Foam stability and rheology are important parameters for foam used as a drilling fluid in underbalanced drilling. A low density fluid is one way to avoid formation damage or borehole fracturing when the reservoir has a low pore pressure, or is depleted. However foamed drilling fluids are not well understood, their stability and rheology are more complex than a conventional drilling fluid. Rheological data have to be applied to model flowing properties within the drill string and the annulus to predict and control the down hole pressure.
In this work we have studied the physico-chemical properties and the flowing properties of aqueous foams formed with a commercial system. Foam stability has been studied as a function of different parameters, including the presence of electrolytes (mono and divalent salts) and contamination by hydrocarbons. Addition of water soluble polymers to increase the stability of the foam has been studied as well as the influence of the hydrodynamic conditions on foam stability.
Flowing properties have been studied by circulation tests in a laboratory transparent flow loop (4 m long and 1 inch internal diameter). Foam circulation experimental data have been used to calculate pressure losses during foam circulation using software based on a published model (Economides 1995). The effect of the quality of the foam on the accuracy of the model has been shown, thus allowing the estimation of the domain of validity of this model. Different methodologies for rheology measuring have been compared. The influence of other parameters such as polymers concentration or foaming agent concentration have been studied also.
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