Rheological Properties of Aqueous Foams for Underbalanced Drilling

Abstract

Abstract Underbalanced drilling is a non-conventional drilling operation where the hydrostatic pressure of the drilling fluid is maintained below the formation pore pressure. Many advantages can be provided by this specific drilling approach: higher ROP, minimised losses through formation, minimised formation damage in low permeability reservoir or depleted wells. Among the existing low-density fluids used in underbalanced drilling, foam is very beneficial due to its good cuttings carrying capacity. However, the knowledge of foam properties and especially of flowing properties is still incomplete. Foam can't be considered as a classical fluid and must be seen as a dispersion of gas bubbles in a continuous liquid matrix. Due to its own nature, foam is a thermodynamically unstable system, contrary to drilling fluids. The lack of knowledge on foam properties mustn't prevent engineers from considering this option in their technico-economical studies. The purpose of this work is to provide innovative procedures for foam rheology characterisation in order to better understand its behavior. The apparent viscosity of a foam is not only a function of its quality (ratio between gas volume and total foam volume) but also of its texture (bubbles size distribution), and of the presence of polymers in the aqueous matrix. A careful experimental procedure was elaborated, which uses a parallel plates geometry. Attention has been paid to foam stability, texture and evolution properties in order to have correct and reproducible measurements. In particular, wall slip phenomena and strain localisation were investigated and controlled. Shear flow properties were thus determined for different foams formulations with a very satisfying repeatability. The presence of a yield stress has been highlighted. Small amplitude oscillatory shearing experiments demonstrate that foam behaves like an elastic solid for small deformations. Use of a vane rheometer and observation of start-up in the creep mode confirm the elastic nature of the material. Excellent coherence was found between the different measurements. As a conclusion, a reproducible methodology for characterising foam rheological properties was elaborated. Those results constitute the first step on our way to accurately predict down-hole pressures while drilling with foam.