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Rethinking Brittleness — How Viscoplastic Stress Relaxation Affects Layer-to-Layer Stress Variations and Vertical Hydraulic Fracture Propagation in Unconventional Reservoirs


Rethinking brittleness - how viscoplastic stress relaxation affects layer-to-layer stress variations and vertical hydraulic fracture propagation in unconventional reservoirs

Shaochuan Xu

Ankush Singh

Mark Zoback

Department of Geophysics

Stanford University

In this paper we consider the linkage between ductility (the degree to which formations are not brittle, as it is normally defined), layer-to-layer variations of the least principal stress magnitudes and its effect on hydraulic fracture propagation and proppant placement. We first review an extensive series of experimental studies (using samples from a number of different shale plays), that documents viscoplastic creep in unconventional reservoirs of varying lithology. In a number of cases, viscoplastic creep was also measured with samples from formations above and below the producing formations. We then review related studies in which a relatively simple constitutive law can be used to predict the way in which the degree of viscoplastic stress relaxation in various layers results in variations of the magnitude of the least principal stress varies with depth. Finally, we examine one case study in detail in which a detailed profile of stress magnitudes is available from DFIT tests to supplement the creep measurements and theoretical stress predictions. Using this profile, we show how these stress variations are likely to affect hydraulic fracture propagation. We demonstrate the interplay between the stress stratigraphy and operational parameters such as the stratigraphic position of the landing zone, the stage length, the number of perforations, perforation diameter, pumping rate, proppant concentrations, etc. Predicting viscoplastic creep dominated layer-to-layer stress variations is found to be very useful to optimize vertical hydraulic fracture propagation.