Flowback-Based Minimum Stress Estimate in Low-Permeability Environment: Procedure, Interpretation, and Application in the Vaca Muerta Shale

Abstract

Low-permeability formations must be hydraulically fractured to produce at commercial rates. A good understanding of the formation stress conditions is critical for completion design, but requires in-situ measurement as calibration to support the geomechanical evaluation. This calibration is commonly done using diagnostic formation injection test (DFIT) methodology by creating a hydraulic fracture and then waiting for it to close through leakoff to the formation. However, in a low-permeability low-leakoff environment, application of this approach might become limited because of the time to fracture closure and the non-uniqueness of the interpretation. This paper demonstrates the applicability of the fracture flowback method to define closure pressure. Although proposed in the 1980s, this method has been underused by the industry. One of the objectives of this paper is, therefore, to advocate through field examples its simplicity of development, interpretation, and repeatability, particularly in low-leakoff reservoirs such organic shale formations. The procedure is composed of a sequence of various cycles of pump-in flowback through a fixed choke, pressure rebound, and fracture reopening. The proposed methodology offers several minimum stress measurements for repeatability and quality check purposes, reduces interpretation non-uniqueness, and can be completed within 1 hour, making it compatible with the hydraulic fracturing operations. Test design considerations, such as well geometry, pump rate, fluid volume, choke size, or perforation requirements, are reviewed to maximize the chance of success. Interpretation of the different possible patterns that can be observed is discussed and illustrated with practical examples from the Vaca Muerta shale. Comparison is made between the pump-in flowback and calibration decline approaches performed over the same interval. Repeatability is evaluated, and discrepancies between cycles are investigated. A direct application of the method is the calibration of a stress profile when applied to a vertical well. However, additional observations related to the fracture closure mechanism or residual fracture conductivity can be drawn by detailing the flowed-back volumes or rate of rebound pressure. These observations can be related to the lithology when the procedure is implemented in different intervals of the same formation.

An Overview of Recent Developments and Understandings of Unconventionals in the Vaca Muerta Formation, Argentina

“…The exact timing of fracture closure during the flowback phase is ascertained through a volume-pressure graph. The intersection point of the extrapolated pre-closure and post-closure flowback curves on this graph pinpoints the exact moment of fracture closure [59,70].…”

“…The deeper zones, often referred to as "La Cocina," show lower closure pr likely due to their increased organic matter, which mitigates the impact of stresses. Conversely, the shallower regions of the Vaca Muerta formation exhibi net pressures, indicating a need for potential adjustments in completion approa improved fracture containment strategies [70]. Areas within the formation that h organic content but are rich in carbonate layers display the highest closure pressu complexity of these stress distributions is exacerbated by the tectonic activity of t cent Andes Mountains, underlining the necessity for a nuanced understanding factors for effective hydraulic fracturing (Figure 7).…”

“…The deeper zones, often referred to as "La Cocina," show lower closure pressures, likely due to their increased organic matter, which mitigates the impact of tectonic stresses. Conversely, the shallower regions of the Vaca Muerta formation exhibit higher net pressures, indicating a need for potential adjustments in completion approaches or improved fracture containment strategies [70]. Areas within the formation that have low organic content but are rich in carbonate layers display the highest closure pressures.…”