Fracture ballooning frequently occurs in deep water drilling and is often confused with a well kick, which may lead to costly well control procedures. Although it has been studied for almost 30 years, less effort has been made with respect to deep water wells. Specifically, they cannot capture fluid loss/gain volume in the fracture driven by dynamic bottom hole pressure. Therefore, a further investigation is required to fill the gap. In this study, an integral mathematical model is developed considering fluid flow both in fracture and borehole. With the pump on and off, the bottom hole pressure is different because of the loss of friction pressure. The governing equation is derived by Bernoulli's equation and Reynold's lubrication theory, and then is solved with finite difference method. The pressure distribution inside the fracture profiles and the fracture aperture are obtained. These parameters have been studied such as friction pressure (or mud circulation pressure), and the rheological properties of drilling fluid. The results show the volume of the fluid losses and gains may be different with dynamic bottom hole pressure, and the friction pressure plays an important role in ballooning effect. With the higher pressure drop, the fracture ballooning is more severe. The fluid rheology properties like yield stress and shear-thinning/thickening effect also have effect on the borehole ballooning, which may get an illustration with friction pressure. Thus, the friction pressure should be calculated accurately, especially in deep water wells. Well control must take ballooning into account with great friction pressure.
Borehole ballooning and/or breathing is the term describes reversible drilling fluid losses and gains during drilling. There are three main mechanisms considered in the literature: 1. Elastic deformation of the borehole walls (Gill, 1989); 2. Variation in temperature of the drilling fluid (Aadnøy, 1996); 3. Opening/Closing of natural fractures intersected during drilling (Lavrov and Tronvoll, 2005). In the first mechanism, the elastic deformation of mudstones and shales is sometimes incorrect to explain fluid losses and gains, because the modulus of elasticity of mudstones and shales at depth typically ranges from a few 100,000 psi. In short, the volume change of rocks is much lower than fluid losses and gains. In the second mechanism, volume changes associated with temperature contraction and expansion of the drilling fluid is primarily related to the rate of heat transfer and the coefficient of thermal expansion of the fluids. Commonly, the magnitude of volume change associated with fluid compression under pump pressure is very low, typically in the order of 2-6 barrels (Tare, Whitfill, and Mody, 2001). Therefore, fracture opening and closing may be the key parameters that affect formation ballooning. In fact, there are many researchers on this issue. Lavrov and Tronvoll (2005) first explained the effect of opening/closing of a single fracture and the consequent intersections during drilling in a naturally fractured reservoir. Analysis was conducted using a coupled analytical model. The fluid flow in and out of the fracture was assessed using different fracture deformation laws and different types of mud rheology, i.e., Newtonian, power-law, and bi-viscous. Majidi et al. (2008) developed a model for fracture ballooning of drilling fluids with Yield-Power-Law. Shahri et al. (2011) developed a theoretical framework for radial flow of mud with Yield-Power-Law rheology and discussed the effect of fracture deformation law. It is concluded that exponential deformation law is more realistic than the simplified linear deformation law. Although fracture ballooning has been studied with different theories and models, it is still not well-understood as these models do not consider fluid flow dynamics inside the borehole. In this paper, this deficiency is alleviated through an integrated model that includes fluid flow dynamics both in the borehole and the fracture.
