Abstract

The pumping of fracturing slurry through coiled and straight tubing can cause considerable wear of tubing and is an issue of industrial concern, both from safety and economic considerations. In hydraulic fracturing operations employing coiled tubing, the tubing wall is affected by non-uniform erosion for the section of the tubing spooled on the reel. The non-uniform wall erosion is caused by the centrifugal forces when pumping sand slurries at high rates. In spite of the large number of fracturing jobs performed in the industry today, research in this area of investigating and evaluating erosion has been limited.

This paper discusses the erosion phenomena and the mechanisms leading to the erosion process. The factors affecting erosion such as pump rate, slurry concentration, and fluid properties are investigated. Newtonian (water) and non-Newtonian (fracturing gel and slurry) fluids have been investigated. The results from the Computational Fluid Dynamics (CFD) simulations for the flow of these fluids in straight as well as coiled tubing are provided. The velocity profiles and sand concentration profiles for each case are presented which provide insight into the particle migration/segregation phenomenon associated with the erosion process. It is found that the fluid properties, flow velocity, and sand concentration all have major influence on the erosion process.

The future plans for the experimental study to understand the erosion in coiled tubing are also presented.

Introduction

Hydraulic fracturing through coiled tubing (CT) has become a cost-effective and economical stimulation technique for wells with multiple zones to be fractured. Fracturing employing coiled tubing has progressed considerably since the first job in 1993. In hydraulic fracturing, the fracturing fluid is pumped at high enough rates to overcome the tensile strength of the rock and thus break the formation open. Sufficient fluid is pumped to ensure the desired dimensions of the fracture. The fracturing slurry containing proppant (solid particles) is also pumped which keeps the created channels open after the treatment ceases and sustains the closure stresses.

During stimulation, coiled tubing should be of sufficient wall thickness to withstand pressures and tensile stresses encountered during fracturing. The tensile stress is needed to allow operation on bottom-hole assembly. The tubing undergoes pressure stress during treatment and sometimes, sudden pressure increases during screen-outs. The coiled tubing is also subjected to fatigue while running in hole, pulling out of the hole, traveling from one localized zone to the next, or stroking the bottom-hole tools for setting. These fatigues can cause external damage and reduce the tubing life by more than 50%.

Besides the mechanical damage to the tubing, the wall can erode from high pressure and high velocity proppant (commonly sand) pumped in the stimulation operation. While pumping sand slurries, the tubing wall is affected by non-uniform wall erosion for the section of the tubing spooled on the reel. The non-uniform erosion is believed due to the centrifugal forces when pumping sand slurries at high rates. The tubing extrados experience more severe erosion than the tubing intrados. Kazakov and Rispler1 estimate a wall loss of 0.004 in. from 100 ton of sand pumped. A loss in wall thickness can decrease the pressure rating, reduce tensile strength and increase fatigue in the coiled tubing. This can create unsafe well-site conditions and jeopardize the personnel safety.

In spite of the large number of fracturing jobs performed in the industry today, research in this area of studying and evaluating erosion has been very limited. A part of this can be attributed to the unique nature of flow involved in this process and the difficulty in establishing a set procedure to characterize this phenomenon. The other reason is that the base fluids used in fracturing are generally highly non-Newtonian. The rheological characteristics of these fluids and the mathematical development of the non-Newtonian slurry flow through CT are much more complex. There is very little information available on the motion/movement of particles within the fluid flowing in CT and also on the wall thickness reduction while pumping fracturing slurries in coiled tubing. Therefore, a systematic study was conducted to help understand the complex phenomena of particle motion and trajectory within the flow field while pumping fracturing slurries in coiled tubing.

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