Diagnostic fracture-injection/falloff tests are the primary test method for prefracture calibration of stimulation treatment designs in unconventional reservoirs. Diagnostic fracture-injection/falloff tests can provide insight into the geomechanical and reservoir properties of a formation using the interpretation of specialized diagnostic plots. Collecting quality injection falloff test data in underpressured reservoirs can be challenging because of the tendency for the wells to go on vacuum. This occurs when the wellbore fluid hydrostatic pressure exceeds the formation pressure. This is true whether the pressure is recorded at surface or bottom hole. A downhole shut-in tool can isolate the formation from the hydrostatic pressure created by fluid above the tool, which helps prevent the well from going on vacuum. However, this method can add to testing costs, increase operational complexity, and increase the potential for test failures.
This paper presents a new method for test design and analysis of diagnostic fracture-injection/falloff tests conducted in underpressured reservoirs. A case study demonstrates a new workflow that combines a hybrid nitrogen fracture-injection/falloff test (NFIT) design using nitrogen as a displacement fluid and a new well-testing type-curve analysis methodology. This specialized workflow helps enable the creation of successful stimulation designs in wells that would otherwise be costly and operationally challenging to test and stimulate effectively.
Diagnostic fracture-injection/falloff tests are the primary method for prefracture calibration of stimulation treatments in unconventional reservoirs. These tests include injecting a small volume of fluid into the formation at a pressure that exceeds the fracture propagation pressure. This is followed by a shut-in period that allows pressure falloff. The injection/falloff sequence is recorded with high-resolution pressure transducers. Then, the data are analyzed to determine geomechanical and reservoir properties for fracture design and reservoir characterization.
The diagnostic fracture-injection/falloff tests analysis workflow uses multiple diagnostic plots, which provide a consistent method of observed data interpretation. The workflow comprises fracture property analysis, flow regime identification, straight-line before- and after-closure analysis, and a relatively new well-testing type-curve analysis. Classical G-function derivative analysis and log-log diagnostics as described by Barree et al. (2007) provide the methods for before-closure fracture property analysis and flow regime identification. Straight-line before- and after-closure analysis uses specialized Cartesian plots of the pressure falloff data with straight lines fitted to the observed data corresponding to the correct flow regimes. Conceptually, the diagnostics and analysis methodology are straightforward. However, observed falloff diagnostics in unconventional reservoirs are often misinterpreted, which can lead to erroneous fracturing parameters, reservoir pressure, and permeability estimates (Barree et al. 2007).