The term “proppant” is indicative of particulate material which is injected into fractures in subterranean formations surrounding oil wells, gas wells, water wells, and other similar bore holes to provide support to hold (prop) these fractures open and allow gas or liquid to flow through the fracture to the bore hole or from the formation. Proppants are commonly used to prop open fractures formed in subterranean formations such as oil and natural gas wells during hydraulic fracturing.
Uncoated and/or coated particles are often used as proppants to keep open fractures imposed by hydraulic fracturing upon a subterranean formation, e.g., an oil or gas bearing strata, or as filtering media in gravel packs.
The uncoated proppants are typically particles of sand, ceramics, glass beads, walnut shells, etc. as known in the art. Particles used to prop fractures generally comprise sand or sintered ceramic particles. The advantage of sand is that it is inexpensive. Its disadvantages are its relatively low strength (high crush values) and lower flow capacities than sintered ceramic particles. Sintered ceramic particles are disadvantageous in that the sintering is carried out at high temperatures, resulting in high energy costs to produce, and expensive raw materials are used.
The coated proppants have individual particles coated with a resin. The individual particles are typically particles of sand, ceramics, glass beads, walnut shells, etc. as known in the art. The proppant coatings may be precured or curable. The precured proppants include a substrate core and a coating of resin cured prior to insertion into the subterranean formation. The curable proppants include a substrate core and a coating of resin cured downhole to form a consolidated proppant pack. Resin formulations typically used for curable coatings on proppant substrates (sand, ceramic, etc.) result in a highly crosslinked coating on the surface of the substrates.
Curable resin coated proppants and precured resin coated proppants have been commercially available for use as propping agents. A curable proppant has a resin coating that includes a resin that is usually at least partially, and but not fully, cured. In contrast, a “precured” proppant has a cured resin coating. The terms “cured” and “curable” are defined for the present specification by three tests historically employed in the art.
a) Temperature Stick Point Test: placing coated material on a heated melt point bar and determining the lowest temperature at which the coated material adheres to the melt point bar. A “sticking temperature” of greater than 350° F., typically indicates a cured material, depending upon the resin system used.
b) Acetone Extraction Test: an acetone extraction method, as described below, to dissolve the fraction of resin within the coating that is uncured. A weight loss of <5% typically indicates that the particle has a procured coating.
c) Compressive Strength Test: no bonding, or no consolidation of the coated particles, following wet compression at 1000 psi at 200° F. for a period of as much as 24 hours, which typically indicates a coating that was precured in the manufacturing process.
Unless otherwise indicated, the terms cured and curable are defined by the Compressive Strength Test.
Proppants are generally used to increase production of oil and/or gas by providing a conductive channel in the formation. Fracturing of the subterranean formation is conducted to increase oil and/or gas production. Fracturing is caused by injecting a viscous fracturing fluid or a foam at a high pressure (hereinafter injection pressure) into the well to create a fracture. A similar effect can be achieved by pumping a thin fluid (water containing a low concentration of polymer) at a high injection rate. As the fracture is formed, a particulate material, referred to as a “propping agent” or “proppant” is placed in the formation to maintain the fracture in a propped condition when the injection pressure is released. As the fracture forms, the proppants are carried into the fracture by suspending them in additional fluid or foam to fill the fracture with a slurry of proppant in the fluid or foam. Upon release of the pressure, the proppants form a pack that serves to hold open the fractures. The propped fracture thus provides a highly conductive channel in the formation. The degree of stimulation afforded by the hydraulic fracture treatment is largely dependent upon formation parameters, the fracture's permeability, the propped fracture length, propped fracture height and the fracture's propped width.
Gravel packing treatments are used to reduce the migration of unconsolidated formation sands/fines into the well bore. In gravel packing operations, the coated and/or uncoated particles suspended in a carrier fluid are pumped into a well bore in which the gravel pack is to be placed. The carrier fluid leaks off into the subterranean zone and/or is returned to the surface while the particles are left in the annulus between the production string and the casing or outside the casing in the subterranean zone adjacent to the wellbore.
Gravel pack operations generally involve placing a gravel pack screen in the well bore and packing the surrounding annulus between the screen and the well bore with the particles. The gravel pack screen is generally a type of filter assembly used to support and retain the particles placed during the gravel pack operation. A wide range of sizes and screen configurations are available to suit the characteristics of a particular well bore, the production fluid, and the subterranean formation sands. Such gravel packs may be used to stabilize the formation while causing minimal impairment to well productivity. The gravel pack acts as a filter to separate formation sands from produced fluids while permitting the produced oil and/or gas to flow into the well bore. The particles act to prevent formation sands from plugging the screen or migrating with the produced fluids, and the screen acts to prevent fines from being produced to the surface and out of the well.
Gravel packing may also be used to protect the well borewall production integrity by employing a tightly packed deposit of aggregate comprising sand, gravel or both between the borewall and the production pipe thereby avoiding the time and expense of setting a steel casing from the surface to the production zone which may be many thousands of feet below the surface. The gravel packing is inherently permeable to the desired hydrocarbon fluid and provides structural reinforcement to the borewall against an interior collapse or flow degradation. Such well completion systems are called “open hole” completions. The apparatus and process by which a packed deposit of gravel is placed between the borehole wall and the production pipe is encompassed within the definition of an “open hole gravel pack system.” Unfortunately, prior art open hole gravel pack systems. for placing and packing gravel along a hydrocarbon production zone, have been attended by a considerable risk of precipitating a borehole wall collapse due to fluctuations in the borehole pressure along the production zone. These pressure fluctuations are generated by surface manipulations of the downhole tools in direct fluid circulation within the well and completion string. Further discussion of gravel packs is presented by U.S. Pat. No. 6,382,319 incorporated herein by reference.
In some situations the processes of hydraulic fracturing and gravel packing are combined into a single treatment to provide stimulated production and an annular gravel pack to reduce formation sand production. Such treatments are often referred to as “frac pack” operations. In some cases, the treatments are completed with a gravel pack screen assembly in place, and the hydraulic fracturing treatment being pumped through the annular space between the casing and screen. In such a situation, the hydraulic fracturing treatment usually ends in a screen out condition creating an annular gravel pack between the screen and casing. This allows both the hydraulic fracturing treatment and gravel pack to be placed in a single operation.
Moreover, sand control is another consideration when extracting hydrocarbons such as natural gas and crude oil from the earth's subsurface formations, from boreholes drilled into hydrocarbon bearing production zones. Production of oil, gas and water from unconsolidated or weakly consolidated formations is normally accompanied by the production of formation sand particles along with the produced fluids. The production of sand with the well fluids poses serious problems such as the erosion of sub-surface and surface production facilities and the accumulation of the sand in the wellbore and surface separators. Several methods such as gravel packing, screens and plastic consolidation have been in use for many years with varying success. However, these methods have several-technical and cost limitations. Further discussion of sand control is presented by U.S. Pat. No. 6,364,019 incorporated herein by reference in its entirety.
When the oilfield industry “fractures” hydrocarbon bearing formations, the use of proppants to retain the high surface area created by the fracture has become common practice. It is highly desirable that the proppant particles are of high performance and can be produced in highly efficient processes (are economically attractive). It is further desirable to develop coated particles that can be produced at remote sites, such as field applied at or near the wellsite.