USING BLENDED PROPPANTS

FIELD OF THE INVENTION

This invention relates to a method of treating subterranean formations and, more specifically, to fracturing treatments for subterranean formations.

BACKGROUND OF THE INVENTION

In oil and gas operations, stimulation or treatment of the subterranean formations using a fluid containing suspended particles, referred to as hydraulic or pneumatic fracturing, may be used to improve production. That is, a fluid, referred to in the art as a fracturing fluid, is pumped or injected through a well bore into a subterranean formation to be stimulated at a rate and pressure such that existing fractures are opened and/or new fractures are formed and extended into the subterranean formation. The fracturing fluid carries particles, referred to in the art as proppant particles, into the fractures. The particles are deposited in the fractures and the fracturing fluid dissipates into the subterranean formation and/or is returned to the surface. The particles function to "prop" open or prevent the fractures from closing completely whereby conductive channels remain through which produced fluids can flow to the well-bore.

More particularly, the fracturing fluid used to initiate and propagate the fracture is commonly known as the "pad". Once the fracture is initiated, viscosified fracturing fluid containing chemical agents such as breakers, and/or containing proppants are pumped into the fracture. The proppant remains in the fracture as a permeable "pack" or partial monolayer that serves to "prop" the fracture open and provide conductive channels for hydrocarbons and/or other formation fluids, such as water, to flow into the wellbore.

Improvements in fracturing techniques are required in order to improve stimulation efficiency and well productivity.

SUMMARY OF THE INVENTION

In an aspect, there is provided a method of fracturing a hydrocarbon-bearing subterranean formation comprising: introducing at least one first type of proppant into a formation and separately introducing at least one second type of proppant into the formation, whereby (i) each are introduced approximately simultaneously into the formation; and/or (ii) while one of said at least one first type of proppant or said at least one second type of proppant is being continuously introduced into the formation, the other is introduced into the formation such that mixing of said at least one first type of proppant and said at least one second type of proppant occurs, wherein said at least one first type of proppant and said at least one second type of proppant are the same or different types of proppants and, if the same type of proppants, said at least one first type of proppant and said at least one second type of proppant differ in composition or at least one physical property.

In another aspect, there is provided a system for conducting a method of fracturing a hydrocarbon-bearing subterranean formation, the system comprising: a first blending unit for at least one first type of proppant in fluid communication with a formation; and a second blending unit for at least one second type of proppant in fluid communication with the formation, wherein said at least one first type of proppant is introduced into the formation and said at least one second type of proppant is separately introduced into the formation, whereby (i) each are introduced approximately simultaneously into the formation; and/or (ii) while one of said at least one first type of proppant or said at least one second type of proppant is being continuously introduced into the formation, the other is introduced into the formation such that mixing of said at least one first type of proppant and said at least one second type of proppant occurs, wherein said at least one first type of proppant and said at least one second type of proppant are the same or different types of proppants and, if the same type of proppants, said at least one first type of proppant and said at least one second type of proppant differ in composition or at least one physical property.

In yet another aspect, the first blending unit is coupled to and in fluid communication with a first pump, the first pump in fluid communication with the formation and the second blending unit is coupled to and in fluid communication with a second pump, the second pump being in fluid communication with the formation.

In a further aspect of the method and system described herein, wherein into the formation includes into a wellbore. In another aspect of the method and system described herein, wherein into the formation includes at a wellhead of the wellbore. In a further aspect of the method and system described herein, wherein the fracturing is hydraulic.

In a further aspect of the method and system described herein, wherein said at least one first type of proppant and said at least one second type of proppant are different types. In a further aspect of the method and system described herein, wherein said at least one first type of proppant comprises one first type of proppant. In a further aspect of the method and system described herein, wherein said at least one second type of proppant comprises one second type of proppant. In a further aspect of the method and system described herein, wherein said at least one first type of proppant is a pre-blend of said at least one first type of proppant and/or said at least one second type of proppant is a pre-blend of said at least one second type of proppant.

In a further aspect of the method and system described herein, wherein said (i) each are introduced approximately simultaneously into the formation. In yet a further aspect of the method and system described herein, wherein each of said at least one first type of proppant and said at least one second type of proppant are introduced sequentially and then approximately simultaneously, or vice versa. In a further aspect of the method and system described herein, wherein either one of said at least one first type of proppant and said at least one second type of proppant are introduced into the formation and then said at least one first type of proppant and said at least one second type of proppant are introduced approximately, simultaneously, or vice-versa.

