Abstract

Xanthan gum has been used extensively as a viscosifier in the oil industry for different applications due to its unique rheological properties. In this paper we introduce two bio-polymers for use in drilling, drill-in, completions, spacer fluids and coiled tubing applications. The first bio-polymer yields higher viscosities and better temperature stability at lower polymer concentrations that welan and xanthan gum due to its higher molecular weight; this polymer is particularly effective in low salt fluids. The second bio-polymer has improved solubility in high density CaCl2 brines.

Viscosity data over a wide shear rate range is presented comparing the new bio-polymers with xanthan and welan gum at temperatures between 75°F to 300°F. Viscoelastic measurement and settling test are presented to demonstrate the effect of elasticity of the bio-polymers on solids transport and suspension capabilities. Fluid-loss, formation damage tests and friction pressure tests in coiled tubing are also shown.

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 Similarly, welan gum (introduced in 1985) has been used in drilling fluids and cement spacers, due to its compatibility with oil field cement formulations. Here two new bio-polymers are introduced which have improved performance in some of the applications where xanthan and welan gum have been traditionally used.

The first polymer is a new bio-fermented polymer produced by a newly isolated naturally-occurring bacterial strain of the Sphingomonas genus. This bio-polymer has been given the generic name of diutan gum. The chemical structure of the monomer is shown in Fig. 1. The tertiary structure is a double-helix. The diutan structure is closer to that of welan gum (Fig. 2) than that of xanthan gum. However, there are important differences. Diutan has an average molecular weight of 5×106, which is much higher than those of welan and xanthan. This is why the length of the diutan molecule is larger than that of welan or xanthan (Fig. 3).

The second polymer is a pyruvate-free variant of xanthan gum, or Non-Pyruvylated Xanthan (NPX) gum, produced from Xanthomonas campestris. The chemical structure of the NPX monomer is shown in Fig. 4. NPX is similar in structure to xanthan gum on all other respects aside from the absence of the pyruvic acid group which reduces anionic character.

The unique structures of these two bio-polymers give them different properties in solution. Some of these properties can be advantageous in the design of water-based drilling, drill-in, completions, coiled tubing and spacer fluids. These properties are presented in this paper.

Experimental

Rheological Measurements.

Rheological measurements were performed using a Brookfield PVS viscometer which can measure shear viscosity between 0.05 to 1,000 s-1, at temperatures up to 350°F and pressures up to 1,000 psi. A pressure of 300 psi was used in all tests. Different geometries can be used with this instrument. The ones used here were the single annulus B1-R1 Couette geometry, and the triple annulus TA5 Couette geometry.

Other viscometers used at ambient temperature were the FANN 35 (B1-R1 Couette) and the Brookfield DV-II (wide gap Couette).

Rheological Measurements.

Rheological measurements were performed using a Brookfield PVS viscometer which can measure shear viscosity between 0.05 to 1,000 s-1, at temperatures up to 350°F and pressures up to 1,000 psi. A pressure of 300 psi was used in all tests. Different geometries can be used with this instrument. The ones used here were the single annulus B1-R1 Couette geometry, and the triple annulus TA5 Couette geometry.

Other viscometers used at ambient temperature were the FANN 35 (B1-R1 Couette) and the Brookfield DV-II (wide gap Couette).

This content is only available via PDF.
You can access this article if you purchase or spend a download.