Automatic Measurement of the Dependence on Pressure and Temperature of the Mass Density of Drilling Fluids

Abstract

Summary Drilling fluids are subject to substantial variations in pressure and temperature while they are circulated in a well. These changes are so great that the mass density of the drilling fluid varies significantly during circulation. Accurate estimation of the hydrostatic and hydrodynamic pressures therefore requires a reliable model of the pressure and temperature dependence of the mass density of the drilling fluid. Usually, the mass density of a drilling fluid is measured manually with a mud balance. The pressure and temperature dependence of the mass density of the fluid [i.e., its pressure, volume, and temperature (PVT) behavior] is then calculated from a PVT model of its components and their relative proportions. However, variations in the composition of the fluid mix and uncertainties in the PVT behavior of each component may lead to inaccuracies. To overcome these limitations, an apparatus has been designed to measure the PVT behavior of the drilling fluid directly and automatically. The setup measures both the mass density and the speed of sound of the fluid at specific conditions of pressure and temperature. It is possible to estimate the adiabatic compressibility from the speed of sound in the liquid mix. The device utilizes a heat exchanger from which the specific heat capacity of the drilling fluid can be derived. The isothermal compressibility can be calculated by combining the specific heat capacity and the adiabatic compressibility. While estimating the specific heat capacity using the heat exchanger, the thermal conductivity of the fluid is also estimated as a byproduct. A PVT model of the drilling fluid is obtained by combining measurements made at different conditions of pressure and temperature. The automatic and continuous provision of drilling fluid PVT behavior enhances the precision of model predictive drilling control systems and, at the same time, can simplify their configuration, therefore making them more accessible to any drilling operation.