In a further aspect of the method and system described herein, wherein said (ii) while one of said at least one first type of proppant or said at least one second type of proppant is being continuously introduced into the formation, the other is introduced into the formation such that mixing of said at least one first type of proppant and said at least one second type of proppant occurs.

In a further aspect of the method and system described herein, wherein at least one other proppant is separately introduced into the fracture prior to, during and/or after introduction of said at least one first type of proppant and said at least one second type of proppant. In a further aspect of the method and system described herein, wherein said at least one other proppant is added singly or as a pre-blend. In a further aspect of the method and system described herein, wherein said at least one other proppant is added sequentially and/or approximately simultaneously with said at least one first type of proppant and said at least one second type of proppant.

In another aspect of the method and system described herein, wherein said at least one first type of proppant and/or said at least one second type of proppant is introduced at a relatively low proppant concentration in order to create a partial monolayer in the fracture. In a further aspect of the method and system described herein, wherein said at least one second type of proppant is introduced at the relatively low proppant concentration in order to create the partial monolayer in the fracture. In a further aspect of the method and system described herein, wherein the relatively low proppant concentration is in kg/m ² and is less than about 1.088(r) (SG) where r is the equivalent radius of the proppant in millimeters and SG is the specific gravity of the proppant. In a further aspect of the method and system described herein, wherein said at least one first type of proppant and/or said at least one second type of proppant is introduced into the formation at a proppant pack concentration. In a further aspect of the method and system described herein, wherein said at least one second type of proppant forms a partial monolayer and said at least one type of proppant forms a proppant pack; said at least one first type of proppant and said at least one second type of proppant forms a proppant pack; and/or said at least one second type of proppant forms clusters and said at least one first type of proppant forms a proppant pack. In a further aspect of the method and system described herein, wherein the proppant concentration of said at least one first type of proppant is about 50 to about 1000 kg/m ³ and the proppant concentration of said at least one second type of proppant is about 25 to about 50 kg/m ³ .

In another aspect of the method and system described herein, wherein said at least one first type of proppant is introduced into a formation and/or said at least one second type of proppant is introduced into the formation at pressures to either propagate/open an existing fracture or at a pressure sufficient to fracture the formation.

In a further aspect of the method and system described herein, wherein the rate of injection of said at least one second type of proppant is the same or different that the rate of injection of said at least one first type of proppant. In a further aspect of the method and system described herein, wherein the rate of injection of said at least one second type of proppant is less than the rate of injection of said at least one first type of proppant. In a further aspect of the method and system described herein, wherein the rate of injection of said at least one second type of proppant and said at least one first type of proppant is about 1 m ³ /min to about 15 m ³ /min.

In a further aspect of the method and system described herein, wherein water recovered is about 40% to about 50% of the amount of water recovered from conventionally treated wells. In a further aspect of the method and system described herein, wherein hydrocarbon recovered is about 15% to about 20% greater than conventionally treated wells.

In a further aspect of the method and system described herein, wherein said at least one first type of proppant comprises at least one regular proppant. In a further aspect of the method and system described herein, wherein said at least one regular proppant has a SG greater than about 2.45 g/cc. In a further aspect of the method and system described herein, wherein said at least one regular proppant comprises sand, ceramic, sintered bauxite and/or resin coated proppant.

In another aspect of the method and system described herein, wherein said at least one second type of proppant comprises at least one proppant having a SG less than or equal to about 2.45 g/cc. In a further aspect of the method and system described herein, wherein said at least one second type of proppant comprises at least one light-weight proppant. In a further aspect of the method and system described herein, wherein the SG of said at least one light-weight proppant is less than or equal to about 2.2 g/cc. In a further aspect of the method and system described herein, wherein the SG of said at least one lightweight proppant is less than or equal to about 2.0 g/cc. In a further aspect of the method and system described herein, wherein the SG of said at least one light-weight proppant is between about 0.9 and about 2.0. In a further aspect of the method and system described herein, wherein said at least one light-weight proppant is a deformable proppant. In a further aspect of the method and system described herein, wherein said at least one light-weight proppant is at least one polymer. In a further aspect of the method and system described herein, wherein said at least one light-weight proppant is selected from at least one of 16/30 LWP ^{I M} HT material, high density polyethylene (HDPE), polyethylene terephthalate (PET), polypropylene (PP), and styrene-divinyl-benzene copolymer.

