A new fracture-injection/falloff type-curve analysis method is presented for reservoirs containing slightly compressible and compressible fluids. Type-curve analysis augments conventional before- and after-closure methods, which are also reformulated in terms of adjusted pseudopressure and adjusted pseudotime to account for compressible reservoir fluids. Unlike before- and after-closure methods which only apply to specific (i.e., small) portions of the falloff data, the new type-curve method allows for analyzing all falloff data from the end of the injection through fracture closure, pseudolinear flow, and pseudoradial flow. Similar to conventional well test analysis, a satisfactory interpretation requires comparable and consistent results between the special analysis methods, before- and after-closure, and type-curve analysis.
Fracture-injection/falloff testing has developed into a standard practice for evaluating reservoir properties prior to hydraulic fracturing.1 Although fracture-injection designs vary, a typical fracture-injection/falloff sequence requires a low rate, small volume injection of treated water or gas followed by an extended shut-in period when the pressure falloff is recorded. The test objectives include identifying hydraulic fracture closure stress, identifying the leakoff type, quantifying the magnitude of pressure-dependent leakoff, estimating the effective permeability to the mobile reservoir fluid, and determining the average or initial reservoir pressure.[1–2]
A fracture-injection/falloff sequence differs from conventional injection/falloff testing in that the pressure during the injection is sufficient to initiate and propagate a hydraulic fracture, and during the pressure falloff, the dilated fracture will contract and close. Provided the time of injection --- and fracture propagation --- is short relative to the response of the reservoir and length of the falloff period, the sequence can be analyzed using methods analogous to conventional pressure-transient tests.
The purpose of this paper is to introduce a type-curve method for analyzing a fracture-injection/falloff sequence in reservoirs containing slightly-compressible or compressible reservoir fluids. Additionally, to account for reservoir fluid compressibility, before-closure pressure-transient analysis[4–5] and after-closure impulse solutions are formulated in terms of adjusted pseudopressure and adjusted pseudotime. Using a low-permeability Mesaverde field example, we demonstrate that, similar to conventional well test analysis, a satisfactory interpretation requires that the results from special and type-curve analyses be consistent and comparable.
Before- and After-Closure Analysis Modifications
Mayerhofer and Economides4 and Mayerhofer et al. developed before-closure pressure-transient analysis while Gu et al. and Abousleiman et al. presented after-closure analysis theories for reservoirs containing slightly compressible fluids. Recently, Soliman et al. redefined the time function used in the after-closure analysis for consistency with the method presented by Soliman for analyzing a pressure buildup test with a short producing time.
Before- and after-closure analysis methods allow only specific portions of the pressure decline during a fracture-injection/falloff sequence to be quantitatively analyzed. Before-closure data, which can extend from a few seconds to several hours, can be analyzed for permeability and fracture-face resistance, and after-closure data can be analyzed for reservoir transmissibility and average reservoir pressure provided bilinear, pseudolinear, or pseudoradial flow are observed.
Before- and after-closure analysis methods assume the reservoir fluid is slightly compressible, but the solutions can also be derived in terms of pseudopressure and pseudotime, or for convenience, adjusted pseudopressure and adjusted pseudotime, to account for reservoir fluid compressibility.