In recent years, Step-Down Tests (SDTs) are being increasingly used for diagnosing completions effectiveness in plug-and-perf (PnP) fracturing in unconventional wells. SDT is primarily used to quantify pressure drop related to perforation friction, near-wellbore tortuosity (NWBT), and to estimate perforation efficiency (PE) i.e. the fraction of active perforations at the end of a hydraulic fracturing treatment of a stage. In the industry, perforation efficiency is generally considered to be the yardstick for evolving limited entry designs and perforating strategies. In a typical SDT, the injection flowrate is reduced in 3 to 4 abrupt steps, each of duration long enough for the rate and pressure to stabilize, to enable interpretation of the rate and pressure response. However, simple as it may appear to be, the interpretation of SDT as a stand-alone diagnostic test has several assumptions and inherent non-uniqueness that are often ignored. This paper presents integrated data, diagnostics, and analysis from multiple completion types across multiple basins that demonstrates the methodology and uncertainties associated with SDT analysis.
In this paper, the SDT methodology was applied to 2 wells with different completion styles, and the interpretation was supplemented with fiber optics and bottomhole pressure gauges (BHPG). In the first well, SDTs were conducted on multiple stages of a cemented single-point entry (CSPE) sleeve completion that had well-defined, erosion-resistant openings to reduce uncertainties in the “perforation” pressure drop solution. In the second example, SDTs were conducted on multiple stages of a PnP well. Each PnP stage had two SDTs – one was conducted post pad but before proppant and another at the end of entire treatment, both with clean fluids.
The authors have highlighted the uncertainties with traditional SDTs and the need for integration with additional diagnostics. The analysis shows that the exponent of flowrate commonly used to quantify pressure drop associated with NWBT is largely uncertain. It also demonstrates the non-uniqueness of the SDT interpretation, and that a range of perforation diameters and a / the number of active perforations can match the SDT unless constrained with fiber optics data and perforation imaging data. The interpretation with constant perforation diameter is found to generally overestimate the PE. The SDTs before and after proppant slurry placement, if correctly interpreted, show an increase in perforation diameter with a reduction in PE post proppant placement. This paper demonstrates that without constraints on either eroded perforation diameter or on a / the number of active perforations, the interpretation of SDT is non-unique.
Distributed Acoustic Sensing (DAS) and Distributed Temperature Sensing (DTS) analysis also illustrates the variable and non-unique tortuosity, and/or complex stimulation domain architecture, in the near-wellbore region. It is therefore recommended that SDTs be interpreted with consideration of the inherent complexities and uncertainties, and preferably supplemented either with perforation imaging or DAS and DTS data for more accurate analysis.
To summarize, accurate interpretation of SDTs requires an interdisciplinary diagnostics approach, which is critical for optimization of limited entry designs.