SPE Member

Abstract

Recent developments in foamed fluids have improved fracturing success on several formations within the North Texas, East Texas, Oklahoma, and Bakersfield, California areas. Laboratory results indicate the foaming of a delayed cross-linking gel increases apparent viscosities, as well as sand transport and fluid loss properties beyond those of foamed linear gels.

A detailed explanation of a 1,100 foot high pressure single pass pipe viscometer and fluid loss apparatus is provided. Results suggest foam cross-linked gels follow power law fluid behavior, possessing 44% higher viscosities than conventional foams. Fluid loss testing also shows a 40% reduction in leakoff beyond conventional foams. Dynamic proppant support for foams is not perfect according to parallel plate observation. Fall rates for conventional foams while affected by foam quality are two orders of magnitude greater than foam cross-linked fluids. Resulting proppant profiles are believed to be superior to conventional foams due to virtually infinite static proppant support.

More than 100 treatments have been completed using a delayed cross-linking foam. A review of local formation characteristics and treatment conditions a re discussed regarding the success of a cross-linked versus conventional linear gel foam.

Introduction

During the past 20 years, numerous technical papers and articles have been written an the subject of foam fracturing. From the foaming of a variety of liquids (water, acid, hydrocarbon, and alcohol) with nitrogen and carbon dioxide to the theoretical analysis of foam rheology and leakoff, virtually every aspect of foam fracturing has been addressed.

This paper focuses on the use of a delayed cross-linking mechanism which has proven to enhance critical foam properties: viscosity, fluid loss control, and proppant transport.

One of the most critical characteristics of any fracturing fluid is its viscosity. Modification of this property can greatly affect overall foam performance. Thus, polymers have been added to the performance. Thus, polymers have been added to the liquid phase of foams to improve apparent viscosity in the fracture. Polymers also have other advantages (fluid loss control an d proppant transport enhancement). For instance, cement nitrogen pumping and vaporizing practices have all but necessitated that proppants, chemicals, etc., be added to the liquid phase of foams. This has resulted in concentrated slurries at the surface. Pneumatic transfer of proppants into a fluid on the Pneumatic transfer of proppants into a fluid on the pump's discharge side has been practiced in some pump's discharge side has been practiced in some areas, but remains in the developmental stages. Therefore, in addition to improving foam performance, the addition of natural or synthetic performance, the addition of natural or synthetic polymers facilitates the transfer of surface polymers facilitates the transfer of surface concentrated slurries which commonly reach 16 ppg proppant concentrations. proppant concentrations. Attempts were made at foaming cross-linked gels soon after their introduction to the industry with little success, indicating that surface cross-linked borate and titanate gels were apparently too viscous and unyielding for adequate foam generation. Yet the recent advent of delayed cross-linked gels, whether time or temperature activated, has permitted foam generation of linear gels at surface while providing the downhole properties of a foamed cross-linked gel.

In addition to testing methods used, this paper discusses rheological, fluid loss and proppant transport characteristics of this unique fluid. Also included is a comparison of treatments using cross-linked and conventional foams during an eighteen month period.

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