Acid-Fracture Conductivity Evaluated for Austin Chalk Formation

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This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 227921, “Evaluation of Acid-Fracture Conductivity of the Austin Chalk Formation,” by Jhonny E. Colina Arias, SPE, A.D. Hill, SPE, and Ding Zhu, SPE, Texas A&M University. The paper has not been peer‑reviewed.

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In this study, downhole cores from the Austin Chalk formation were used to evaluate the performance of acid fracturing under realistic reservoir conditions. The results show that acid fracturing can generate sufficient conductivity comparable to or higher than that achieved with 100-mesh proppant at 0.05 lbm/ft2. The formation’s layered geologic structure strongly influenced results, and uneven etching patterns contributed more to conductivity than total etched volume alone. Supporting tests on Austin Chalk outcrop samples confirmed similar behavior. These findings reinforce the argument that acid fracturing is a strong alternative stimulation method to improve productivity in the Austin Chalk formation.

Introduction

The Austin Chalk is a naturally fractured, low-permeability carbonate reservoir with high carbonate content and a long history of development. While modern completion strategies rely heavily on multistage proppant fracturing, acid fracturing still is considered a potential alternative in this formation. Its potential lies in its ability to enhance existing natural fracture networks without the logistical challenges and costs associated with proppant hydraulic fracturing. However, several uncertainties remain regarding how acid treatments behave under actual reservoir conditions, especially in formations with heterogeneous mineralogy and variable surface textures.

This study aims to evaluate the etching behavior and conductivity generated by acid fracturing using downhole core samples from the Austin Chalk.

Methodology

The procedure consists of six main stages:

- Sample preparation for acid injection

- Preacidizing surface scanning

- Acid injection

- Post-acidizing surface scanning

- Sample preparation for conductivity testing

- Fracture conductivity measurement

Four downhole core samples from an Eagle Ford well in south Texas were cut into rectangular blocks with rounded edges. Saw-cut fractures were used instead of induced fractures to minimize variability and preserve the limited core material. Each sample was coated with a silicone rubber compound to ensure a proper fit and seal within the test cell.

Baseline fracture surface topography was measured using a laser profilometer or computed tomography (CT) scanner before acid treatment. Samples were then inserted, in vertical orientation, in a modified conductivity cell. A fracture aperture of 0.15 in. was kept during acid injection. Following sample scanning, acid injection was conducted using an etching apparatus under downscaled field conditions. A schematic of the acid-injection experimental setup is shown in Fig. 1. Post-acidizing fracture surfaces were scanned and surface differences before and after acid injection were used to calculate etched volume and visualize the resulting etching pattern.