Abstract

Hydraulic fracturing through coiled tubing (CT) has become an effective and economical stimulation technique for wells with multiple zones to be fractured. Yet, prediction of friction pressure losses of fracturing slurries in coiled tubing presents a unique challenge in the fracturing job design. This paper presents an experimental study of friction pressure losses of fracturing slurries in coiled tubing using a full-scale coiled tubing test loop which consists of 1000 ft 2–3/8" coiled tubing and 200 ft 2–3/8" straight tubing sections. The fracturing slurries were prepared with guar-based gels at various polymer and sand concentrations. Based on the experimental results, the characteristics of the flow behavior of fracturing slurries as affected by sand concentration, base gel rheology and centrifugal forces are discussed. A new correlation has been developed and can be used to predict the friction loss increase over base gel friction.

The coiled tubing erosion mechanisms (sliding abrasion, random particle impact, and directional particle impact) have been discussed. The abrasion by sliding sand bed is believed to be the dominant erosion mechanism.

Introduction

Hydraulic fracturing through coiled tubing has become an effective well stimulation technique for wells that have multiple zones to be fractured.1,2One of the design challenges is the difficulty of predicting the friction pressure losses in the coiled tubing string. The small diameter and the tubing curvature cause excessive friction pressure losses. Another important aspect of fracturing through coiled tubing is the tubing erosion and wall loss due to the abrasive sands and the centrifugal forces. Reports1,3have indicated that this tubing erosion is non-uniform along the tubing length and the circumference, thereby reducing the effective life of the coiled tubing and affecting its safe operation.

The flow of fracturing slurries in coiled tubing shares some common characteristics with conventional hydrotransport processes that are widely used in coal, iron ore, and other industries4. Hydraulically, slurries can be categorized as homogeneous and heterogeneous systems, each exhibits different flow behavior, as is illustrated in Fig. 1. For homogeneous flow, the slurry mixture behaves like a single phase liquid. Slurries or suspensions of fine particles in viscous fluid or small particles in turbulent flow are generally homogeneous systems. For a heterogeneous system, on the other hand, carrier fluid and the particles to a large extent retain their separate identities. This will lead to solids settling and non-uniform solids concentration distribution. A critical parameter of heterogeneous system is the deposition velocity (VD) which is defined as the critical velocity at which a bed of particles begins to form. Lowering flow velocity below the deposition velocity will increase the friction pressure losses.

The unique feature of fluid flow in coiled tubing is the curved flow geometry which results in centrifugal forces and secondary flow5. The centrifugal force is the driving force for the sand segregation in coiled tubing while in horizontal slurry pipeline, solids (if heavier than the carrier fluid) settle because of gravity. In coiled tubing, sands tend to migrate to the outer wall or "settle" in the radial direction. Under flow conditions of typical pump rate in hydraulic fracturing, the centrifugal acceleration is much higher than gravitational acceleration. Also due to the centrifugal force in coiled tubing flow, the wall erosion will be more severe than in straight slurry pipelines.

This paper presents experimental results of friction pressure losses of fracturing slurries using a full-scale coiled tubing test facility and discusses the characteristics of the flow behavior of fracturing slurries in both coiled tubing and straight tubing. The mechanisms of tubing erosion during fracturing through coiled tubing are also briefly discussed. The experimental results and the theoretical analysis will provide valuable insight into the complex phenomena of fracturing slurry flow in coiled tubing.

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