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.

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