In a further aspect of the method and system described herein, wherein the SG differential between said at least one first type of proppant and said at least one second type of proppant is at least 0.2 g/cc. In a further aspect of the method and system described herein, wherein the SG differential between said at least one first type of proppant and said at least one second type of proppant is at least 0.2 g/cc and either said at least one first type of proppant and/or said at least one second type of proppant comprises at least one lightweight proppant. In a further aspect of the method and system described herein, wherein both said at least one first type of proppant and said at least one second type of proppant comprise at least one light-weight proppant. In a further aspect of the method and system described herein, wherein said at least one first type of proppant is at least one regular proppant and said at least one second type of proppant is at least one light-weight proppant.

In a further aspect of the method and system described herein, wherein the particle size of said at least one first type of proppant is different from said at least one second type of proppant. In a further aspect of the method and system described herein, wherein the particle size of said at least one first type of proppant and said at least one second type of proppant is from about 12/20 US mesh to about 40/70 US mesh.

In a further aspect of the method and system described herein, wherein said at least one first type of proppant is introduced as a slurry comprising at least one first fracturing fluid and said at least one first type of proppant and said at least one other type of proppant is introduced as a slurry comprising at least one second fracturing fluid and said at least one second type of proppant.

In a further aspect of the method and system described herein, wherein said at least one first fracturing fluid and said at least one second fracturing fluid are the same or different. In a further aspect of the method and system described herein, wherein each of said at least one first fracturing fluid and said at least one second fracturing fluid are selected from at least one of slick water, fresh water, liquid hydrocarbons, foams, viscous gels and nitrogen or other gases.

Other features and advantages of the present invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and the specific examples while indicating embodiments of the invention are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from the detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

Certain embodiments of the present invention will now be described more fully with reference to the accompanying drawings, wherein like numerals denote like parts:

FIG. 1 is a schematic of an embodiment of a system for conducting a method for fracturing;

FIG. 2 is a schematic of another embodiment of a system for conducting a method for fracturing;

FIG. 3 is a graph showing hydrocarbon production vs. days using a method described in Example 1 ; and

FIG. 4 is a graph showing water recovery % vs. hours using the method described in

Example 1.

DETAILED DESCRIPTION

Generally, the present invention provides a method and system for propping a fracture in a subterranean formation, such as in hydraulic/pneumatic fracturing operations in the oil and gas production industry used to fracture underground reservoirs bearing oil and gas, to provide or enhance flow channels to improve the fluid conductivity of the formation to provide increased oil and gas production rates. Such a method and system, as described herein, may improve the yield of hydrocarbon recovery in comparison to water recovery.

The methods and systems described herein are applicable to a wide variety of fractures, including (but not limited to) substantially vertical, substantially horizontal, and dendritic (or branched) fractures. The systems and methods may be used for

hydraulic/pneumatic fracturing operations using equipment commonly used for conventional fracturing operations known to one skilled in the art.

Definitions

"Simultaneously" and "concurrently", as used herein, are interchangeable and are understood to mean "at the same time". "Approximately, simultaneously" or "approximately, concurrently" as used herein, are interchangeable and are understood to mean at the same time or nearly at the same time.

"Introducing" a component into the formation, as used herein, is understood to mean pumping, injecting or any suitable means known to those skilled in the art for introducing the component into the formation.

By "type" of proppant, it is meant that the proppant(s) of the same type have similar specific gravities (SG).

"Proppant concentration ^" , as used herein, refers to the amount of proppant per unit area of fracture wall (measured on one side only). In U.S. customary units, it is expressed in pounds of proppant per square foot of one wall of fracture. In SI units, it is expressed in kilograms per square meter of one wall of the fracture face.

"Conventional proppant" and "regular proppant", as used herein, are interchangeable and are understood to mean a proppant that is not a light-weight proppant and, typically, has a density greater than 2.45 g/cc.

"Conventionally" treated well is defined as a well that has been treated with a conventional proppant, such as sand.

Methods

In general, the method comprises comprises introducing at least one first type of proppant into a formation and separately introducing at least one second type of proppant into the formation, whereby (i) each are introduced approximately simultaneously into the formation; and/or (ii) while one is being continuously introduced into the formation, the other is introduced into the formation such that mixing of said at least one first type of proppant and said at least one second type of proppant occurs. In other words, the at least one first type of proppant and the at least one second type of proppant are not pre-blended together in a blending unit prior to their introduction to the formation (e.g. wellbore). For example, a slurry having one first type of proppant and fracturing fluid is introduced into the formation and a slurry having one second type of proppant and fracturing fluid is separately introduced into the formation, whereby (i) each are introduced approximately simultaneously into the formation, typically, at about the wellhead; and/or (ii) while one is being continuously introduced into the formation, typically, at about the wellhead, the other is introduced into the formation such that mixing of the proppants occurs. The at least one first type of proppant and the at least one second type of proppant may be the same or different types. Typically, they are different types. If they are the same type, they would not be identical either in composition or physical properties. For example, they could be the same composition but be different sizes and/or shapes.

