Mechanical properties and microscopic characteristics of acid etching softening layer in limestone fractures

Abstract

Carbonate reservoirs are often stimulated by acidizing and acid fracturing techniques， but the microstructure of the rock surface is corroded and destroyed after acid etching， and the mechanical properties of the rock are changed significantly， which further affects the final stimulation effect. The experiment on the mechanical properties， the study on surface morphological characteristics by scanning electron microscopic （SEM）， and the microscopic pore structure characterization by nuclear magnetic resonance （NMR） are carried out before and after limestone acid etching to clarify the influence of acid etching on the mechanical properties and microscopic pore structure changes of limestone. The results show that acid etching softening layers with a certain thickness of about 11.61 μm appear on the rock surface after acid etching. Remarkably， the compressive strength， elastic modulus， and surface hardness of limestone are all reduced to about 40% before acid etching due to the acid etching softening layers. Dissolved pores of different sizes are formed inside the rock surface after acid etching and stress concentration is induced around the dissolved pores， changing the stress distribution of the acid etching softening layers. Under high closure stress， the dissolved pores are easier to crush deformation or collapse failure， which is also the main reason for the change of mechanical properties of acid etching softening layers. According to the T2 spectrogram of NMR， it is found that there is the largest change in the number of pores with pore sizes between 1 nm and 1 μm， and the acid dissolution alters the internal pore structures of the acid etching softening layers， hence further affecting the mechanical properties of rock after acid etching.

Keywords

• limestone
• acid etching softening
• pore characteristic
• mechanical property
• acid fracturing