Field Validated Hydraulic Model Predictions Give Guidelines for Optimal Annular Flow in Slimhole Drilling

Abstract

Abstract Accurate prediction of annular flow is of primary importance in slimhole drilling. A model was developed that better represents actual annular configurations. It accounts for axial variations of drillpipe eccentricity and for effects arising from drillpipe rotation. The model is validated with data obtained from 3 industrial slimhole wells (4"3/4 and 3"3/8 phases). Based on model predictions, guidelines are given to define flow parameters and to adapt fluid rheology in order to obtain optimal hole cleaning under safe drilling conditions. Introduction Flow in the reduced annular geometries of slimhole wells may give pronounced pressure losses at moderate or even low flow rates, generating thus substantially higher equivalent circulating densities than in conventional wells. Accurate control of wellbore hydraulics and adequate choice of fluid rheology and flow parameters are therefore important issues of the slimhole technology, as far as avoiding excessive pressure against the formation and efficient hole cleaning are concerned. Early field practice and more recent laboratory studies showed that the usual hydraulic models are not able to correctly predict pressure losses in slimhole annuli. These models assume a concentric annular geometry and ignore drillpipe rotation. However, the flow is extremely sensitive to the precise boundary conditions, so drillpipe actual configuration and rotation must be accounted for. The need to better understand and control annular flow was recognised at the early stages of the Euroslim project, a collaboration between Forasol, Elf, DBS and IFP. The objective of this project was to demonstrate the technical reliability and economic impact of the slimhole technology. As reported elsewhere, several industrial slimhole wells (terminal hole diameters of 4"3/4 and 3"3/8) were drilled successfully with a purpose built slimhole rig and drillstring A global 30 to 50% costs reduction was obtained, due to a large extent to integration of services by the drilling contractor. Extensive research was performed at IFP on solids free fluid formulations and optimisation of annular flow. The project on annular flow aimed at developing a flow simulator for field use and comprised the following steps:–obtain the necessary theoretical understanding and construct an appropriate flow model based on realistic assumptions on fluid rheology and wellbore geometry,–validate model predictions, first in the laboratory and then by experiments in field scale pilots,–confront model predictions to field data obtained under industrial conditions. The paper briefly describes model assumptions and basic trends as well as their validation under controlled conditions with specially designed field scale hydraulic experiments. Interpretation of field data obtained under industrial conditions is one of the objectives. It will be shown that the model correctly reproduces the measured stand pipe pressures and that it can provide reliable information on flow regime and pressure distribution in the different parts of the well. The ability of the model to predict annular flow in the whole range of parameters makes it an attractive tool to use in order to further optimise annular flow. The other objective of the paper is, therefore, to examine under what conditions axial velocity differences in the cross section of the annulus can be minimized, which can considerably improve the cuttings transport. The hydraulic model The model addresses the flow of shear-thinning fluids in complex annular geometries. P. 727