Transport of Cuttings in Directional Wells

Abstract

Abstract The aim of this research is to define easily applicable quantitative relationships for optimizing flow rates and drilling-fluid rheology depending on the inclination of the well. The research project involves various aspects:laboratory experiments,experiments in wells during drilling,building a numerical model based on experimental data obtained in both the laboratory and wells. Laboratory experiments performed in tubes and annular-space models have analyzed the influence of various parameters such as inclination, rotation of the drill string and fluid rheology, including thixotropy. The results of these experiments have led to equations describing the minimum fluid velocity required for evacuating cuttings versus their density, the density of the fluid and its rheological properties. In the numerical model, with a view to applying these equations to the well, they have been adjusted with the help of results obtained by cheeking the arrival of cuttings at the drill site during the drilling out of hardened cement in the cosine above the casing shoe after cementing jobs. Among the conslusions reached, the following points can be noted:The influence of inclination becomes apparent as soon as the well becomes slightly deviated (10 deg.).Evacuation of cuttings is more difficult in slanting zones, depending on the case, with an angle between 30 and 60 deg..High viscosity of the drilling fluid is favorable for the cleaning of the well in the nearly vertical part but may reduce the transfer capacity in the slanting parts.High thixotropy of the drilling fluid is very unfavorable in the slanting parts, especially if the rotation speed of the drill string is low.The increase in the density of the drilling fluid facilitates transport. Introduction Publications already made on the transfer of cuttings in deviated wells emphasize the complexity of the problem. As soon as the inclination angle approaches 10 deg., the behavior of cuttings changes gradually as the angle increases. Remarkable experiments performed in a pilot installation simulating a borehole have described the transfer modes that occur according to the inclination and the fluid flow rate. They also lead to observations on the influence of parameters such as the eccentricity and rotation speed of the drill string, the rheology and the flow regime of the drilling fluid. In some ranges of inclinations and flow rates, cuttings gather along low generatrices of the borehole and form a bed, which may cause great friction of the drill string or even sticking. A recently-developed model analyzes the formation of this bed and predicts its thickness, thus enabling flow rates to be determined to prevent the bed from forming. However, according to the results described, this model cannot currently be used by drill-site operators as an instrument for computing. Indeed, it takes into consideration parameters that, in practice, are inaccessible and cannot be controlled, such as eccentricity, solid-solid friction and solid-liquid interfacial friction. The research described here has adopted a different procedure and has been guided by the effort to make the results exploitable by drill-site operators. It has attempted both to determine the most important adjustable parameters governing cuttings transport during drilling and to give at least a qualitative idea of their influence. At the same time, it has aimed at building a numerical model that is simple to implement, as a quantitative way of optimizing fluid flow rate and rheology according to the geometry of the borehole (diameter and profile). P. 293^