A Study of Gel Degradation, Surface Tension, and Their Impact on the Productivity of Hydraulically Fractured Gas Wells

Abstract

High pH borate gels have been used in fracturing deep gas reservoirs in Saudi Arabia. Guar and hydroxypropyl guar are used at various concentrations up to 45 lb/1000 gals. A breaker (regular or encapsulated oxidizers, guar-specific enzyme, or combinations of these breakers) is usually used to degrade the gel after the fracturing treatment. Field results indicated that the cleaning time following fracturing treatments was too long. Unbroken gel was noted in the flow back samples of some wells. This study was conducted to assess the effectiveness of various breakers that are used in the field, and determine other parameters that may affect the time needed to clean-up fractured wells. This paper presents the results of a detailed study done to evaluate the performance of several breakers at typical field conditions. The apparent viscosity of various borate gels was measured as a function of breaker type and concentration. Gel degradation was followed in a high temperature/high pressure see-through cell. The surface tension of various gel filtrates was measured as a function of temperature up to 150°C. Viscosity measurements indicated that all oxidizers degraded high pH borate gels, however the time needed to degrade the gel was found to be a function of breaker type and concentration; temperature and polymer loading. All guar-based gels produced a residue after reacting with the breaker. This residue was noted irrespective of the type and concentration of the breaker used. The residue was noted with gels formed using either guar gum or hydroxylproply guar. This residue may adversely affect the conductivity of the fracture. Surface tension measurements indicated for the first time that the surface tension of borate gels is high, which will enhance water blockage and hence reduce gas production. This paper examines factors affecting gel degradation, surface tension of borate gel filtrate, and their impact on well productivity. Introduction Hydraulic fracturing is often necessary for oil and gas wells to enhance well productivity. The 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. To make the fracturing operation successful, the fracturing fluids need to possess certain properties such as sufficiently viscous to suspend and transport proppants, suitable at pumping temperature, having low friction pressure, having moderate efficiency, resistant to shear degradation, can be removed efficiently from the fracture, and economically realistic.1 Guar and guar 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 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 This study, however, will focus on guar gum and HPG polymers cross-linked with monoborate ions. Pre-Khuff sandstone reservoirs in Saudi Arabia produce sweet gas for more than ten years. Many of wells drilled in these reservoirs are hydraulically fractured.4 Guar gum and HPG are used to prepare the gels used in hydraulic fracture treatment. Both polymers are cross-linked with monoborate ions. The source of this ion is either boric acid or borax or organoborate salts. The sandstone reservoirs are clean sand with illite as the main clay present in the formation. Potassium chloride is used all drilling and completion fluids to avoid fines migration problems. The reservoir temperature and pressure are 300°F and 8,535 psi, respectively. 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 the gels used in these treatment did not break completely. They also indicate that the surface tension of gel filtrate was high, which resulted long cleaning time. The detrimental effects of fracture conductivity reduction resulting from incomplete fracturing fluid flowback/cleanup are well documented in literature.5 Until recently, characterization of fracturing fluid cleanup could only be simulated in the laboratory and can't be monitored in the field.6

Review on High-Temperature-Resistant Viscoelastic Surfactant Fracturing Fluids: State-of-the-Art and Perspectives

“…The study has shown that the VES-based fluids produce less surface tension than borate-cross-linked gel. 94 In addition, friction reduction due to the VES gel is a known phenomenon, 95,96 and VES's use as the FR is a topic of interest to researchers. 30,97 The principle of reducing friction in the surfactant systems is similar to that of high-molecularweight polymers.…”

Chemical and Reactive Transport Processes Associated with Hydraulic Fracturing of Unconventional Oil/Gas Shales

“…Many additives are organic in nature, , and the rates and stoichiometry of their chemical transformations are not well described. For example, guar gum is commonly used as a gelling agent, and while the timing of degradation is well constrained, the effects of the degradation biproducts on solution chemistry as well as the transport properties of such a viscous material are poorly described. , Complex organics may also be sourced from the shale kerogen itself. Organics can be altered by exposure to fracture fluids, (section ) and many additives are proprietary.…”

Avaliação do potencial uso de bioglicerina como base para formulação de fluidos de perfuração aquosos para poços de petróleo e gás

“…Além disso, o uso destas bioglicerinas como base para fluidos reduziria os gastos com produtos específicos e de alto custo, como surfactantes especiais que normalmente são utilizados em fluidos nos quais a água é a base. [44][45][46][47] …”