Abstract

A recent research project was conducted to determine whether a stable, fine-textured, 95% gas content foam could be made. The viscosities of 95-quality N2 foams were measured in a recirculating flow loop viscometer.

Only foam prepared from 2% of an anionic surfactant with plain water had uniform, fine-bubble structure (texture) at 95 quality. All other combinations of additives or other foamers (nonionic, amphoteric or anionic, and/or 0.24% guar) produced unstable foams at 95 quality and stable foams at lower quality.

Foams that were unstable at 95 quality typically contained large slugs of N2 gas within the foam structure. Unstable, high-quality foams did not invert phases to form a mist. Instead, such foams were mixtures of very small and very large bubbles. The net viscosity and stability of those fluid systems were lower than that of a uniform, fine-textured foam.

Yield points were measured for fine-textured foams at 70 to 95 quality. These new yield points were higher than yield points in earlier data.

Introduction

Foam fracturing is a standard technique used in North America to stimulate low-permeability reservoirs, including shales and coal seams. Foams are attractive because the water content of a foam fluid is very small, reducing the damage potential to sensitive formations. A foam fluid consists of a high percentage of a gas-internal phase and a lesser percentage of a liquid-external phase that includes a stabilizing surfactant called a foaming agent. Foams are viscous fluids used for creating fracture geometry and transporting proppant into the fractures. Foam at nitrogen qualities of 70 to 90 have been applied effectively in the formation types mentioned above. Alternatively, even pure N2 without proppant will stimulate gas production from these low-permeability formations in some areas. Liquid CO2 with a small concentration of proppant has also been reported to stimulate gas production.

One way to minimize the amount of water injected is to use a foam quality higher than 90. Although the water content of a 90-quality foam is only 10%, 95- or higher quality foam contains even less water (5%). The danger of increasing the quality is that at some point the liquid will stretch to cover so much surface area of bubbles that the foam may collapse or even invert phases to become a mist. Assuming sufficient foam stability, 95- quality foam has enough viscosity to place proppant.

This paper shows the requirements for building high-quality (95% gas content) foam fracturing fluids. The effects of external phase composition, foaming agent type and concentration, shear rate history, and gas type (N2 and CO2) on viscosity development were examined in the study.

Experimental

Base liquids consisted of either tap water or 0.24% guar polymer in tap water. Foaming agent was mixed into the base fluid at a concentration of 2% (by volume) unless otherwise noted. Liquid phase composition and foamer types are listed in Table 1. The foamer surfactants that were tested are typical of commercial materials used in foam fracturing service work; two anionics, one nonionic, and one amphoteric were tested. Anionic surfactant number one was used for all experiments except where noted.

The base liquid was pumped into the recirculating flow loop viscometer and pressurized to 1,000 psi. Nitrogen gas was bled into the pipeline loop while the loop was being recirculated at 1,000 sec-1. To generate 95-quality N2 foam, N2 was pumped into the loop, displacing 613 mL of liquid from the 645-mL volume of the loop through a backpressure regulator. Liquid displacement was measured by trapping the effluent in a beaker on a digital balance. When gas was added, viscosity was monitored and foam was observed through the visual port for the appearance of large gas bubbles.

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