Long-term (multi-year) buildup tests conducted for multi-fractured horizontal wells (MFHWs) completed in shale reservoirs offer the unique opportunity to study and analyze flow-regimes sequences that are not commonly observed with typical buildup test periods. In this study, two buildup periods (including a rarely-observed, nearly 5-year buildup), and the preceding extended flow tests, were analyzed for a MFHW completed in an Australian shale gas reservoir within the Beetaloo Basin. The objectives of the analyses were to a) identify the sequence of flow regimes observed for each test (flow/buildup) period; b) extract estimates of reservoir permeability and hydraulic fracture properties; and c) study the evolution of these properties with each subsequent test.
A MFHW, the Amungee NW-1H, completed in the Velkerri B shale in Australia, was analyzed. Due to a casing deformation, and inability to mill out plugs beyond this, most of the flow contribution was from the heel stages of the well. The first flow/buildup period was conducted from 2016-2021 (a nearly 5-year buildup), while the second flow/buildup was initiated in 2021 (buildup is currently continuing). The extended (> 1 month) production tests (EPTs) preceding the buildups were analyzed using RTA methods (flow-regime identification/straight-line/type-curve analysis) modified for shale gas properties (e.g., desorption), while the buildups were analyzed using classic PTA methods.
The first (~5-year) buildup period (BU #1) revealed a sequence of bilinear-linear-elliptical-pseudoradial flow followed by a second linear flow period. The first two flow regimes are interpreted to be associated with inter-fracture flow, while the latter is assumed to correspond to linear flow to the well. Elliptical/radial flow around fractures is rationalized to occur due to interpreted relatively short fracture half-lengths (corresponding to the high-conductivity portion of the fractures). Permeability estimates are in good agreement with DFIT analysis. Flow-regime interpretations for the other test periods (EPTs #1 and #2, BU #2) are largely consistent, although EPT #1 flow-regime interpretation was challenged by noisy data. Permeability values derived from EPT #1 and #2 are smaller than from buildup tests, suggesting stress-sensitivity caused by drawdown. Properties derived from the analysis of BU #1 and #2 are in good agreement, suggesting that any effects caused by stress-sensitivity of reservoir parameters are largely reversible. Permeability derived from all tests are much larger than those obtained from laboratory data, leading to the interpretation that natural fractures are elevating system permeability. Fracture half-lengths are also much shorter than those typically reported for MFHWs.