Summary
Fluids based on chelating agents have been developed for matrix stimulation
of high-temperature sandstone formations. These fluids dissolve sizeable
amounts of calcite and clays and maintain high levels of dissolved metal in
solution over time with minimal precipitation. A series of field samples from
high-temperature (149°C) sandstone reservoirs in a West African formation bear
carbonate concentrations ranging from 2% to 37% (w/w). The effects of matrix
treatment using a chelating agent-based system on these field samples were
studied using coreflood and slurry reactor experiments.
Linear coreflood test data show dramatic increases in the formation
permeability after treatment with the chelating agent-based fluid. The
improvement in permeability is ascribed to the removal of carbonate minerals
and soluble clays, without secondary metal precipitation. Slurry reactor tests
elucidated the kinetics of mineral dissolution in mechanically ground field
samples. Treatment with acidic chelant fluids generated high levels of
dissolved calcium, silicon, and aluminum that remained in solution over time.
For comparison, conventional mineral acid treatment of the field samples
generated high levels of metals in solution that declined over the same period
of time, which is indicative of secondary precipitation. The effectiveness of
the chelant fluid for stimulation of this high temperature formation was
confirmed through increased formation permeability and high levels of dissolved
minerals.
Introduction
Typically, the purpose of acidizing a carbonate formation is to remove
near-wellbore damage and to produce “wormholes” to increase the permeability of
the critical matrix. However, because of the rapid reaction between
hydrochloric acid (HCl) and carbonates, diverting agents such as ball sealers,
viscoelastic surfactant diverters, and foams (Coulter and Jennings 1997) are
used to direct some of the acid flow away from large channels that may form
initially and take all of the subsequent acid volume. Fredd and Fogler (1998a,
1998b, 1998c) have proposed the use of ethylenediaminetetraacetate (EDTA)
chelating agents as the primary active components in fluids used to stimulate
limestone and dolomite formations. By adjusting the composition and pH of these
fluids, it is possible to customize the chelant solutions and target specific
well conditions to achieve maximum wormhole formation with a minimal volume of
solvent. Control over reaction kinetics is vital when acidizing carbonate
formations at high temperatures at which high reaction rates can overwhelm some
treatment fluids.
For comparison, sandstone reservoirs undergo matrix acidizing treatments to
remove damaging aluminosilicate minerals and reduce the skin value. The
precipitation of silica is thought to be the major reason that
sandstone-acidizing jobs fail to produce the anticipated decrease in skin,
especially at temperatures > 150°F or in the presence of acid-sensitive
clay. Acid treatment of sandstone at high temperatures, therefore, requires a
retarded acid. Additionally, conventional acid treatment of sandstone
formations (such as a mud acid treatment) involves many stages of fluid, which
increases the complexity of the treatment. An alternative approach uses
chelating agents combined with acids as the main treatment agent.
Chelating agents are materials that are used to control undesirable
reactions of metal ions. In oilfield applications, chelating agents (Frenier et
al. 2000) are frequently added to acidic stimulation fluids to prevent
precipitation of solids as the acid spends on the formation. The use of
chelating agents is one proposed approach to stimulation because they can
complex many of the metal ions found in sandstone formations. Chelating agents
are also used as components in many scale-removal and scale-prevention
formulations (Frenier 2001). EDTA fluids have been used extensively to control
iron precipitation and to remove scale. For example, disodium-EDTA has been
used as a scale-removal agent in the Prudhoe Bay field of Alaska (Shaughnessy
and Kline 1982). In this application, calcium carbonate scale had precipitated
in the perforation tunnels and in the near-wellbore region of a sandstone
formation. High decline rates followed conventional HCl treatments, but 17
wells treated with disodium-EDTA maintained production after these
treatments.
Hydroxychelating agents have also been proposed as metal-control agents,
scale-removal fluids, and agents for matrix stimulation of carbonates (Fredd
and Fogler 1998a; Frenier et al. 2000; Frenier 2001). The materials evaluated
include hydroxy-aminopolycarboxylic acids (HACA) such as
hydroxyethylethylenediaminetriacetic acid (HEDTA) as well as other types of
chelating agents. In addition, formulations based on nitrilotriacetic acid
(NTA) and diethylenetriaminepentaacetic acids (DTPA) have found use in advanced
chelant designs.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
15 February 2005
- Meeting paper published:
25 May 2005
- Revised manuscript received:
8 February 2007
- Manuscript approved:
25 April 2007
- Version of record:
20 February 2008
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