Arthur Bale

Deformation bands and their influence on fluid flow

Deformation bands represent a common type of strain localization in deformed porous sandstones and occur as single structures, as clusters, and in fault damage zones. They show from zero to six orders of magnitude reduction in permeability and may therefore potentially affect fluid flow. We here present mathematical calculations indicating that uncommonly high permeability contrasts and/or exceptionally high band concentrations are required for deformation bands to significantly affect production rate. We also present field observations showing rapid variations in porosity and permeability along deformation bands and deformation-band zones alike. Furthermore, many paleofluid fronts seen in the field are unaffected or only gently affected by deformation bands. Together, these calculations and observations suggest that their function during reservoir production is small or negligible in most cases. Structural complications caused by subseismic faulting and complex fault anatomy are more likely to cause production problems, in addition to stratigraphic and diagenetic effects. Nevertheless, the arrangement and orientation of deformation bands may have an effect on the flow pattern and reservoir sweep. In cases where deformation bands do cause production problems, it may be possible to resolve these by means of hydraulic fracturing.

Enhanced 2D Proppant Transport Simulation: The Key To Understanding Proppant Flowback and Post-Frac Productivity

The goal of a hydraulic fracture treatment is to create a large flow area exposed to the formation, and connected to the wellbore along a conductive path. The only goal of hydraulic fracture models is accurately predicting this final proppant placement! This goal is well understood. However, most of the theoretical, modeling, and experimental effort in this area has historically focused on understanding and predicting only gravity effects on proppant placement. However, for proppant laden, viscous fluid, slurry flowing along a fracture, other forces are always more important than gravity, and can easily cause proppant to move upwards, both during pumping and during fracture closure. Among others, these forces include: I) Differential Fracture Closure - A fracture growing vertically generally penetrates zones with higher/lower closure stress. After shut-in, higher stress zones close first, squeezing out proppant laden slurry there, often upwards, to lower stress zones. 2) Fluid Loss - After shut-in of a propped fracture treatment, all fluid must leak-off into permeable formations penetrated by the fracture. Until closure, viscous fluid continues to transport proppant (possibly upward) towards fluid loss layers, often corresponding to pay. 3) Slurry Rheology - As proppant is introduced to the fluid, the resulting slurry has a higher density and tries to move downward. However the solids also act to increase viscosity, and the more viscous slurry prefers the wide, middle, of a fracture. This serves to keep proppant near the middle of the fracture, which is often right in the pay zone. This paper will discuss combined effects of these forces on proppant placement. This discussion is placed in a context of post-frac analysis of several field treatments. The analysis used a fracture model including rigorous, numerical, 2-D material transport, and the often unexpected results are compared to supporting evidence from post-frac well performance. In many instances, the combined effect of proppant placement forces is beneficial, with more proppant placed across the pay than suggested by simple models. In other cases, post-shut-in proppant redistribution can (and did) cause catastrophic job failure.

Comprehensive Mini-Frac Testing in the Gullfaks Field as a Tool for Characterization of Reservoir Structure and Rock Mechanics.

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