Hole Cleaning Modelling: What's 'n' Got To Do With It?

Abstract

SPE and IADC Members Abstract Efficient hole cleaning is vitally important in the drilling of directional and extended-reach (ERD) wells. In previous works, the importance of fluid velocity/pump output has been emphasised, while in others, the importance of individual fluid rheological parameters has been stressed. Drillpipe eccentricity and rotation directly affect hole cleaning as well. Recent work on particle settling velocity has accelerated the understanding of the behaviour of drilled cuttings in wellbores. This paper presents a new model to the industry, which combines recent developments in the fields of particle settling and rheology. This model provides a useful tool for the planning of hole cleaning for highly deviated wells through the use of Lift Factors under the eccentric drillpipe Information from deviated wells in the field is used to illustrate the usefulness of this hole cleaning model. Importantly, analysis of the data indicates that fluid 'n' factors calculated using the yield-power law [Herschel-Bulkley] rheological model play a greater role in promoting good hole cleaning in ERD wells than has previously been recognised. Introduction Several hole cleaning models have been published. It is worth reviewing these works prior to describing the new model. Bern et al include in their model a dimensionless Rheology Factor based on the API plastic viscosity [PV] and yield point [YP] from the Bingham plastic rheological model. Included in the model are calculations for critical flowrates (CFR) required to drill deviated wellbores It is stated that under laminar flow conditions, muds with either very high or very low YP values are preferred for hole cleaning and that YP is the dominant factor in deviated wells. In their earlier paper, fluid rheologies were described using the power law model, but as the PV and YP terms are simpler and more commonly used, the authors applied them in the later paper. It is noted that this model assumes a rotating drillstring. Clark and Bickham use a mechanistic model to describe hole cleaning. This model considers the various mechanisms involved in the transport of cuttings out of a well (i.e., rolling, lift, and particle settling). This paper concurs with industry opinion in classifying flow rate as the most important factor in hole cleaning, and it considers fluid density and rheology as the most important drilling fluid properties that affect hole cleaning. The Herschel-Bulkley rheological model is used in this paper with the fluid's yield stress being the dominant factor. The influences of the other rheological parameters (ie. consistency index and flow index) are unclear. Rasi investigated the rate of cuttings bed development and in his modeling efforts focused on predictions of cuttings bed height in eccentric wellbore. In an undefined manner, Rasi used a dimensionless friction factor (which included factors for fluid rheology), well geometry and design, and pump output to calculate the height of the free zone above the cuttings bed. The API PV and YP and the viscometer 6 rpm term (which approaches the yield stress term in the Herschel Bulkley model) were mentioned as input parameters. It is uncertain whether the author used the three terms to develop a flow behaviour curve across the shear rate spectrum. P. 479