However, with respect to the at least one first type of proppant, as implied, there may be more than one first type of proppant and this may exist as a pre-blend/mixture. Similarly, with respect to the at least one second type of proppant, there may be more than one second type of proppant and this may also exist as a pre-blend/mixture. The distinction being that the at least one first type of proppant and the at least one second type of proppant may not be pre-blended/mixed together in a blending unit prior to their introduction to the formation. The at least one first type of proppant typically contains at least one regular proppant. The at least one second type of proppant may be at least one light-weight proppant. The rate of injection of the at least one first type of proppant into the fracture may be the same or different from the rate of injection of the at least one second type of proppant. The at least one first type of proppant and the at least one second type of proppant may be in the form of a slurry; a combination of the proppant and at least one fracturing fluid.

In addition to the at least one first type of proppant and/or the at least one second type of proppant, other type(s) of proppant(s) may be separately introduced to the formation, which may include (i) being introduced, approximately, simultaneously with the introduction of the at least one first type of proppant and/or the at least one second type of proppant; and/or (ii) while one or more is being continuously introduced into the formation, the others are introduced into the formation such that mixing of the proppants occur. Although, in embodiments, the method includes the at least one first type of proppant and the at least one second type of proppant not being pre-blended/mixed together in a blending unit prior to their introduction to the formation, there may be additional step(s) that include the introduction of other blended proppants (e.g. pre-blended/mixed proppants of the same type or different type) which may include (i) being introduced, approximately, simultaneously with the introduction of at least one first type of proppant and/or the at least one second type of proppant; and/or (ii) while one or more is being continuously introduced into the formation, the blended proppants are introduced into the formation such that mixing of all of the proppants occur.

The introduction into the formation of the proppants are done at pressures to either propagate/open an existing fracture or into a formation at a pressure sufficient to fracture the formation. As an example of the method, a hydrocarbon-bearing subterranean formation may be hydraulically/pneumatically fractured by first introducing into the formation at least one fracturing fluid with or without proppant and may be introduced at a pressure sufficient to initiate a fracture. Alternatively, it may be introduced into the fracture after the fracture has been propagated/initiated. This may then be followed by fracturing the subterranean formation with a subsequent fluid with proppant, as decribed herein. The number of successive proppant stages introduced into the fracture is determined by the preferences of the operator.

In certain embodiments, the density differential between the at least one first type of proppant and the at least one second type of proppant is at least 0.2 g/cc, typically at least 0.50 g/cc, most typically at least 0.80 g/cc. If the at least one first type of proppant is at least one of the regular proppant and the at least one second type of proppant is at least one lightweight proppant, the density differential between at least one of the regular proppant and the light-weight proppant may be at least 0.2 g/cc, typically at least 0.50 g/cc, most typically at least 0.80 g/cc. Most typically, the density of the at least one second type of proppant is less than the density of the at least one first type of proppant.

In a typical embodiment, the at least one first type of proppant comprises at least one regular proppant, such as sand, ceramic, sintered bauxite or resin coated proppant and the at least one second type of proppant comprises at least one light-weight proppant having a density less than or equal to 2.45 g/cc. The density differential between the at least one first type of proppant and the at least one second type of proppant may be at least 0.2 g/cc.

In certain embodiments, the particle size of the proppant of the at least one first type of proppant may be different from the particle size of the proppant of the at least one second type of proppant. Accordingly, the particle size of the proppant of the regular proppant may be different from the particle size of the light-weight proppant.

In certain embodiments, the rate of injection of the at least one second type of proppant into the formation may be the same or different from the rate of injection of the at least one first type of proppant. Typically, the rate of injection of the at least one second type of proppant is equal to or lower than the rate of injection of the at least one first type of proppant. The rate of injection may be at a pressure and a flow rate sufficient to open a fracture (creating a new fracture or opening an existing fracture) in the formation.

Typically, the rate of injection of a proppant into the formation is from about 0.5 m ³ /min to as high as about 33 m ³ /min and any ranges therebetween. Generally, the rate of injection is no greater than about 18 m ³ /min. Ranges more typically include, for example, about 1 m ³ /min to about 15 m ³ /min.

