Summary
Matrix acidizing is a commonly used well stimulation technique in which acid
is injected into the formation in order to dissolve a portion of the rock and
therefore recover or enhance the permeability in the near-wellbore region. In
carbonates, when acid is injected, selective dissolution of the rock takes
place, creating highly permeable flow channels, called wormholes. In general,
for a matrix acidizing treatment in carbonates, the deeper the wormholes
penetrate into the formation, the better the outcome, as characterized by a
lower skin factor. Thus, the deeper the wormhole penetration achieved with a
given volume of acid, the more efficient the treatment. In this paper, we
present laboratory results that show how wormhole propagation is affected by
the presence of immiscible phases (gas or oil) when the acid is injected into
the rock.
As wormholes are created, not all of the acid reaches the wormhole tip, with
a significant portion of the acid being lost as fluid loss to the surrounding
matrix. If this fluid loss can be controlled, then more of the acid can reach
the wormhole tip, and therefore the penetration of wormholes can be increased.
This paper investigates the effect of having gas or oil saturation in the
carbonate rock prior to the injection of acid. By reducing the relative
permeability to the acid in the matrix surrounding the wormhole, the presence
of an immiscible phase can reduce the fluid loss from the main wormhole, thus
allowing for deeper penetration of wormholes with a given acid volume.
It was found that gas injection prior to acid injection does significantly
reduce the volume of acid required to propagate wormholes through cores. This
effect is observed both at room temperature and at higher temperatures. The
presence of gas in the core reduced the acid volume needed for wormhole
propagation through the core by a factor of up to 3. The paper presents an
extensive set of experimental results for both gas- and liquid- saturated
carbonate cores subsequently treated with strong HCl solutions. We show that
the acid volumes are generally smaller for the nitrogen-saturated cores, and
that the wormholes created are narrower and less branched than in the case of
water-saturated rocks. We also show that the presence of oil saturation at
residual water saturation in the core has a similar beneficial effect on
wormhole propagation to gas injection. On the other hand, oil present at its
residual saturation had little effect on the acidizing process. All of these
results confirm the strong influence that fluid loss from a propagating
wormhole has on the efficiency of the acidizing process in carbonates.
Introduction
Acid stimulation with strong hydrochloric acid solutions is a common method
to increase well productivity in carbonate reservoirs. When hydrochloric acid
is injected into the formation, the acid dissolves carbonate rocks in a highly
nonuniform pattern, creating large channels known as wormholes. Because
wormholes are very large compared with the pores in a nonvugular carbonate, the
wormholes provide a highly conductive flow path in the near-wellbore vicinity.
Effectiveness of acid treatments is mainly determined by the geometry and
pattern of the wormholes created. In general, the effectiveness of a
matrix-acid treatment in carbonates depends strongly on the depth of
penetration of wormholes into the formation. Because the conductivity of the
wormholes is almost infinite compared with the matrix, if wormholes penetrate
beyond any near-wellbore formation damage, the post-stimulation skin factor is
given by
Eq. 1
where rwh is the radial distance to which wormholes have
penetrated and rw is wellbore radius. For example, for a
wellbore with a radius of 0.25 ft, wormholes penetrating 6 in. beyond the
wellbore result in a skin factor of –1.1, whereas wormholes penetrating 2 ft
beyond the wellbore yield a skin factor of –2.2. In a typical vertical well,
the 2-ft-long wormholes would increase the well productivity by approximately
20% more than the 6-in. wormholes. Thus, an acid treatment in carbonates is
improved if wormhole penetration can be increased. A long, relatively narrow
wormhole with little branching is the ideal structure to maximize wormhole
penetration distance.
© 2006. Society of Petroleum Engineers
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History
- Original manuscript received:
12 August 2003
- Revised manuscript received:
2 November 2005
- Manuscript approved:
11 April 2006
- Version of record:
20 September 2006
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