Experimental Investigation on Mechanical Characteristics of Shale Under Water-Rock Interaction
Authors: Cao Gang , Ma Tianshou , Zhang Chao , Liu Yang , Shi Yufan 
Published: September 2022

ABSTRACT:
Shale mechanical properties under water-rock interaction have a significant impact on wellbore stability, casing integrity, and hydraulic fracturing for shale oil and gas reservoirs. Most previous studies focused on the influence of fluid type and soaking time on shale mechanical properties, but the influence of soaking conditions (temperature and pressure) was seldom investigated. In order to investigate the influence of temperature, pressure, and soaking time on shale mechanical properties, the shale specimens were soaked under different temperatures and pressures, a large number of uniaxial compression experiments were carried out on the soaked shale specimens. The elastic modulus, Poisson’s ratio, and peak strength of shale rock were selected as the response factors, and the second-order response surface model was employed to analyze the interaction relationship among mechanical properties and soaking conditions. The results indicated that the influence degree of a single factor on shale mechanical properties follows the order: soaking time &gt; temperature &gt; pressure. The response surface analysis results indicated that the influence degree of multi-factor interaction on shale mechanical properties follows the order: soaking time and temperature &gt; soaking time and pressure &gt; temperature and pressure, which confirmed that the interaction of soaking time and temperature has a significant impact on shale mechanical properties. The experimental results can help us better understand the evolution of shale mechanical properties under water-rock interaction, and it can also provide guidance for wellbore stability, casing integrity, and hydraulic fracturing for shale oil and gas exploitation.

1. INTRODUCTION
In recent years, shale gas and coalbed methane, as a low-carbon and clean unconventional natural gas, have attracted more and more attention (Jia et al. 2012, 2016; Soeder 2018; Dong et al. 2018; Nie et al. 2021). The gas shale reservoir is usually featured by low porosity, low permeability, low abundance, and low productivity, horizontal well and hydraulic fracturing are the key technologies to exploit shale gas (Hu et al. 2012; Li et al. 2019). However, due to shale is rich in clay minerals and micro-fractures, which make it easy to occur water-rock interaction when contact with downhole working fluids, such as water-based drilling fluids and slick water fracturing fluids, which will change the mechanical properties of shale and have a significant impact on wellbore stability, casing integrity, and hydraulic fracturing of shale oil and gas reservoirs (Ma and Chen 2014; Ghanbari and Dehghanpour 2015; Favero et al. 2016; Ma et al. 2016, 2018, 2020, 2021; Lyu et al. 2018). In order to investigate the mechanical characteristics of rocks under water-rock interaction, a large number of experiments had been conducted, and previous experimental results indicated that the influencing factors that degrade the mechanical properties of rocks by water-rock interaction are mainly mineral composition, fluid type, formation temperature, and pressure (Eeckhout 1976; Hsu and Nelson 2002; Youn and Tonon 2010; Lyu et al. 2015; Ma and Chen 2015; Liu et al. 2020). Wong (1998) conducted the soaking and swelling tests of shale rock with different concentrations of salt water at normal temperatures and pressures, and the results indicated that the strength and elastic modulus decreased with increasing the degree of water immersion, while the Poisson’s ratio varied independently. Tang (2000) and Tang et al. (2002) studied the time effect of different rock mechanical parameters under the action of different hydro-chemical solutions through mechanical tests at normal temperature and pressure, analyzed the effect of hydro-chemical damage, and discussed the quantitative characterization method and mechanism of chemical damage. Al-Bazali et al. (2008) studied the influence of water-based fluids on the compressive strength and acoustic velocity of different types of shale rocks, and the results indicated that the ionic content of water-based fluid has a significant impact on the change of shale properties. Talal (2013) studied the effect of ion diffusion on the compressive strength of dry shale, wet shale, and wet shale with chemical equilibrium, the results indicated that the ion diffusion and infiltration cause to decrease of compressive strength, and the mechanical stability is therefore affected adversely. Ma and Chen (2014) proposed a quantitative evaluation method of mesodamage of shale hydration by using CT scanning technology, and the evolution of shale hydration damage was analyzed, and Ma et al. (2016a) further proposed the damage constitutive model of hydrated shale according to the evolution equation of shale hydration damage. Gui et al. (2018) measured the acoustic velocity of shale with different coring angles under different confining pressure and soaking time of drilling fluid, and analyzed the effects of stress and hydration coupling on acoustic velocity, elastic parameters, anisotropy and damage of shale. Zhang et al. (2020) investigated combined effect of anisotropy and hydration on compressive strength of shale rock under four different confining pressures, the results indicated that with increasing moisture content, the compressive strength and axial strain at peak stress decrease, apparent elastic modulus decreases nonlinearly, and average apparent Poisson’s ratio increases approximately linearly. Zhao et al. (2021) analyzed the micro-structure and crack evolution law of shale samples under normal temperature and pressure soaking conditions, obtained the degradation law of mechanical parameters of shale samples under different confining pressures, and revealed that under the action of hydration Damage effects on the degradation of shale mechanical properties.