determining the flow behavior. Moreover, we introduce the capillary force in the cross flow term of the triple-medium model to characterize the imbibition effect, develop a two-phase flow simulation model of shale gas considering the fracturing fluid imbibition retention, and analyze the two-phase flow behavior by considering the imbibition effect of the fracturing fluid retention in the shale gas reservoir. The impacts of the fracturing fluid imbibition and complexity of the fracture system on the two-phase flow are still unclear. We propose systematic experiments to overcome this difficulty, which could provide valuable indicative information on the two-phase flow. Valuable experiment data are provided, which can be used to validate analytical equations for gas/water flow in the shale fracture–matrix system.

1. Introduction

The steady domestic economic growth has led to an increase in demand for oil and gas. The conventional oil and gas resources cannot meet the high energy demand (Wang et al. 2020). The Chinese shale gas resources are widely distributed and have abundant reserves. The accumulated geological reserves of shale gas in the marine strata in the Sichuan Basin and its periphery are 764.3 × 109 m3(Zhang and Liu 2019). The shale reservoirs exhibit ultralow porosity and permeability (Du and Nojabaei 2019, Chai et al. 2019). In addition, the matrix permeability is generally of nD grade and the pore size is considerably smaller than those of sandstone (Javadpour et al. 2007). Various types of shale pores with multiple scales exist, including intragranular pores, microfractures, and fractures (Zou et al. 2013). The organic-rich shale has various hydrocarbon occurrences, mainly adsorbed gas and free gas, with a small amount of dissolved gas (Sheng et al. 2020). The above characteristics hinder the economic production from shale gas reservoirs (Yuan et al. 2015). The development of hydraulic fracturing technology in recent years has led to the developing value of shale gas considering the current oil price level (Zhou et al. 2015, Sheng et al. 2019).

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