Abstract

This paper presents a method to determine optimum drilling fluid properties and flow rates to minimize cuttings bed height and circulation time in high angle and horizontal wells. The method uses empirical models relating the cuttings bed height and the bed erosion time to drilling fluid properties and flow rates.

Bed erosion tests have been conducted using a cuttings transport facility available at the University of Tulsa. Cuttings bed height as a function of time has been investigated by using variable flow rates (200 – 400 gpm) and four different drilling fluid compositions. Experimental results were used together with a non-linear regression analysis program to establish a functional relationship among drilling fluid properties, flow rate, cuttings bed height and the time required to circulate the borehole clean.

A numerical example is provided to explain the field application of the method. The sequential calculations involved in determining optimum combination of the Power Law viscosity parameters n and K, and the flow rate to minimize the cuttings bed height and circulation time are also given.

Field implementation of the proposed empirical correlations and the new method can aid optimization of circulation practices before tripping, and so reduce the associated risk of non-productive time.

Introduction

With increasing measured depths and horizontal displacements in extended-reach (ERD) wells, good hole cleaning remains a major challenge. It has been recognized for many years that removal of the cuttings from the wellbore during drilling of high-angle wells poses special problems. The cuttings can settle by force of gravity along the bottom of the hole. As the cuttings settle in the drilling fluid, a "bed" of solids is formed along the bottom of the hole. Failure to achieve sufficient hole cleaning can cause severe drilling problems including: excessive over pull on trips; high rotary torque; stuck pipe; hole pack-off; excessive ECD (equivalent circulating density); formation break down; slow rates of penetration and difficulty running casing and logs. The most severe of these is sticking of the drill string. This condition can be very expensive to remedy. A single stuck pipe incident may cost over one million dollars. Bradley et al. (1991)1 reported that stuck pipe costs for industry, previously was in the range of 100 to 500 million US dollars per year.

In attempt to avoid such problems drilling operators often include such practices as "washing and reaming," wherein the drilling fluid is circulated and the drill string is rotated as the bit is introduced into the wellbore, and "back reaming," wherein the drilling fluid is circulated and the drill string is rotated as the bit is withdrawn from the wellbore. Other operations such as "wiper trips" or "pumping out of the hole" are often performed to attempt to control the amount of cuttings accumulated in the wellbore. All these operations require time and can very significantly add to the cost of drilling a high-angle well. Therefore, there is a strong need to understand how different drilling variables influence cuttings bed erosion behavior which would also lead to development of models for better prediction of circulation time needed to clean the borehole from cuttings.

To date, modeling of cuttings bed erosion has not been well addressed in the drilling literature. Martins et al., (1997)2, conducted a study on the cuttings bed erosion. They used an exponential concentration decay model to explain solids bed erosion in wellbores. In their study, however, they did not consider the effect of drilling fluid rheology on the cuttings bed erosion. In a recent study, Adari (1999)3 investigated bed erosion process by considering the effect of drilling fluid rheology and flow rate.

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