Foam-assisted underbalanced drilling technique is advantageous over the traditional overbalanced drilling near the productive water-sensitive formations due to its reduced formation damage, improved rate of penetration, higher cutting-transport capacity, and lower circulation losses. However, the complicated nature of foam rheology has been a major impediment to the optimal design of field applications.

Earlier studies with surfactant foams without oils and polymers show that foam flow in pipe can be represented by two different flow regimes: the low-quality regime showing either plug-flow or segregated-flow pattern, and the high-quality regime showing slug-flow pattern. The objective of this study is to investigate foam flow characteristics in horizontal pipes at different injection conditions, with or without oils, by using polymer-free and polymer-added surfactant foams.

The results of this study were presented in two different ways: (i) steady-state pressure drops (or, apparent foam viscosity, equivalently) measured by multiple pressure taps and (ii) visualization of bubble size, size distribution and flow patterns in transparent pipes. The results with surfactant foams and oil showed that (i) oil reduced the stability of foams in pipes, hence, decreasing the steady-state pressure drops and foam viscosities, and (ii) the presence of oil tended to lower the transition between the high-quality and the low-quality regimes (i.e., lower foam quality at the boundary, or lower fg* equivalently). In addition, the results with surfactant foams with polymer showed that (i) polymer thickened the liquid phase and, if enough agitation was supplied, could make foams long-lived and improve foam viscosities, and (ii) the system sometimes did not reach the steady state readily, showing systematic oscillations. In both cases, though, the experiments carried out in this study showed the presence of two distinct high-quality and low-quality flow regimes.

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