On Track for Refrac: Targeting Under-Stimulated Stages and Assessing Casing Integrity Defects with High-Resolution Acoustic Imaging
Authors: Pehlke Trent , Simpson Greer , Maxwell Tamara , Bourgeois Charles , Stibbs Joshua , Littleford Thomas 
Published: January 2024

Over the past decade, advancements in fracturing technology and industry knowledge have revealed the feasibility and value of refracturing wells that were originally stimulated using legacy methods. Refracturing wells plays an integral role in the overarching pad-level production strategy, averting the depletion of parent wells following the fracturing of child wells. High-resolution acoustic imaging technology is a prevalent tool for assessing all facets of refracture operation. In a single run, this technology captures intricate corrosion and wall loss details, mitigating risks associated with subsequent refracture injection pressures and rates. While a new casing can be presumed to adhere to an API specification thickness within 12.5% of the nominal thickness, legacy wells, with potential age spans of up to two decades, lack such assurances. These assets are often exposed to corrosive environments containing H2S, mechanical wear, and previous exposure to high-rate slurry. The diagnostic imaging technology guides critical decisions using submillimetric datasets. It helps determine the necessity of running a protective tie- back string, establishes the safe depth for the liner hanger, identifies potential fracture-driven interaction (FDI) between neighboring wellbores through casing and liner deformation analysis, and conducts perforation erosion analysis for the initial stimulation proppant placement. This equips operators with insights to tailor the refracture program efficiently. It targets undrained reservoirs, mitigates corrosion or deformation concerns from the original casing, and minimizes the risk of total loss of parent wells due to FDI.

This paper discusses the application of high-resolution acoustic imaging for assessing pre-refracture well integrity, optimizing refracture designs, and boosting production rates. Assets were inspected for corrosion, ovality, mechanical wear, and connection defects. Using the submillimetric defect dimensions for wall loss events, Effective Area, RSTRENG, Modified B31G, and Barlow burst pressure calculations are conducted to determine the safe pressure envelope of the existing casing. This enables a definitive decision on whether to run a costly tie-back string to protect a damaged or corroded casing from failure during subsequent stimulation. The novel imaging technology enables operators to target under-depleted zones for restimulation, where outlying stages with low or nonuniform perforation growth are identified by capturing precise measurements of the initially stimulated perforations. Once identified, these understimulated stages can be selectively targeted in the refracture design by adjusting proppant poundage, selecting appropriate diverters, defining mesh sizing, optimizing slurry rates, or altering exit-hole area and phase, by adding perforations in the same or new orientation phase.

Finally, this paper presents two field-based case studies demonstrating the real-world deployment of this technology. It offers operators with a single-pass evaluation of well integrity and facilitates the optimization of production rates.