In a further embodiment, there is provided a method of fracturing a hydrocarbon- bearing subterranean formation which comprises introducing into a formation at least one first type of proppant and separately introducing into the formation at least one second type of proppant, whereby during fracture treatment, each of the at least one first type of proppant and the at least one second type of proppant is introduced into the formation as follows: i) each are introduced approximately simultaneously into the formation;

ii) while one is being continuously introduced into the formation, the other is introduced into the formation such that mixing of said at least one first type of proppant and said at least one second type of proppant occurs;

iii) each are introduced sequentially and then approximately simultaneously, or vice versa; and/or

iv) either one of the at least one first type of proppant and the at least one second type of proppant are introduced into the formation and then the at least one first type of proppant and the at least one second type of proppant are introduced approximately, simultaneously, or vice-versa.

Other proppant(s) may be added singly or as pre-blends; sequentially and/or approximately simultaneously. For example, there is provided a method of fracturing a hydrocarbon-bearing subterranean formation which comprises introducing at least one type of proppant (A) into a formation to either propagate/open an existing fracture or into a formation at a pressure sufficient to fracture the formation; and, subsequently, introducing at least one first type of proppant (which could be the same as (A) or be different) and separately introducing into the formation at least one second type of proppant at a pressure to propagate/open an existing fracture, whereby (i) each are introduced approximately simultaneously into the formation (e.g. the wellbore); and/or (ii) while one is being continuously introduced into the wellbore, typically, at about the wellhead, the other is introduced into the wellbore such that mixing of the proppants occurs.

In another embodiment, the method comprises introducing into the formation at least one first type of proppant and separately introducing at least one second type of proppant, whereby (i) each are introduced approximately simultaneously into the formation; and/or (ii) while one is being continuously introduced into the formation, the other is introduced into the formation such that mixing of the proppants occurs. The at least one first type of proppant or the at least one second type of proppant is introduced into the formation at a relatively low proppant concentration in order to create a partial monolayer in the fracture. The partial monolayer is, typically, formed using at least one light-weight proppant. The light-weight proppant reduces embedment of proppant particles into the formation. In SI units, the Proppant Concentration in kg/m ² < 1.088(r) (SG) where r is the equivalent radius of the proppant in millimeters and SG is the specific gravity of the proppant. In U.S customary units, the Proppant Concentration in Ibm/ft ² < 5.647(r) (SG) where r is the radius of the proppant in millimeters and SG is the specific gravity of the proppant.

While not intending to be bound by any theory and with respect to certain

embodiments of the methods described herein, it is believed that, in one scenario, when (a) at least one type of proppant (e.g. regular proppant) is initially introduced to the fracture, the proppant settles in the fracture as most likely a pack (depending on the concentration). If this is then followed by (b) the introduction of at least one first type of proppant (e.g. regular proppant) into the formation and separately introducing at least one second type of proppant (e.g. light-weight proppant) into the formation, whereby (i) each are introduced approximately simultaneously into the formation; and/or (ii) while one is being continuously introduced into the formation, the other is introduced into the formation such that mixing of said at least one first type of proppant and said at least one second type of proppant occurs, as decribed herein, the mixture settles out on top of the previously settled regular proppant from (a). The way it settles may include 1 ) said at least one second type of proppant (e.g. light-weight proppant) forming a partial monolayer on top of said at least one type of proppant from (a) and/or said at least one first type of proppant from (b); 2) said at least one second type of proppant (e.g. light-weight proppant) forming a pack on top of said at least one type of proppant from (a) and/or said at least one first type of proppant from (b); 3) clusters of said at least one second type of proppant (e.g. light-weight proppant) forming on top of a pack of said at least one type of proppant from (a) and/or said at least one first type of proppant from (b), or combinations thereof. It is believed that step (a) propagates the fracture and step (b) assists in increasing the fracture width. The total volume of fluids produced from such a fracture is equal to a regular proppant pack; however, there appears to be an increase in the amount of hydrocarbons produced in comparison to other fluids produced, such as water. Therefore, the overall ratio of hydrocarbons to other fluids appears to be increased. In certain embodiments, water recovered is about 40% to about 50% of the amount of water recovered from conventionally treated wells and/or hydrocarbon recovered is about 15% to about 20% greater than conventionally treated wells.

