Accounting for Exterior Flow Using the Modified Logistic Growth Model for Unconventional Geopressured Shale Gas Reservoirs

Abstract

The scope of this work is to assess the flow beyond boundary-dominated flow, defining the existence of a third flow regime (after transient linear flow and boundary-dominated flow) that we call "exterior flow" for further use in decline curve analysis in unconventional reservoirs. Thus, exterior flow is defined as the linear flow of gas from the non-stimulated matrix feeding into the edges of the depleted stimulated reservoir volume (SRV), at late times, far away (predicted by Lee 2021; and anticipated by Marder et al. 2021). Sometimes referred as "flow beyond the tips" or post-SRV flow (Blasingame 2019). Because of the existence of this third flow regime, the production curves of over-pressured shale gas reservoirs, such as the Haynesville formation, seem to not be fitted by a single hyperbolic model or any of the modern rate-time relations (Power law exponential, Stretched exponential, Duong, etc.). We believe that the over-pressured condition of the formation—close to the lithostatic gradient in the Haynesville Shale for example—yields such high pressure drawdowns that all flow regimes (transient linear flow, boundary-dominated flow, and exterior flow) occur sooner compared to other basins. Additionally, this paper shows that a new member of the logistic growth family of curves, called the "modified Logistic Growth Model" in its 2023 version (m-LGM 2023) solves the problem of curve-fitting production data from horizontal wells in the Haynesville Shale. Furthermore, two novel diagnostic plots are presented for rate-time analysis to obtain the characteristic time of switching from boundary-dominated to exterior flow, which enables the prediction of additional volume to be produced under this flow regime. Finally, given that the current literature of the SPE-PRMS 2022 does not provide specific guidelines for the categorization of probable reserves (P2) in shale gas, we believe this work could signify a contribution for future reserves using this category for unconventional formations. The volume expected from exterior flow could be justified as probable reserves (P2) in shale gas wells.