Abstract

Currently, acid etching and conductivity experiments mostly use smooth rock samples to simulate the formation of acid fractures. There are few reports on the influence of fracture rough morphology on acid etching behavior and conductivity. In this paper, digital carving technology is used to repeatedly produce artificial carbonate rock samples that are consistent with the natural fracture surface morphology, and the acid injection parameters are controlled to perform single-factor acid etching tests. The initial roughness of different fracture surfaces and the effect of acid injection rate are studied. The study shows that the carved carbonate rock sample has good consistency with the original natural fracture surface. Acid has the effect of smoothing the high points and enlarging the valleys or deeper zones on the rough fracture surfaces. Initial roughness causes non-uniform etching, resulting in uneven fracture surface mismatch is the main reason for the higher acid fracture conductivity at low closure stress. Increasing acid speed will not be accompanied by linear roughness, mismatch, and aperture increasing all the time. Reasonably optimizing acid injection rate can obtain better etching morphology and conductivity at certain closure stress.

Introduction

Acid fracturing is one of the most effective stimulation technology to obtain industrial productivity in carbonate reservoirs. This method includes two stages: fracture formation in rock and etching of fracture surfaces (Melendez et al., 2007). The main objective of acid fracturing is to make the acidized fracture a permanent conductive flow path that endures the overburden pressure and effectively connects the reservoir to the wellbore (Lu et al., 2017; Malagon et al., 2008).

Laboratory measurements on acid fracture conductivity are important for acid stimulation design (Anderson & Fredrickson, 1989; Deng et al., 2009). Previous experimental studies suggest that fracture conductivity are mainly dominated by etching channels and rock strength after acid etching (Beg et al., 1996; Pournik et al., 2009), which are affected by formation type (Beg et al., 1996; Hill et al., 2007), acid type (Pournik et al., 2010; Pournik et al., 2009), acid concentration(Pournik et al., 2013), acid-rock contact time (Domelen & M.,), formation type (Hill et al., 2007), and temperature (Anderson & Fredrickson, 1987), etc. Beg et al. (1996) indicated that deep channels created along the fracture face would obtain very high conductivity at high closure stress. Dong et al. (1999) studied the etching morphology was in turn function of aperture and there appeared channel etching pattern with a larger aperture. Hill et al. (2007) conducted an acid etching test at different times with diverse acid systems. The narrow channel etching pattern with greater roughness was created by viscoelastic acid yielded for a short time, which yielded the highest conductivity.

This content is only available via PDF.
You can access this article if you purchase or spend a download.