Multistage hydraulically fractured horizontal wells provide an effective means to exploit unconventional reservoirs. The current industry practice in the interpretation of field response often utilizes empirical decline curve analysis or pressure/rate transient analysis (PTA/RTA) for the characterization of these reservoirs and fractures. These analytical tools are based on many simplifying assumptions and cannot provide a detailed description of the evolving reservoir drainage volume from a well, the understanding of which is essential for unconventional reservoir and fracture assessment and optimization.
In our previous study, we developed a novel "data-driven" methodology for the production analysis of shale gas and shale oil reservoirs. The approach is based on the high frequency asymptotic solution of the diffusivity equation in heterogeneous reservoirs. It allows us to determine the drainage volume from a well, and the instantaneous recovery ratio (IRR), which is defined as the ratio of the produced volume to the drainage volume, directly from the production data. In addition, in a manner analogous to the diagnostic plot in PTA, a new w(τ) plot has been proposed to provide better insight into the depletion mechanisms and the fracture geometry.
In the current study, we extend this novel approach to the interpretation of the characteristics of the (potentially) complex fracture systems and drainage volume. We have utilized the w(τ) and IRR plots to the identification of field signatures that imply complex fracture geometry, formation linear flow, partial reservoir completions, fracture interference and compaction effects during production. The w(τ) analysis gives us the fracture surface area and diffusivity information while the IRR analysis provides additional information on fracture conductivity.
The major advantages of this current approach are the model free analysis without presumptions of flow regimes, and a simple and intuitive understanding of the drainage volume and fracture conductivity. The results of the analysis are useful for well and hydraulic fracturing operation design optimization and matrix and fracture parameter estimation.
Unconventional reservoirs such as shale oil and shale gas play a significant role in the US and the world energy market (Holditch, 2013). For these low permeability reservoirs, long horizontal wells with multistage hydraulic fracturing have been proven to be an effective way to stimulate the formation in most cases. However, due to large uncertainty in fracture complexity and reservoir heterogeneity, the characterization of the fracture systems and the prediction of well performance are always of paramount interest.