ABSTRACT:

This paper establishes an analytical model for dolomite acid-etched damage at the laboratory scale with the combination of reaction kinetics of acid-rock and damage mechanics, where the damage variable is defined as the reduction of effective bearing area. In addition, the expression of damage variable will be derived in calculus and simplified by the definition of dimensionless variable. The microscopic mechanism of acid-etched damage process has been revealed analytically, with a theoretical preliminary exploration of the coupling effect of acid damage and mechanical parameters. Combined with Lemaitre's equivalent strain principle to visualize dynamic change of the elastic modulus, and the deterioration dynamics of elastic modulus has been described quantitatively after plotting the damage trend graph under different dimensionless variables, which can provide reference for the prediction of fracture pressure and guide the design of deep acid fracturing.

1. Introduction

The acid fracturing technology degrades the rock mechanics parameters of the reservoir through the dissolution between the acid and rock, and can effectively reduce the fracture pressure, which is one of the efficient measures to develop deep carbonate reservoirs (Wei et al., 2018). The acid rock reaction changes the microscopic material composition and structure inside the rock, which changes the macroscopic mechanical characteristics of the rock. During the acid-etched process, chemical dissolution occurs at the interface between the acid and the rock, causing uneven dents on the rock surface. On the one hand, it causes damage to the reservoir rock, which manifests the deterioration of mechanical parameters; on the other hand, it also forms high-speed channels of oil and gas which can increase diversion capacity. At present, scholars have conducted a lot of research on rock damage during acid-etched.

The experimental research has been carried out firstly and found that water has a significant effect on the fracture of single crystal quartz (Atkinson et al., 1981), and the adsorption of water vapor reduces the free energy on the mineral surface; Through theoretical analysis, the chemical process, dissolution and solid-state diffusion and alteration in the water-rock system all have important effects on the rheology of crustal rocks (Kirby, 1984). These early explorations indeed provided a basis and reference for future research on acid rock; Hutchinson et al. (1993) used limestone as the experimental object, and used different chemical solutions to simulate the acid type. It is found that limestone dissolves in acidic solution, while it follows the law of physical dissolution in sodium salt solution, which has begun the research of acid rock action. However, the dissolution of rock in acid is only the most basic experimental phenomenon, and further exploration is needed for the research of rock mechanical properties. Haneef et al. (1993) used electronic probes and other techniques to conclude that the corrosion of granite and other materials is more serious than that of a single case when they are in the grounding position. This research result confirms the dissolution effect of rocks under the action of acid rock from a microscopic point of view; The triaxial compression experiment is conducted to study the influence of different concentrations of salt solution on the friction coefficient of sandstone cracks systematically (Feucht et al., 1990). The research method of triaxial compression test began to show its advantages for the study of acid rock action, which has been widely used in subsequent related research.

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