Effect of Particle Density and Size on Solids Transport and Hole Cleaning With Coiled Tubing

Abstract

Abstract Particulate material to be removed from a wellbore can have a vast range in size and density. Typical materials are formation sand, drilling cuttings and various fracture proppants such as resin-coated sands, ceramics, Bauxite and ultra light-weight materials. The characteristic size, shape and density of the particles greatly influence their dynamic behavior in flowing media. Terminal velocity, drag and gravity forces and shear stresses are affected by particle properties and the rheology of the circulation fluid. This paper presents the results of a solids transport study on four different density proppants with same particle size (20/40 mesh) and three different diameter particles ranging from 0.15 to 7 mm. The specific gravity of the proppants varied from 1.25 to 3.6. The tests were performed using a sophisticated flow loop. Findings indicate that particle density and size have a significant effect on the solids transport. For a given flow rate, higher density solids result in higher in-situ solids concentrations and lower wiper trip speed (the wiper trip speed is the coiled tubing pull-out-of-hole (POOH) speed) and reduced transport efficiency. The solids transport for different particle sizes is strongly influenced by wellbore deviation angle. In a near-vertical wellbore larger particles have the lower transport efficiency while in a horizontal wellbore the medium sized particles have the lowest transport efficiency. New correlations have been developed from the experimental data to predict solids in-situ concentration, solids carrying capacity and optimum wiper trip speed for these tested solids under a given operating condition. Introduction Drilling or wellbore cleanouts using coiled tubing (CT) techniques are prone to quality failures due to a lack of knowledge about solids transport. Poor solids transport can negatively impact the rate of penetration (ROP) when drilling a well. A well's production may be reduced due to blocked perforations or flow restriction if a cleanout does not remove all the solids. However, CT can be a very cost effective technology when the overall process is well designed and executed.Highly deviated and horizontal wells have placed a premium on having a reliable body of knowledge about solids transport in single and multi-phase conditions. In a typical solids cleanout the CT tags the top of the fill, and is run into the hole to atarget depth while jetting into the solids (penetration stage). The hole can then be cleaned either by circulating a fluid while keeping the CT stationary (circulation stage) or by pulling the CT out of the wellbore with continuous circulation (wiper trip stage), or by a combination of these stages. Solids or particle transport in coiled tubing operations is a complex problem that is affected by numerous parameters. Predicting effective solids transport requires all of these parameters to be considered simultaneously. In our previous study[1–5] comprehensive tests were conducted to evaluate solids transport with single-phase and two-phase fluids. The effect of liquid/gas volume flow rate ratio, in-situ liquid velocity, ROP, inclination angle and circulation fluid properties on solids transport was investigated. A computer program was developed based on the test results[4]. In this study the effect of particle size and particle density are investigated to gain a more in-depth understanding of solids transport for coiled tubing workovers, cleanouts and drilling applications. Tests were conducted with four proppants of varying density and the same particle size (20/40 mesh) and three particle types with different diameters. The empirical results were incorporated into a solids transport simulator which provides a practical means to evaluate solids transport under downhole conditions.