Systems

In general, a system for conducting the method described herein comprises a first blending unit for at least one first type of proppant in fluid communication with a formation; and a second blending unit for at least one second type of proppant in fluid communication with the formation, wherein said at least one first type of proppant is introduced into the formation and said at least one second type of proppant is separately introduced into the formation, whereby (i) each are introduced approximately simultaneously into the formation; and/or (ii) while one is being continuously introduced into the formation, the other is introduced into the formation such that mixing of said at least one first type of proppant and said at least one second type of proppant occurs.

In a more specific embodiment, the system for conducting the method described herein comprises a first blending unit for at least one first type of proppant, the first blending unit coupled to and in fluid communication with a first pump, the first pump in fluid communication with a formation; and a second blending unit for at least one second type of proppant, the second blending unit coupled to and in fluid communication with a second pump, the second pump in fluid communication with the formation, wherein said at least one first type of proppant is introduced into the formation and said at least one second type of proppant is separately introduced into the formation, whereby (i) each are introduced approximately simultaneously into the formation; and/or (ii) while one is being continuously introduced into the formation, the other is introduced into the formation such that mixing of said at least one first type of proppant and said at least one second type of proppant occurs.

An embodiment of a system for conducting the method is shown schematically in Figure 1 and indicated generally by numeral 10. The system 10 comprises proppant storage tanks 12 and 14 coupled to and in fluid communication with blending units 16 and 18, respectively, via pathways 20 and 22, respectively. With respect to pathways 20 and 22, this is conducted using dump trucks to transfer the proppant to the blending units 16 and 18. Fluid tanks 24 and 26 are coupled to and in fluid communication with the blending units 16 and 18, respectively, via flow lines 28 and 30, respectively. The blending units 16 and 18 are coupled to and in fluid communication with high pressure pumps 32 and 34, respectively, via flow lines 36 and 38, respectively. The high pressure pumps 32 and 34 are in fluid communication with the wellbore (not shown) via wellhead 40 via flow lines 42 and 44, respectively. In operation, proppant(s) from the proppant storage tank 12 is transferred into the blending unit 16 via the flow line 20 and fluid from the fluid tank 24 is transferred into the blending unit 6 via flow line 28. Similarly, proppant(s) from the proppant storage tank 14 is transferred into the blending unit 18 via the flow line 22 and fluid from the fluid tank 14 is transferred into the blending unit 18 via flow line 30. The proppant(s) and fluid in blending unit 16 are mixed/blended together to form proppant slurries. The proppant(s) and fluid in blending unit 18 are mixed/blended together to form proppant slurries. The proppant slurries from each of the blending units 16 and 18 are transferred to the high pressure pumps 32 and 34, respectively, via flow lines 36 and 38, respectively. The high pressure pumps 32 and 34 transfer the proppant slurries to the wellbore (not shown) via flow lines 42 and 44, respectively. Each proppant slurry is introduced approximately simultaneously into the wellbore at about the wellhead 40, where the proppant slurries mix together at the wellhead 40 and are pumped into the wellbore together as a combined slurry; and/or while one slurry is being continuously introduced into the formation, the other slurry is introduced into the wellbore at about the wellhead 40 such that mixing of the slurries occurs at the wellhead 40.

Another embodiment of a system for conducting the method is shown schematically in Figure 2 and indicated generally by numeral 100. It functions similarly to the system 10 shown in Figure 1.

The system can be adapted to accommodate any of the methods described herein. With respect to each type of component used in the system, any type of suitable component/conventional equipment utilized in fracturing of subterranean formations may be used. For example, any suitable blending unit may be used, such as, and without being limited thereto, conventional blending units that meter and measure a single proppant type. With respect to the pathways and flow lines, any suitable means (e.g. any conventional means) that allows for fluid communication is acceptable. The pathways may involve the transfer of the proppant either mechanically, hydraulically and/or pneumatically via vehicles, such as dump trucks; flow lines; and/or conveyors (e.g. conveyor belts) etc. The flow lines described herein can be any means for suitable fluid communication.

Systems and Methods

The systems and methods described herein allow the introduction of at least two proppant types with more accurate control and measurement of each proppant into the formation. Operationally, if the different types of proppants were pre-blended together in a single blender prior to introduction at the wellhead, proppants having similar SGs may separate out and may not result in a uniform, consistent blend as the blend is transported to the wellbore. In addition, if the different types of proppants are pre-blended in, for example, the same blender, there would be little ability to alter the ratio of the proppants in the pre- blend. It is, therefore, advantageous to not pre-blend in advance. This allows, for example, pumping of at least one first type of proppant or at least one second type of proppant, as well as pumping both, separately, but approximately, simultaneously into the formation to create a blend thereof in any desired ratio. It also allows for pumping at least one first type of proppant continuously into the formation while the other is introduced into the formation such that mixing occurs.

