Proppant Placement Using Reverse-Hybrid Fracs

Abstract

Summary In conventional gel-fracturing treatments, the damage induced by the gel can have a significant impact on well performance, particularly in low-permeability gas formations. Slick-water fracs have been shown to be more successful in some tight gas formations because of reduced gel damage and limited height growth. Proppant placement is a major concern in water fracs. Hybrid water fracs (i.e., using slick water as the pad fluid and gel to place the proppant) provide improvements to the performance of water fracs. This paper proposes a new method, reverse-hybrid fracs (RHF), for the efficient placement of proppant deep into created fractures while minimizing gel-induced damage. Experiments were conducted in a simulated fracture (i.e., slot cell) to study the transport of proppant. Slick water was injected first into the slot, followed by gel. Finally, slick water containing proppant was injected to displace the gel. The water that carried the proppant quickly formed viscous fingers through the gel. The gel was observed to form long, thin layers that effectively hindered proppant settling and helped transport the proppant further into the slot. This resulted in the formation of proppant packs above the gel layers. In this paper, experimental results are presented to show how the gel layers distribute in the slot and how proppant distribution is affected by the gel layers. A transparent cell composed of rough walls was set up to investigate the effect of fracture wall roughness. The effects of fluid viscosity ratio, fracture wall roughness, and gel pad volume were investigated. Based on the experiments and scaling relations, recommendations are made for the pumping sequence and the size of the gel and slick-water stages in reverse-hybrid fracture treatments. This method of proppant placement requires less gel than conventional gel fracs. Other possibleadvantages include:less gel damage to the proppant pack,limited height growth, andless penetration of the pad fluid into the formation resulting in shorter cleanup time following fracture treatment. Introduction In a fracturing treatment, the effective fracture lengths achieved (e.g., measured by matching the production response or from pressure-transient tests) can quite often be significantly smaller than the created fracture lengths (e.g., measured by fracture-mapping techniques). This difference can be attributed to inadequate proppant transport and/or insufficient fracture cleanup. In hybrid fracs, low-viscosity and slick water are used to create the fracture, while a high-viscosity gelled fluid is used in the proppant stage. In some fields, the use of hybrid fracs has resulted in a significant increase in effective fracture lengths and well productivity (Sharma et al. 2004). The use of hybrid fracs can, under certain conditions, result in the increased possibility of tip screenout. Because the fracture is being created with a high leak-off, low-viscosity fluid, the smaller fracture widths can result in premature tip screenout. This is the primary motivation for using reverse-hybrid fracs. As the name suggests, the sequence of fluid injection is the reverse of what is used in hybrid fractures; a high-viscosity polymer (e.g., linear or cross-linked) fluid is used to create the fracture while the proppant is pumped, behind this high viscosity pad, into a low-viscosity fluid. This kind of treatment is referred to as reverse-hybrid fracs (RHF) in this paper.

Experimental study of the growth of mixing zone in miscible viscous fingering

“…The onset and evolution of instabilities is referred to as viscous fingering. 1-4 Viscous fingering has been a subject of extensive study over the past years because of its applications in many processes which include flow through porous media, 4 secondary and tertiary oil recovery, [5][6][7] proppant placement in hydraulic fractures, [8][9][10] and flowback from hydraulic fractures. 11 In case where the displaced and displacing fluid are miscible with each other, the formation of instabilities is referred to as miscible viscous fingering.…”

Effects of salinity and temperature on drag reduction characteristics of polymers in straight circular pipes

“…Slick-water treatments have been shown to be more successful because of reduced gel damage and limited height growth (Liu et al, 2006). Production and economic analysis have shown that high rate slick-water fracturing treatments are more effective for enhancing long term production.…”

Proppant Placement Using Alternate-Slug Fracturing

“…Sharma et al (2004) showed that hybrid fractures result in longer proppedfracture lengths and higher well productivity compared with wells treated with slickwater or crosslinked-gel-based fluids (conventional fractures). Reverse-hybrid fracturing uses a sequence of fluid injection, reverse of that used in hybrid fractures (Liu et al 2007). A high-viscosity fluid (linear or crosslinked gel) is used to create the fracture, and the proppant is pumped behind it in a low-viscosity fluid (such as slickwater).…”

“…The method was developed to improve on the idea of "pipelining" introduced by Ely et al (1993). Liu et al (2007) showed that the low-viscosity fluid, laden with proppant, fingers through the high-viscosity fluid, potentially resulting in the deeper placement of proppant in the fracture. The proppant accumulates in the fingers, and the gel layers prevent the particles from settling.…”

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New fracturing techniques, such as hybrid fracturing (Sharma et al 2004), reverse-hybrid fracturing (Liu et al 2007), and channel (HiWAY) fracturing (Gillard et al 2010), have been deployed over the past few years to effectively place proppant in fractures. The goal of these methods is to increase the conductivity in the proppant pack, providing highly conductive paths for hydrocarbons to flow from the reservoir to the wellbore.

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