Abstract

Many studies have been conducted to determine the optimal hydraulic conditions when conventional fluids are used in drilling operations. However, little information is available for cuttings transport when gasified fluids are used as a drilling fluid. Since deviated and horizontal-well drilling are becoming more common and are frequently combined with the use of non-conventional drilling fluids, such as gasified fluids or aerated muds, a better understanding of the cuttings transport phenomena is needed for these applications.

This study was conducted to help filling this gap and to gain more in-depth understanding of cuttings transport in horizontal and highly-inclined wells when using gasified fluids. Extensive experiments were performed in a unique field-scale low-pressure flow loop (8" × 4. 5", 90' long) at horizontal and inclination angle of 80 degrees from vertical. Gravel with 3. 29 mm average diameter was used to simulate drill cuttings and water and air were used as the liquid and gas phase. The three phases were injected into the test section of the flow loop, at different volumetric flow rates combinations, for the horizontal and 80 degree inclination angle. Pressure drop, annular pressure, temperature and cuttings accumulation were recorded in each experiment through a data acquisition system. Gas and fluid-interface distribution were visually observed and reported for each experiment. Likewise the solid-liquid distributions were classified into three different patterns and reported for each experiment. The effects of gas and liquid flow rates, drilling rate, inclination angle, pressure drop and flow patterns on cuttings transport were analyzed in this study.

It was observed in the experiments, for the range of volumetric flow rates used during these tests, that cuttings are only transported by the liquid phase. It was also found that it is possible to define a boundary for the minimum air and water velocities required to avoid the formation of a stationary cuttings bed. These minimum requirements exist in the intermittent flow region for the gas and liquid interface distribution. It was observed that the minimum requirements for air and water injection rates are also a function of the solids injection rate. It is postulated in this study that there is a minimum energy required for solids transport, and this is constant for a given solids injection rate. It was observed that the inclination effect for angles close to horizontal is negligible.

Introduction

One of the primary functions of a drilling fluid is to transport efficiently the particles generated in the drilling process (drill cuttings) to the surface through the wellbore annulus. The ability of a fluid to transport cuttings is called the "carrying capacity" of drilling mud. During the past two decades, especially in recent years, many studies have been conducted to obtain a better understanding of the cuttings transport phenomena. Several mechanistic models and empirical correlation have been developed in order to give the drilling engineer better tools to help him design efficient hydraulic programs and improve the economical success of drilling a well1–10. As is well known, improper hole cleaning can create problems such as stuck pipe and increased torque and drag which costs the oil industry millions of dollars in losses. In times when the oil industry demands the use of new alternatives to develop its reservoirs optimally, under-balanced and near-balanced drilling techniques have become excellent option to reduce drilling costs and increase well and reservoir productivity.

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