Wells – Processes – Specific propping feature
Reexamination Certificate
2000-07-14
2001-12-11
Yoon, Tae H. (Department: 1714)
Wells
Processes
Specific propping feature
C166S293000, C166S295000, C507S219000, C523S130000
Reexamination Certificate
active
06328105
ABSTRACT:
BACKGROUND
1. Field of the Invention
This invention relates to proppants, and more specifically, the use of proppants in hydraulic fracturing of subterranean formations.
2. Description of Related Art
Hydraulic fracturing is a process used to stimulate oil and gas production in subterranean formations. A fracturing fluid is injected into the well casing or tubing to produce a buildup of well bore pressure. When the well bore pressure is large enough to overcome compressive earth forces, fractures form. Continued injection of the fracturing fluid increases fracture length and width.
The fracturing fluid typically includes a viscous fluid, such as, for example, a linear gel or a crosslinking gel, that carries hydraulic fracturing particles commonly known as proppants. Once the fracturing fluid transports the proppant inside the fracture, the viscosity of the fracturing fluid breaks down leaving the proppant particles in place. The fracturing fluid is then dissipated through the formation or recovered via the well bore.
Proppant particles are commonly made of sand, glass, bauxite, ceramic, and shells and range in size from about 6 to 200 mesh (U.S. Sieve Series scale) . Many of these proppant particles are optionally supplied with a resin coating. A resin-coated proppant particle generally has a greater crush resistance than the core substrate material and can be used at greater depths or may be used at the same depth with increased relative conductivity. For example, resin-coated sand can be used at closure stresses up to about 10,000 psi, as compared with about 6,000 psi. for uncoated sand. The coating also serves to trap free fines from fragmented or disintegrated substrates under high closure stress.
Compressive earth stresses, or closure stresses, encountered in subterranean formations subjected to fracturing can range from 500 psi to 25,000 psi or even higher, depending upon the depth of the fracture. Accordingly, the proppant selected for a particular subterranean formation must be strong enough to resist the compressive earth forces in that formation, thereby keeping the fractures open and allowing fluid flow therethrough. Thus, sand proppant particles are generally used where closure stresses are up to about 6,000 psi, ceramics are generally used at closure stresses up to about 15,000 psi, and bauxite is generally used at closure stresses greater than about 15,000 psi.
The productivity of a subterranean fracture is dependent upon, among other things, the conductivity of the fracture. Conductivity is defined as the permeability of the proppant times the width of the fracture. Permeability, in turn, refers to the permeability of a mass or “pack” of the proppant particles and is governed by the size and shape of the proppant particles and the size of the interstitial spaces between the particles. These interstitial spaces become the passageways for the flow and recovery of subterranean fluids. Accordingly, a proppant consisting of coarse proppant particles can form a proppant pack having larger interstitial spaces to achieve higher conductivity relative to a proppant consisting of finer proppant particles.
Despite the advent of modern proppants, conductivity of proppant-filled fractures continues to be a problem. In formations at shallower depths where closure pressures are typically about 4000 psi or less, coarser proppant particles greater than about 40 mesh are commonly used. However, coarser proppant particles are susceptible to bridging in fractures during stimulation treatment. Bridging occurs when the coarse proppant particles plug narrow fractures. The result is a reduction in the overall productivity of the well bore or premature screen-out. Moreover, the stress encountered even in shallower formations may produce fines that migrate and plug the interstitial flow passages between the proppant particles, thereby drastically reducing the conductivity of the fracture.
In fractures located at greater depths where closure pressures are higher, e.g. 10,000 psi or more, proppants composed of finer particles are typically used. However, because the interstitial spaces between these finer particles is smaller, permeability and hence conductivity are inherently less.
It is desirable to have a method of increasing the conductivity of a subterranean fracture. It is also desirable to develop a proppant having increased permeability for a given particle size. It is also desirable to replace coarse proppant particles with finer proppant particles in subterranean fracturing operations while maintaining or improving the conductivity of the fracture.
SUMMARY OF THE INVENTION
The present invention provides an improved proppant and a method for improving the conductivity of fractures in subterranean formations using such proppant. The inventive proppant comprises a mixture of bondable particles and removable particles. The bondable particles, when in place in a subterranean formation, adhere to one another to form a substantially permanent, self-supporting matrix that is interspersed with removable particles. The removable particles, after formation of the matrix, can then be removed from the matrix by being dissolved or otherwise entrained in the fluid or fluids subsequently processed in the formation. As a result of this removal, the interstitial spaces in the matrix left behind increase in size, thereby achieving higher matrix porosity and hence higher conductivity in the fracture as a whole.
In accordance with a preferred embodiment of the invention, the removable particles are selected to have a size, shape and density similar to the size, shape and density of the bondable particles. With this feature, the mixture of bondable and removable particles comprising the inventive proppant remains substantially uniform during the fracturing operation.
In accordance with an especially preferred embodiment of the invention, the bondable particles include a resin compound capable of softening and preferably curing under ambient conditions encountered in the well fracture. This allows adjacent bondable particles to adhere together through both physical and chemical means during formation of the self-supporting matrix, which promotes formation of an especially strong matrix.
DETAILED DESCRIPTION
In accordance with the present invention, fracture conductivity of a subterranean formation is increased by placing in a subterranean formation a proppant comprising a mixture of bondable particles and removable or dissolvable particles.
Materials which can be used as the bondable particles in the inventive proppant can comprise any particulate material which will serve as a bondable proppant. Such materials are well known in the art, and any such material can be employed. In general, bondable proppants include any free-flowing, particulate material which can be charged down a well under the conditions of pressure, temperature and chemical environment encountered during fracturing and which, when in place in the fracture, exhibit suitable crush strengths and porosities to enhance fluid recovery. Such particles also need to be bondable under ambient conditions encountered in the formation. By being bondable is meant that the particles can join together to form a substantially permanent, self supporting matrix.
Preferred bondable proppants comprise particulate materials in which the individual particles have been provided with a coating which enhances the surface adhesion of the substrate material. Typical coating materials for this purpose comprise curable resins, that is, resins capable of being cured to a higher degree of polymerization. Single-coated, double-coated or a multiple-coated particles can be used. In double or multiple-coated particles, the outer coating can be substantially cured such that little or no cross-linking takes place upon exposure to down-hole conditions. In this type of particle, the outer coating is adapted to rupture under temperature and pressure revealing the inner coating which is capable of curing.
Examples of single-coated bondable particles which can be used in accor
Calfee Halter & Griswold LLP
Technisand, Inc.
Yoon Tae H.
LandOfFree
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