Volumetric Behavior of Drilling Muds at High Pressure and High Temperature

Abstract

Abstract When drilling HPHT wells, the control of the downhole pressure is very important because of the narrow margin between the pore pressure and the formation fracture pressure. The temperature combined with the high mud density preclude the use of pressure measurements with a MWD tool. For this reason the pressure must be calculated from models that require, first of all, knowledge of downhole mud densities. Analysis of data from laboratory studies conducted for HPHT wells demonstrate discrepancies between measured densities and those calculated through using a compositional model. We have found that the source of these discrepancies could be the assumption of a constant density of the solids fraction. A special cell able to measure volumetric changes of high density muds at high pressure and high temperature, up to 1500 bars and 200 C, was developed as part of a HPHT research programme. This cell was used to measure the densities of a heavy oil base mud and its fluid components. The study conducted with this cell confirmed that at high pressure and high temperature we can not assume that the solids have a constant density. In this paper we present the data from laboratory experiments conducted on OBM and WBM. We also present the mathematical model used to describe the volumetric behavior of the fluid components at HPHT conditions as well as the compositional model integrating the volumetric behavior of solids used to predict the density of the mud. We conclude this paper with the comparison of results from this model with data obtained from the literature. Introduction During experimental work conducted for an HPHT well in the North Sea (Well A), we measured the density changes of a heavy OBM (2240 Kg/m3) as well as the density changes of its liquid phase with pressure and temperature. The results of the measurements are given in Tables 1 and 2. Comparison of mud experimental data against mud densities calculated from liquid phase data with the compositional model is given in Fig. 1. Discrepancies observed between measured and calculated densities are higher than the uncertainty of the measurements. The most likely cause of these discrepancies is the assumption of constant density of solids used in the compositional model. Therefore, we decided to conduct a HPHT study to help understand and model the exact behavior of fluids, solids, and their combination to predict with accuracy the bottom hole mud density in HPHT drilling wells. Our goal was a final accuracy level lower than 10 kg/m3 for the bottom hole mud density. For an OBM, this accuracy will depend on the accuracy of measurements and modeling of the brine, the oil, the solids behavior as well as on the exactitude of the rules used to compose them. Therefor, it is necessary to obtain a very good accuracy on each experimental set of data as well as on the model. The compositional model is expressed: (1) (2) (3) (4) In these equations the subscripts m, e, o, b, s refer to the mud, the emulsion, the oil, the brine, and the solids, V refers to the volume, and M to the mass. Assumptions that needed to be verified at HPHT conditions were:The volume variation of the emulsion is the linear combination of oil and brine volumes (eq. 2)The volume variation of solids is negligible (eq. 4) P. 157