Underbalanced Coiled Tubing Approach Targets Natural Fractures in Tight Sandstones
Authors: Carpenter Chris 
Published: June 2024

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This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper IPTC 23188, “Underbalanced Coiled Tubing Technology in Tight Sandstones: A Success Story by Integrating Petrophysics, Geophysics, Flow Data, and Pressure-Transient Analysis To Target Natural Fractures,” by Ali R. Al-Nasser, Ali J. Al-Solial, SPE, and Abdulrahman Y. Abushal, SPE, Saudi Aramco, et al. The paper has not been peer reviewed. Copyright 2024 International Petroleum Technology Conference.

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The complete paper describes the use of underbalanced coiled tubing drilling (UBCTD) technology in tight sandstones, using an integrative approach that incorporates petrophysical, geophysical, and reservoir engineering data. The primary objective is to distinguish between high matrix permeability and natural fractures, focusing on a localized high-permeability region subject to detailed analysis before implementing a UBCTD operation. The integrative methodology examines various data sets, including log data, production-logging-tool (PLT) results, seismic interpretation, well rates, and pressure transient analysis (PTA).

Predrilling Analysis Approach
The success of any well is heavily contingent on strategic placement, a critical factor magnified in the context of UBCTD. Misplacing a well in UBCTD can have catastrophic consequences for productivity and severability. While targeting areas of high flow capacity (kh) is desirable, steering clear of unstable zones and materials prone to creep is equally crucial. UBCTD emphasizes the need for meticulous analysis and a profound understanding of the development area.
UBCTD wells are crafted as producers with the sole purpose of efficient production rather than extensive evaluation. Fracture networks, generally perceived as a risk in conventional drilling, are less problematic in UBCTD. Intentionally targeting zones of loss circulation becomes a viable approach to enhance productivity. This analytical focus on targeting fractures for increased productivity can be extrapolated to target high-permeability streaks and optimize deposition in UBCTD wells.
The proposed workflow for similar projects entails a thorough investigation leveraging multiple data sources to assess various reservoir parameters. The key components include production rates, porosity and permeability evaluations from logs and cores, kh determination through PTA, and examination of fracture responses through image logs. The authors stress that PTA is not merely a criterion for UBCTD but is a valuable reservoir characterization tool.
Analyzing scenarios with high flow rates, high porosity or permeability, and moderate kh—or, conversely, high flow rates, moderate porosity or permeability, and exceptionally high kh—can be relatively straightforward. However, complications arise in scenarios with poor production rates and high kh coupled with fractures visible in image logs, indicating inadequate vertical lift performance. Another challenging scenario involves poor production rates, low kh, and fractures visible in seismic and image logs. In this case, the observed fractures signify issues such as near-wellbore damage, restricting fluid flow and affecting reservoir productivity.
The UBCTD approach is tailored to consider these challenges, drawing insights from the understanding of reservoir conditions.