Efeito da variação da temperatura no reinício do escoamento de fluidos de perfuração

Abstract

The fluid flow start-up is a significant problem in deepwater and in long wells such as those of the Brazilian subsalt layers. Due to the narrow operating window (operating pressure range) in deepwater and depending on the wells geometry and fluid properties, significant pressure peaks may take place affecting the wellbore structure. Besides that, between the wellbore and the surface of the ocean there is a significant difference of temperature and the effects of this temperature variation can change the rheological properties of the fluid and, consequently, and the pressure variation in the wellbore. This can damage the pressure control during the drilling process, especially during the flow start-up, where operating pressures are higher because of the breaking of the gel phase. The objective of this study is develop a mathematical model to simulate the compressible and transitory flow, with heat transfer, in order to predict the profiles of pressure and temperature along the well and the variation temperature effects on the rheological properties of drilling fluids during the flow start-up. The geometrical model was defined as a rigid tube, with a constant cross section area, and the drilling fluid is treated as non-newtonian Bingham fluid. The model is based on the conservation equations of mass and momentum, which are solved by the method of characteristics, and conservation of energy, solved by the finite difference method. The resulting algebraic equations are solved iteratively through an algorithm written in FORTRAN computer language. The model is only able to simulate fluid flow with very low thermal expansion coefficient and compressibility, in which the variation of these parameters cannot be observed. The effects of the dimensionless parameters λ , Bi and Φ are more evident in the fluctuations in the levels of the peaks of pressure waves and λ , Bi have more dissipative effect, and a gradual increase of Bi raises the pressure in steady state.