Summary
Hydrochloric (HCl) acid is used to stimulate carbonate formations in both
matrix and fracturing treatments. However, the reaction rate of the acid with
calcite is fast. In addition, the viscosity of regular HCl solutions is
relatively low. Acid-soluble polymers are usually added to the acid to increase
its viscosity, which is needed to enhance acid diversion during matrix
acidizing and reduce acid leakoff rate during acid fracturing. Gelled acids are
extensively used in matrix and acid-fracturing treatments performed in
carbonate formations. However, a few studies examined the impact of these
polymers on the reaction of HCl acids with calcite.
This paper uses a rotating disk instrument to measure the dissolution rate
of calcite by use of gelled acids. Measurements were conducted over a
temperature range of 25 to 65°C, a pressure of 1,000 psi, and rotational speeds
of 100 to 1,000 rpm. Acid formulations that are typically used in the field
were examined. Polymer concentration was varied from 0.5 to 2 wt%. The apparent
viscosity of the gelled acid was measured with a Brookfield viscometer.
Measurements were done for the same solutions tested with the rotating disk
instrument. The temperature was varied from 25 to 100°C, while the pressure was
maintained at 300 psi. The shear rate was varied from 57 to 1,700
s–1.
Evidence of reverse and toroidal flows was noted for the first time by
examining the etching patterns of the reacted disks. The etching pattern on the
surface of the disk depended, among other factors, on the disk rotational speed
and polymer concentration.
There was a significant increase in the apparent viscosity of gelled acids
and a major decrease in the dissolution rate as the polymer concentration was
increased from 0.5 to 1.5 wt%. The reaction of gelled acids with calcite was
controlled by a surface reaction at 25°C, and by mass transfer at 65°C.
Temperature increased the dissolution rate of calcite at all conditions
examined. It did also reduce the viscosity of the gelled acid, which affected
the way the acid reacted with calcite.
Introduction
Carbonate reservoirs are heterogeneous, with large variations in rock
permeability. Stimulation fluids, in matrix acidizing, will flow through the
path of the least resistance where the permeability is high or the damage
(skin) is low. There is a need for proper fluid diversion to enhance the
outcome of matrix acid treatments. One way to enhance diversion is to increase
the viscosity of the acid (Woo et al. 1999). High viscosity is also needed in
acid-fracturing treatments to achieve deep acid penetration and longer
fractures (Deysarkar et al. 1984).
Gelled (Pabley et al. 1982; Johnson et al. 1988; Crowe et al. 1989;
Nasr-El-Din et al. 2002a) and in-situ gelled acids (Mukherjee and Cudney 1993;
Magee et al. 1997; Yeager and Schuchart 1997; Buijse et al. 2000; Saxon et al.
2000; Taylor and Nasr-El-Din 2003) have been used to increase the viscosity of
the acid on the surface or in the formation. An acid-soluble polymer is
typically added to the injected acid to increase its viscosity on the surface.
A suitable polymer, a crosslinker, and a breaker are added to the acid to form
a gel in the formation over a certain pH range. To overcome some of the
concerns raised about polymer-based acids, visco-elastic surfactants
(Nasr-El-Din et al. 2006) were introduced to replace high-molecular-weight
polymers, which are thought to cause formation damage (Lynn and Nasr-El-Din
2001).
Similar to other acid additives, polymers can affect the way the acid reacts
with the rock. Several authors reported that the addition of polymers to the
acid decreased the dissolution rate of rock by the acid (Taylor et al. 2004a)
and the diffusivity of H+ (Hansford and Litt 1968; Mishra and Singh
1978; de Rozieres et al. 1994). There are several ways that polymers can affect
the reaction of the acid with the rock. The polymer will increase the viscosity
of the acid, which will reduce the diffusion rate of H+ from the
bulk solution to the surface of the rock. Polymer molecules can adsorb on the
rock surface and form a barrier that reduces acid reaction with the rock.
Finally, polymers can change the flow pattern close to surface of the rock, and
therefore, affect the way the acid reacts with the rock.
The present study uses the rotating-disk instrument to examine the reaction
of gelled acids with calcite. This instrument has been extensively used to
investigate the reaction of acids and chelating agents (Newtonian fluids) with
carbonate rocks (Boomer et al. 1972; Lund et al. 1975; Anderson 1991; Fredd and
Fogler 1998a, 1998b, 1998c; Conway et al. 1999; Gautelier et al. 1999; Alkattan
et al. 1998, 2002; Frenier and Hill 2002; Taylor et al. 2004b, 2006; Lungwitz
et al. 2007). It has been also used to study mass and heat transfer into
non-Newtonian fluids (Hansford and Litt 1968; Mishra and Singh 1978; de
Rozieres et al. 1994).
The reaction between acid and rock is a three-step process that involves the
following:
- Transport of the H+ from the bulk solution to the rock
surface
- Reaction at the surface
- Transfer of the reaction products away from the surface
The slowest step controls the overall reaction rate (de Rozieres et al.
1994).
The objectives of the present study are to (1) examine the effect of polymer
concentration and disk rotational speed on the etching pattern on the surface
of the rock; (2) assess the effect of polymer concentration, temperature, and
disk rotational speed on the dissolution rate of calcite by use of gelled
acids; and (3) determine the relationship between the apparent viscosity of
gelled acids and the dissolution rate of calcite rock.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
30 October 2006
- Meeting paper published:
5 December 2006
- Revised manuscript received:
10 January 2008
- Manuscript approved:
21 February 2008
- Version of record:
15 August 2008
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