New Proppant for Deep Hydraulic Fracturing
Authors: Underdown David R. , Das Kamalendu 
Published: December 2007

Much work has focused on developing and evaluating various materials for use as proppants for hydraulic fracturing. Sand is used most often as a fracturing proppant in shallow wells. Deep wells with high closure stresses require a proppant, such as sintered bauxite, that will not crush under adverse conditions. Ceramic and zirconium oxide beads and resin-coated sand proppants also have been developed for deep hydraulic fracturing. A new fracturing proppant has been developed that exhibits the properties necessary for use in deep hydraulic fracturing. This proppant is produced by precuring a specially modified phenolformaldehyde resin onto sand. The new proppant maintains conductivity and resists crushing much better than does sand. The new proppant was compared to intermediate-density sintered bauxitic proppants and cured-in-place proppants and the tests were confirmed by an independent laboratory.

Introduction

Sintered bauxite commonly is used as a proppant in deep wells, where materials such as sand or glass beads would be crushed. Sintered bauxite resists crushing much better than sand does at high closure stresses, but it is much more expensive and harder to place because of its high bulk density. Several types of materials are used as proppants. Sand is one of the more common materials used because of its availability and cost. However, sand is limited to use in relatively shallow wells where closure stresses of 8,000 psi [55.2 MPa] or less are encountered. When sand does collapse as a result of closure stress, a great volume of fines is generated, which causes an irreversible decrease in conductivity of the fracture. This phenomenon also is observed with glass beads. A new proppant has been developed that resists crushing and maintains conductivity much better than regular fracturing sand. The new proppant is made from sand coated with a modified phenolformaldehyde resin. The resin is cured around each of the sand grains. When the sand grains are coated with the proper amount of resin and cured to the proper degree, the resin adds substantial strength to the sand grain because of the physical characteristics of the resin once it is cured. Results presented in this paper show that sintered bauxite and the new proppant generate very few fines when exposed to the same closure stress. Flow tests also show that the precoated sand grains, which are cured individually, show much less decrease in conductivity than does uncoated sand when confined at closure stresses greater than 6,000 psi [41.3 MPa].

Theory

The use of sands coated with phenolformaldehyde resins (i.e., thermoset) has been under development for several years. The basic application has been placement of such a material into a wellbore in an uncured state. The natural temperature of the well or injected steam then causes the resin on the sand to "flow" and set, thus producing a high-strength, consolidated porous medium. This type of material has been used in conjunction with screens to produce a strong, consolidated porous medium in a fracture. The application of sand coated with uncured thermoset resin for both gravel packing and fracturing has met with varied success. This type of process has several limitations to successful application. Sand coated with an uncured thermoset resin must be placed at a relatively low temperature. Exposure to elevated temperatures (i.e., greater than 160 to 200 deg. F [greater than 71 to 93 deg. C]) for long periods during placement may cause the resin to cure partially before placement is complete. Exposure of the sand coated with an uncured thermoset resin to elevated temperatures during placement could be detrimental to the conductivity of the final fracture or gravel pack. This could result from the resin-coated sand becoming "tacky" during placement, thus allowing small particles within the placement fluid to adhere to the coated sand grain. Placement of this type of "dirty" particle within a fracture or gravel pack could decrease the final conductivity substantially. One major concern with using sand coated with an uncured thermoset resin as a proppant is how it is cured once it is in place in the fracture. Test results show that crushing similar to that observed for sand occurs for certain resincoated sands if the closure stress is not kept off the proppant until it is fully cured (Fig. 1). Tests with certain modified thermoset resins have shown that even though a strongly consolidated porous medium can be obtained by using sand coated with the uncured resin, as previously described, a unique property is imparted to sand if the coated sand grains are cured individually under proper conditions. Once the resin-coated sand grains are cured individually, the resulting material has a resistance to crushing similar to that of sintered bauxite.

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