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: (1) less gel damage to the proppant
pack, (2) limited height growth, and (3) less 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.
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
6 March 2006
- Meeting paper published:
15 May 2006
- Revised manuscript received:
25 August 2006
- Manuscript approved:
21 September 2006
- Version of record:
20 August 2007
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