Abstract

Foam diversion can effectively solve the problem that acid distribution among layers of different permeability is uneven during matrix acidizing. Based on gas trapping theory and mass conservation equation, mathematical models for foam slug diversion acidizing and foamed acid diversion acidizing were established. Design method for foam slug diversion acidizing was given. The mathematical models were solved by computer program. The results show that total formation skin factor, and pressure of wellhead and bottomhole increase with foam injection, and decrease with acid injection. Volume flow rate of high-permeability layer decreases, and that of low-permeability layer increases during foam injection, which can divert subsequent acid to low-permeability layer from high-permeability layer. Under the same formation situation, the effect of foamed acid diversion acidizing is better than foam slug diversion acidizing. In foamed acid diversion acidizing process, operation time is longer, and pressure of wellhead and bottomhole is higher. Field application shows that foam slug diversion acidizing can effectively block high permeability layer, and improve intake profile obviously. It is fit to acidizing for heterogenous formation.

Introduction

Matrix Acidizing Process. Foams have been used routinely in matrix acidizing treatments in the petroleum industry, but with inconsistent results(Smith C. L., et al., 1969; Kennedy D. K., et al., 1992). Matrix acidizing is a treatment for damaged sandstone or carbonate formations. Formation damage can be caused by drilling, completion, or production processes. A damaged formation has plugged or constricted pore spaces, reduced permeability and productivity. In an acidizing process, acid is used to dissolve material in the matrix, thus increase porosity, permeability and productivity.

A formation may have layers with different porosity, permeability and rock types. Layers may also differ in degree of damage. In matrix acidizing, all damaged layers need to be treated, especially the most damaged layers. However, fluid naturally flows into the most-permeable, least-damaged layers, and may leave less-permeable and more-damaged layers under-treated. To solve this problem, foam is used to partially block the high-permeability and undamaged layers and divert acid into the less-permeable layers (Mohammed Zerhboub, et al., 1994; Cheng L., et al.).

Foam for Acid Diversion. Many studies (Bernard, G. G., et al., 1965; Friedmann, F., et al., 1986; Huh, D. G., et al., 1989; Sanchez, J. M., et al., 1989; de Vries, A. S., et al., 1990; Friedmann, F., et al., 1991) agree that foam does not alter water (or acid) viscosity or the relation between water relative permeability and water saturation in steady foam flow. Foam does directly reduce gas mobility, thus indirectly reducing water saturation, relative permeability and mobility. Foam reduces gas mobility in part by trapping a large percentage of gas in place; up to 80–99% of gas is trapped even if foam flows at high pressure gradient (Bretherton, F. P., 1961; Hirasaki, G. J., et al., 1985; Falls, A. H., et al., 1988; Rossen, W. R., 1990). Foam reduces gas mobility also by increasing the effective viscosity of flowing gas (R. K. Prud'homme, et al., 1996; Gillis, J. V., et al., 1990; Hill A. D., et al.; Gdanski R. D., et al., 1993). These two effects are related: both depend on capillary forces that give foam an apparent yield stress and trap bubbles in place. The presence of foam in a high-permeability or undamaged layers reduces liquid saturation there, reducing relative permeability of liquid, and reducing acid into the foam-saturated layers.

On the other hand, foam is less stable in low-permeability layers (Hill A. D., et al.; Gdanski R. D., et al., 1993). Therefore it is possible to block high-permeability zones preferentially during acid injection. At the same time, only weak foam is present in low-permeability or damaged zones. Acid can then be diverted into the lower-permeability or damaged zones without zonal isolation. Use of a surfactant pre-flush also increases diversion by helping to place more foam in high-permeability or undamaged layers (Gillis, J. V., et al., 1990).

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