Summary
Viscoelastic-surfactant (VES) -based diverting products have been used
successfully in matrix acidizing in the Gulf of Mexico (GOM) in recent
years. The uses of VES diverters range from remedial matrix-acid or
nonacid-cleanup treatments to use before gravel- or frac-packing operations to
clean up long intervals after perforating. Success or failure of these
treatments is often related to the efficiency of diverting agents, especially
for acid treatments on wells with long, heterogeneous intervals or
multiple-zone completions. A VES diverting agent is of particular interest to
remedial treatments of frac-/gravel-packed wells, because damage to the
near-wellbore area and completion should be minimized for optimum production.
Laboratory studies and field applications have demonstrated the nondamaging
properties of a VES fluid.
This paper reviews the properties of the vesicular-type VES diverting fluid,
reviews the operational considerations, and presents several case histories
with VES diverting agents in the GOM. The bottomhole static temperatures
(BHSTs) of these cases range from 140 to 290°F for both gas and oil
wells. With an internal breaker system, the diverter does not require
contact with formation fluid, brine, or acid to clean up and provide optimum
production. In some cases, as many as four stages of diverters were pumped in
the treatment, and successful diversion was observed for each stage. The paper
outlines field-operational considerations and detailed fluid properties, which
were tailored to specific well conditions and formation characteristics.
Details and pressure responses of the treatments are discussed.
Introduction
Matrix-acid stimulation has been frequently used in the GOM to remove
formation damage caused by drilling-mud invasion, clay swelling and clay
migration, and production impairment from inorganic scaling. When acid is
injected into the reservoir below the fracturing rate and pressure, acid will
typically enter the region with the highest permeability. Proper diversion is
required to direct the treatment fluid into the damaged zone to achieve maximum
benefit of the acid stimulation.
Another area that employs acid treatment in the GOM completions is damage
removal before gravel packs or frac packs. Gravel packing has been extensively
used as a standard sand-control treatment in the GOM for several decades.
Before or after the placement of gravel with completion fluids, low-density
brines, or linear gels, an acid treatment is usually pumped to remove the
near-wellbore formation damage or high skins encountered in wells perforated
overbalanced (Unneland and Waage 1996; Matherne and Hall 1996). In
other cases, a clay-acid package is often pumped into the formation before the
gravel pack to stabilize the residual clays. The effectiveness of these
treatments is often directly related to the ability of the acid treatment to
remove the near-wellbore damage and connect the wellbore to the formation. In
addition to determining the most effective combination of acid blends and
volumes for each particular reservoir, treatment design and planning are done
to ensure that the correct procedure is followed to place the acid across the
entire interval. Staging of the treatment is used to force acid across the
entire interval, treating the damaged portion of the completion.
The successful acid placement in matrix treatments of openhole horizontal
wells are even more difficult because of the length of the zone treated and
potential variation of the formation properties. A successful diversion
technique is critical to place the acid at the location where damage exists. A
variety of diversion techniques have been practiced in acid stimulations (Rae
and di Lullo 2003). Diversion sometimes can be achieved by simply increasing
pumping rate during the acid treatment. Viscous diversion techniques such as
hydroxyethyl cellulous (HEC) gelled brine and nitrogen foam are often used,
especially when the temperature of the well is not very high. Other materials
used as an acid-diverting agent include acid-insoluble particulates. A mechanic
method such as a ball sealer also has been successful when it is
practical. Most of these methods have some limitation to their
application, especially in a completion with a sand-control method installed.
VES-based diverting products have been developed and widely used in recent
years, and their use has found application beyond the limitations of the
previously mentioned techniques.
© 2006. Society of Petroleum Engineers
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History
- Original manuscript received:
7 June 2004
- Meeting paper published:
26 September 2004
- Revised manuscript received:
2 February 2006
- Manuscript approved:
3 February 2006
- Version of record:
20 November 2006
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