Acid Fracturing: New Insigths on Acid Etching Patterns from Experimental Investigation

Abstract

Nowadays, it is commonplace to say that acid fracture conductivity depends on the fracture face asperities. Does it really depend on it? Almost thirty years ago, someone wrote, "We believe the conductivity measured in these tests is mainly due to the smoothing of peaks and valleys on the rough fracture faces, and is independent of rock heterogeneities due to the small sample size." Moreover, almost one year ago, one wrote, "More asperities touch and deform as the closure stress increases. The channels become even shorter and fewer openings are left." Between these two extremes, the asperities came to be pointed out as an essential factor to generate acid conductivity. Many published results from small, wet sawed and leveled carbonate rock samples support such claim. We did the same. Our experimental investigation on small scale carbonate samples with sawn faces, both from outcrops and well cores, reconfirm the existence of three main acid patterns namely uniform, channels and roughness. The design of experimental apparatus prevented that those etching patterns were artifact patterns. The acid etching patterns determine different conductivity behavior under confining stress. However, hydraulic fractures are tensile fractures and they are naturally rough. In nature, there is no such thing as a leveled fracture face. Tensile fracture faces could be rougher than fracture face after acid reaction. In fact, the first experimental results show that after acid reaction, linear roughness of tensile fractures can be larger, equal or less than linear roughness before acid reaction. This paper presents experimental results and discusses the asperities paradigm.

Building Acid Frac Conductivity in Highly-Confined Carbonates

“…In addition, they can help to characterize the different etching patterns as defined elsewhere (Antelo et al, 2009;Pournik et al, 2009) namely channels, roughness, cavity, uniform and turbulent. These images are in the companion paper (Neumann et al, 2012).…”

Comparative Experimental Study of Fracture Conductivity of Carbonate Rocks under Different Stimulation Types

“…The experimental results showed that viscoelastic acid generated the highest conductivity at the stress of low closure and emulsified acid resulted in the largest retained conductivity at higher loads. Neumann et al 31 experimentally demonstrated that the fracture conductivity of elastic fracture mainly depended on the spacing between the "concave" and "convex" surfaces formed by acid etching. Al-momen et al 32 used dolomite cores to study the effects of contact time, acid type, and temperature on fracture conductivity.…”

A method for predicting fracture conductivity based on three‐dimensional spatial morphology features

“…Rodrigues et al found that the surface morphological parameters of the acid dissolution were used only to characterize the preflow capacity of the fracture, and conductivity of the fracture depends on the width of the fracture, 13 and the roughness and torsional effect of the fracture is less than the smaller one. Neumann et al found that the acid dissolution pattern determines the change of the fracture flow capacity under closure stress, and the fracture roughness may be greater than or less than the original split fracture 14 . Mehrjoo et al developed three models of three different acid types.…”

“…Neumann et al found that the acid dissolution pattern determines the change of the fracture flow capacity under closure stress, and the fracture roughness may be greater than or less than the original split fracture. 14 Mehrjoo et al developed three models of three different acid types. The electrical conductivity of rock after acid etching is predicted according to the injection rate, rock strength, and closure stress.…”