A Study of Gel Degradation, Surface Tension and their Impact on the Productivity of Hydraulically Fractured Gas Wells
Authors: Nasr-El-Din Hisham A. , Al-Mohammed Abdullah Mohammed , Al-Fuwaires Omar A. , Al-Aamri Ali 
Published: February 2016

Guar and hydroxypropyl guar are used to prepare high pH borate gels at various concentrations up to 45 lb/1000 gals. Oxidizers are usually used to degrade the gel after the fracturing treatment. This study was conducted to assess the effect of polymer type and loading on gel degradation; and determine other parameters that may affect the time needed to clean-up fractured wells.

Guar gum and HPG were used prepare the gel. Sodium bromate and chlorous acid were used as breakers. Gel degradation was examined at typical field conditions. The apparent viscosity of various borate gels was measured as a function of breaker type and polymer loading. Gel degradation was followed in a high temperature/high pressure see-through cell.

Viscosity measurements indicated that sodium bromate and chlorous acid were able to degrade high pH borate gels at different polymer loadings. All guar-based gels produced a residue after reacting with the breaker.

The amount of residue was measured after heating the sample in the see-through cell at 140°C and 300 psi after four hours. The results indicated that the guar gum produced higher amount of residue compared to gel prepared with HPG polymer.

Surface tension measurements indicated that the type of oxidizers did not affect surface tension of gel filtrate. Suitable nonionic surfactants were able to reduce surface tension of gel filtrate. Surface tension of gel filtrate decreased with increasing temperature.

Introduction

Fracturing fluid is one of the most important components in hydraulic fracturing treatments.1 The fluid is used to create fracture and transport proppant down the created fracture. Guar and its derivatives; HPG (hydroxypropyl guar), and CMHPG (carboxymethyhydroxypropyl guar), are the most commonly used polymers to prepare water-based fracturing fluid.2 High viscosity is generated by crosslinking the polymer molecules with a crosslinker (B(III), Ti(IV), or Zr(IV)).1

Borate gels have been used in the oil industry as fracture fluids and zone isolation.3 Guar gum and HPG are cross-linked with monoborate ions B(OH)4-. The source of this ion is boric acid, borax, or organoborate salts. Analysis of well flow back samples following fracture treatments highlighted the presence of very viscous fluids and gel fragments. Also, the time needed to clean the fractured wells was too long. These trends indicated that the gels used in these treatments did not break completely.

The objectives of this paper are to:evaluate the performance of two oxidizers at typical field conditions andstudy the effect of changing the polymer loading and polymer type on the rate of degradation of these gels,determine the effect of oxidizer type and temperature on the surface tension of gel filtrate.

Background

Guar gum is a naturally occurring polysaccharide that is used extensively as a water-based viscosifier. Guar is a linear polymer with a backbone composed of mannose units connected by ß-1,4 acetal linkages. This backbone has single unit branches of galactose connected by a-1,6 acetal linkages.4

Boric acid or borate salts have been used as a source of monoborate ions. It has been suggested that the cross-linking reaction occurred due to either the formation of borate esters,5 hydrogen bonding,6 or p-bond overlap.7