Tool-Free Fishbone Fracture Acidizing Stimulation Implemented in Openhole Horizontal Well

Abstract

Abstract

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 228992, “Pioneering 25-Stage Tool-Free Fishbone Fracture Acidizing Stimulation in Openhole Horizontal Well: Field Implementation in Low-Permeability Carbonate Reservoir in Central Iraq,” by Yong Xiao, Cheng Ma, and Liping Jiang, China Zhenhua Oil Company, et al. The paper has not been peer-reviewed. _ The irregular wellbore trajectory of openhole horizontal wells poses significant challenges to tool accessibility, especially for efficient and low-cost multistage stimulation. The authors share their experience with tool-free fishbone-like fracture acidizing stimulation and detailing the methodology and performance in unlocking the potential of a reservoir with reduced capital expenditure. The authors write that this strategy delivers the industry’s first tool-free fishbone fracture stimulation technology by sand jetting and acidizing in openhole wells, providing a technical solution for the low-permeability and complex lithology carbonate reservoirs in the Middle East. Petrophysical Characteristics The low-permeability carbonate reservoir is in central Iraq. Its first well was drilled in 1975, with 17 wells drilled before the current operator took over. Four layers of the carbonate reservoir were confirmed: Hartha, Sa’adi, Tanuma, and Khasib. Since the official development of the oil field, openhole horizontal wells with acidizing completions have constituted the main development strategy. Core description and geological logging analysis reveal that these reservoirs are buried at depths of 1,800 to 2,000 m, with normal pressure and temperature gradients, an average porosity of 20%, and an average permeability of 1.73 mD. The lithology primarily consists of light gray to gray packstone, grainstone, and wackestone, with abundant biofragments. The reservoir spaces are dominated by vugs, interparticle pores, dissolution, and moldic pores. Overall, pore connectivity is relatively poor. Systematic petrophysical tests show that the reservoirs can be classified into two categories: medium porosity and low permeability in three of the reservoirs, and low porosity and low permeability in Sa’adi. The pore-throat characteristics of the Sa’adi reservoir are inferior to those of the other three formations. The presence of insoluble minerals within this reservoir adversely affects the performance of conventional acidizing treatments. Methodology of Tool-Free Fishbone Fracture Acidizing Stimulation The Sa’adi formation is a low-porosity, low-permeability, tight, muddy carbonate reservoir with a temperature of 77°C and a pressure coefficient of 1.16 at true vertical depths of 2,200 m. Conventional dragging matrix acidizing for skin removal fails to achieve an economic production rate of at least 700 B/D in openhole horizontal wells. Standard coiled tubing (CT) acid fracturing cannot maintain sufficient energy downhole to break down the reservoir because of large friction losses, making it impossible to form a high-conductivity channel initiating tens to hundreds of meters into the formation. Consequently, the stimulation methodology must shift from damage removal and productivity restoration to constructing high-conductivity artificial fractures for production enhancement. Openhole multistage stimulation faces inherent limitations: The irregular wellbore trajectory limits the entry of large tools, mechanical isolation tools are unsuitable because of the rough openhole wellbore wall, and the exposed long openhole section causes uncontrolled fluid intake that prevents precise staged stimulation.