By using the systems and methods described herein, one can perform a scour ahead of the introduction of a mixture of proppants. For example, if one blender unit contains a slurry of at least one first type of proppant and another blender unit contains a separate slurry of at least one second type of proppant, by pumping the proppants using individual blenders, rather than pre-blending both types of proppants in a single blender and pumping together, one can more easily perform a scour ahead of the introduction of the proppants using the method described herein. If both types of slurries are being introduced

approximately, simultaneously, one can stop the introduction of one slurry and continue running the other slurry to perform a scour ahead of the introduction or re-introduction of both slurries. In a specific example, if one first type of proppant is a regular proppant (e.g. sand) and the other type of proppant is a light-weight proppant and both are being separately, but approximately simultaneously, introduced to the wellbore, one can stop the introduction of light-weight proppant to the wellbore and continue running sand (as a "tail in"). This allows more flexibility in the fracture treatment schedule, and one can tailor the introduction of the light-weight proppant (e.g. vary the ratio of light-weight proppant:sand, rather than relying on it being pre-set, and invariable).

With respect to both the sytems and methods described herein, concentration ranges of the at least one first type of proppant (e.g. regular proppant) may be about 50 to about 1000 kg/m ³ for conventional proppant and any ranges therebetween. Typically, the concentration of the at least one second type of proppant (e.g. light-weight proppant) is such that a partial monolayer forms, a light-weight proppant pack or clusters of light-weight proppant. For a partial monolayer to form, the concentration may be, for example, about 25 to about 50 kg/m ³ and any ranges therebetween.

With respect to the fluid(s) used in fracturing, proppant placement is known to be a function of fluid SG versus proppant SG, as well as fluid viscosity. Typically, the proppant's SG is matched to the fluid's SG for "floating" the proppant into place, in particular, the lightweight proppant. However, small restrictions in aperture in the fracture may "strip out" the proppant from a low viscosity fluid system, since there is limited inertia in the proppant to overcome these restrictive forces. This is where a competent (higher viscosity) fluid may be desirable. With respect to the regular proppant, it is typical that the fluid viscosity not be too high so the regular proppant is able to settle.

Various particle sizes of proppants are used. In typical embodiments, the particle size of the proppant of the at least one second type of proppant (e.g. light-weight proppant) is greater than the particle size of the at least one first type of proppant (e.g. regular proppant). Regular proppant is usually 40/70, 30/50 or 20/40 US mesh. Light-weight proppant may be 16/30 US mesh. While it is desirable to use the largest light-weight proppant possible, smaller sizes may be required in deeper/higher stress environments due to limitations in generated fractured widths. Typically, the particle size of the proppant with the proppant system/method is from about 8/12 US mesh to about 100 US mesh. Most typically, the particle size of the proppant with the proppant system/method is from about 12/20 US mesh to about 40/70 US mesh.

Other proppants may be pumped into the fracture prior to/during/after introduction of the mixture, as descibed herein. The number of additional proppant steps chosen would be understood from an operational perspective.

The fracturing fluid may include any conventional fluid treatment such as crosslinked gels, linear gels, slickwaters, gelled hydrocarbons, gases and foams. The fracturing fluid may further contain a fine particulate, such as sand, for fluid loss control, etc.

In certain embodiments, the initial (first) fracturing fluid may contain a breaker.

Further typically, however, is the use of slick fluids, such as those exhibiting reduced water friction. Other proppant steps may optionally contain a breaker. The breaker can be any conventionally employed in the art to reduce the viscosity of the fracturing fluid including, but not being restricted to, thermostable polymers. Depending on the application, a breaker of predictable performance may be incorporated into the initial fracturing fluid or any of the proppant steps referred to herein.

A "spearhead" fluid may further precede the introduction of the fracturing or pad fluid. A spearhead stage is also referred to as an acid stage and is generally a mix of water with diluted acid, such as hydrochloric acid. This serves to clear debris that may be present in the wellbore providing a clear pathway for fracture fluids to access the formation.

The initial fracturing fluid, as well as any of the proppant steps referred to herein, may also contain other conventional additives common to the well service industry such as surfactants, biocides, gelling agents, cross-linking agents, curable resins, hardening agents, solvents, foaming agents, demulsifiers, buffers, clay stabilizers, acids, or mixtures thereof. The fracturing fluid may be any carrier fluid suitable for transporting proppant(s) into a subterranean formation. Such fluids include, but are not limited to, carrier fluids comprising salt water, fresh water, liquid hydrocarbons, slick water, foams and/or nitrogen or other gases.

