Study on the Performance of High-Temperature Resistant Cross-Linked Acid for Acid Fracturing of Tight Carbonate Reservoirs
Authors: Wang Qing , Zhou Fujian , Zou Yeping , Yao Erdong 
Published: September 2022

ABSTRACT:
Acid fracturing is commonly used in tight carbonate reservoirs to increase production. The efficiency of acid fracturing is greatly influenced by the type of acid systems. With the development of carbonate reservoirs towards ultra-deep and ultra-high temperatures, the acid system faces greater challenges. In this paper, the temperature resistance, retardation, and conductivity of the new cross-linking acid were evaluated by rotating disk instrument, Haake Mars III rheometer, and FCS-842 conductivity meter, respectively. The results show that the viscosity of the cross-linked acid system is stable at 80 mPa.s in the temperature range of 120°C to 140°C, the acid-rock reaction rate is half lower than that of similar cross-linked acids, and it has good temperature resistance and retardation characteristics. The acid etching and conductivity experiments show that the new cross-linked acid etched rock slab to form channels with high initial conductivity, which is a good choice for acid fracturing in high temperature and deep carbonate reservoirs.

1. INTRODUCTION
According to HIS statistics, carbonate oil and gas resources account for about 70% of the global oil and gas resources, and about 60% of the world’s total oil and gas production comes from carbonate oil and gas reservoirs (Li et al., 2018). Chinese marine carbonate rocks are characterized by wide distribution, old age, deep reservoir depth, and high temperature. Deep carbonate reservoirs have low permeability and complicated pore structures. It generally composed of dissolved caves, fractures and dissolved pores with substantial heterogeneity (Zhao et al., 2008; Zhang et al., 2020). Low-permeability and tight carbonate reservoirs have no natural production capacity or low natural production capacity and require stimulation measures such as acidizing or acid fracturing.
Acidification means that the acid solution is injected into the formation below the fracture pressure of the formation. And the acid solution dissolves the rock to form wormholes, penetrates the pollution zone, and restores the formation permeability. Acidification is generally used in low-permeability carbonate reservoirs with polluted near-well-bore area, where the stimulation scale is small and the stimulation distance is limited (Mateus P. S. et al., 2020). Acid fracturing is similar to hydraulic fracturing in the stage of fracture generation (Yue et al., 2021), which uses high-viscosity fluid to open up the reservoir. Then, the acid fluid reacts with the rock, dissolves part of the rock, and makes the fracture surface non-uniform. Different from hydraulic fracturing (A. Suleimenova et al., 2016), acid fracturing does not use proppant. After the operation, the fracture props itself open with the relatively undissolved regions acting as proppants, and provides channels for oil and gas flow shown in Fig. 1. In order to obtain good performance, large scale of acid fracturing is generally used in tight carbonate reservoirs stimulation.