Abstract

Xanthan gum has been used extensively as a viscosifier in coiled tubing due to its unique friction reduction properties. In this paper we describe the friction pressure properties of a new, previously introduced, biopolymer for use in coiled tubing applications. The name of the polymer is diutan. Diutan yields higher low shear rate viscosities at lower polymer concentrations than xanthan gum due to its higher molecular weight. Diutan is more shear thinning that xanthan gum. As a consequence, the high shear rate viscosities of diutan are lower than those of xanthan gum at comparable low shear rate viscosities.

Large scale friction pressure tests in coiled and straight tubing were conducted to observe if the lower high shear rate viscosities of diutan translated into lower friction pressures. The results of these tests showed that both xanthan and diutan exhibited significant drag reduction. However, the percentage drag reductions of diutan were larger than those of xanthan gum.

Introduction

Since its introduction in 1964 xanthan gum has been used extensively in the oil industry as a viscosifier for different applications due to its unique rheological properties. These applications include drilling, drill-in, completions, coiled tubing and fracturing fluids.1 Navarrete et al.2 introduced new biopolymers which have improved performance in some of the oil field applications where xanthan gum has been traditionally used.

One of the new biopolymers is a biofermented polymer produced by a newly isolated naturally occurring bacterial strain of the Sphingomonas genus.3–5 The biopolymer name is diutan. The diutan structure is different from that of xanthan gum and the diutan molecule is also longer than that of xanthan gum (Fig. 1).

The unique structure of diutan gives the polymer different properties in solution, which are advantageous in the design of water-based drilling, drill-in, completions, coiled tubing and spacer fluids. Among those properties are higher Low Shear Rate Viscosity (LSRV) at low polymer loadings and better thermal stability.2,6 This paper focuses on the friction pressure properties of diutan fluids and their comparison to those of xanthan fluids. Friction pressure tests were conducted in both straight and coiled tubing. The results are presented and discussed in relation to the rheological behavior of diutan and xanthan gum.

Experimental

Rheological Measurements.

Rheological measurements were performed using a FANN 35 (B1-R1 Couette) and a Brookfield DV-II (wide gap Couette).

A controlled stress rheometer, the Bohlin CVO 120 HR was used for High Shear Rate Viscosity (HSRV) measurements. Measurements were performed using a 60 mm diameter parallel plate geometry with a gap of 150 µm. High shear rate tests >1,000 s-1) were performed by applying shear stresses above 5 Pa and below 65 Pa at ambient temperature.

Fluid Preparation.

Fluids for laboratory testing were prepared on a Hamilton Beach mixer. The polymer was initially dispersed in water at 3,000 rpm for about 1 min. Then the speed was increased to 11,000 rpm. Diutan was mixed in ASTM sea water for 45 min. Dispersible clarified xanthan required adjusting the pH to 8.2 with a NaOH solution before applying 11,000 rpm for 30 min. When hydrating polymers in 3% KCl, the polymers were first hydrated in tap water for 20 to 30 min after which the KCl was added and mixed for another 10 min at 11,000 rpm.

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