Increasing Lubricity of Downhole Fluids for Coiled-Tubing Operations
Authors: Carpenter Chris 
Published: February 2015

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 168298, ’Increasing Lubricity of Downhole Fluids for Coiled-Tubing Operations,’ by Silviu Livescu, SPE, and Steven Craig, SPE, Baker Hughes, prepared for the 2014 SPE/ICoTA Coiled Tubing and Well Intervention Conference and Exhibition, The Woodlands, Texas, USA, 25-26 March. The paper has not been peer reviewed.

While increasing coiled-tubing (CT) diameter remains a theoretical option to improve reach, doing so creates logistical challenges. Although fluid-hammer tools and downhole tractors have extended the reasonable operational range of CT significantly, they also increase circulating pressures and operational complexity. To reach a 10,000-ft lateral, the use of metal-on-metal lubricants will be required to work in conjunction with the other systems. Arising from a series of trials is a new lubricant that reduced the linear coefficient of friction (CoF) by approximately 40 to 60% (0.10 to 0.14) under downhole conditions.

Introduction

In this study, the results of an extensive set of laboratory linear-friction measurements aimed at mimicking downhole conditions are reported. Temperature was the factor identified to most influence linear CoF. As far as the authors of the complete paper are aware, the relationship between temperature and CoF has never been investigated before for CT operations, either experimentally or theoretically. Several in-house laboratory rotational-friction tests were performed in the past for understanding the wet-dynamic- friction mechanisms for CT operations. CoF values as low as 0.05 were obtained. Similar low values were also reported by other investigators. However, none of these CoF values has been confirmed in the field (typical field values are closer to the generic CoF value of 0.24 than to the aforementioned laboratory values).

Lubricants are used in CT operations for two primary reasons. The first is the prejob planned use of lubricant either to reach farther in a lateral or to reduce pulling weights. The second, and by far the most common, application for lubricants is to reduce the contact friction to reach slightly farther in the well or to release tubing or tools stuck in hole. These situations, termed operational recovery, typically use small slugs of lubricant to reduce frictional drag locally.

In both cases, engineers are frequently asked to quantify the benefits of using the lubricant. This is achieved by post-job matching of the observed weights to computer-model predictions. This task is significantly easier and far more accurate when used in the planned phase. In these situations, a greater section of the well is lubricated and steady-state weight records are available over a longer lateral section. Additionally, there is often the chance to match the base-case unlubricated results. A quantitative lubricant benefit can be derived when this level of data is available. In operational-recovery situations, there are typically multiple changes occurring over a short period of time, making an accurate derivation of friction changes very challenging.