Accounting for Nonuniform Induced Properties in Production Analysis of Unconventional Reservoirs
Fuentes Cruz, Gorgonio
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Several conceptual models for unconventional reservoirs have been proposed in recent years based on extensions of well-studied analytical/semi-analytical models for conventional reservoirs. The standard semi-analytical approaches assume uniform properties in the reservoir. In this study, we develop new models for production data analysis of hydraulically fractured wells based on the concept of nonuniform induced properties, in particular, the induced permeability field and the induced interporosity flow field. In the induced permeability field approach, we consider the case when the hydraulic fracturing operation alters the ability of the formation to conduct fluids throughout, but in varying degrees depending on the distance from the main hydraulic fracture plane. In the induced interporosity flow field, we assume that, as a result of the hydraulic fracturing treatment, the density of micro-fractures (natural and induced) is high near the hydraulic fracture face, but gradually decreases away from it. We also address common issues related to variations in the wellbore pressure, desorption effects, and non-linearity caused by gas flow, with the intent to provide a simple, yet clear understanding of their effects on the production performance. The methods used in this work include mostly semi-analytical techniques (Laplace transform and numerical inversion to the time domain). Analytical (formulae in the time domain) and numerical simulation (finite difference) techniques are also used to validate the results from the new models. The results indicate that the maximum and minimum induced permeabilities may provide the key to evaluate the overall completion efficiency in unconventional formations, where the extent and quality of the stimulated volume are equally significant. Also, the closely spaced micro-fractures have a strong impact on well performance, even when their density is diminishing toward the far parts of the stimulated reservoir. We conclude that the new models preserve the typical linear-flow signature of commonly observed well performance of unconventional shale reservoirs; however, the extrapolation of the production behavior departs from the standard models significantly. This research contributes to the understanding of the production behavior of unconventional reservoirs to characterize the quality of the stimulated reservoir and to consider often neglected factors effecting forecasts of well performance.