The initial fracturing fluid may be pumped at a rate sufficient to initiate and propagate/create a fracture in the formation and to place the proppant into the fracture and form a bank. During the actual pumping the pH may be adjusted by the addition of a buffer, followed by the addition of an enzyme breaker, crosslinking agent, proppant or additional proppant and other additives, if required. After deposition, the proppant material serves to hold the fracture open, thereby enhancing the ability of fluids to migrate from the formation to the wellbore through the fracture.

Typically, viscous gels or foams are employed as the fracturing fluid in order to provide a medium that will adequately suspend and transport the solid proppant, as well as to impair loss of fracture fluid to the formation during treatment (commonly referred to as "filterability" or "fluid loss").

The proppant steps may include carrier systems that are gelled, non-gelled, or that have a reduced or lighter gelling requirement as compared to carrier fluids employed with conventional fracture treatment methods.

Regular proppants may be used, which may be any conventional proppant in the art. Such proppants include, for instance, quartz, glass, aluminum pellets, silica (sand) (such as Ottawa, Brady or Colorado Sands), synthetic organic particles such as nylon pellets, ceramics (including aluminosilicates such as "CARBOLITE," "NAPLITE" or

"ECONOPROP"), sintered bauxite, and mixtures thereof. In addition, protective and/or hardening coatings, such as resins to modify or customize the density of a selected base proppant, e.g., ground walnut hulls, etc., resin-coated sand (such as "ACME BORDEN PR 6000" or "SANTROL TEMPERED HS"), resin-coated ceramic particles and resin-coated sintered bauxite may be employed.

Typical regular proppants are sand, ceramic, sintered bauxite and resin coated proppant. Such proppants typically exhibit a high density, for instance greater than 2.5 g/cc. Typically, sand or synthetic fracture proppants are used. Such proppants are normally used in concentrations between about 50 to about 2000 kg/m ³ pounds per gallon of fracturing fluid composition and concentrations therebetween, but higher or lower concentrations can be used as required.

Light-weight proppant may be relatively lightweight or substantially neutrally buoyant materials. By "relatively lightweight" it is meant that the material has a density that is less than a regular proppant employed in hydraulic or pneumatic fracturing operations, e.g., sand or having a density similar to these materials. The light-weight proppant is defined as having a density less than or equal to about 2.5 g/cc. Generally, the density of the lightweight proppant is less than or equal to about 2.2, more typically less than or equal to about 2.0, even more typically between about 0.9 and about 2.0. Such proppants are less subject to settling and can be more easily transported to provide greater effective propped fracture length. Greater effective propped fracture length translates to improved stimulation efficiency, well productivity and, reservoir drainage.

Typically, the light-weight proppant is resistant to chemical reaction. The proppant is typically adapted to be substantially non-soluble in a formation fluid, and vice versa. The proppant may be substantially non-permeable. Examples of light-weight proppant include a material selected from 16/30 LWP™ HT material from the 3M Company, high density polyethylene (HDPE), polyethylene terephthalate (PET), polypropylene (PP), or styrene- divinyl-benzene copolymer. The light-weight proppant may have a crush resistance of more than substantially 50 MPa. More typically, the light-weight proppant has a crush resistance of more than substantially 80 MPa. In other embodiments, the light-weight proppant has an undeformed maximum cross sectional measurement of about 5.0 mm. Typically, the proppant is formed to have a pre-deformed initial shape, the initial shape comprising a disk, rice-shape, cubeoid, spheroid, or toroid (donut).

The light-weight proppant may be a deformable proppant of a unitary material or may include a core surrounded by a shell. The core may be a fluid (liquid), such as water, hydrocarbon, or other fluid known in the industry. This composite (liquid with shell) design provides a less compressible base and increases the elastic limit while allowing the shell to deform, reducing embedment into the formation. The core may be a gas. This composite (gas with shell) design provides reduced specific gravity.

By "substantially neutrally buoyant", it is meant that a material having a density sufficiently close to the density of an ungelled or weakly gelled carrier fluid (e.g., ungelled or weakly gelled completion brine, other aqueous-based fluid, or other suitable fluid) to allow pumping and satisfactory placement of the proppant using the selected carrier fluid. For example, urethane resin-coated ground walnut hulls having a specific gravity of from about 1.25 to about .35 grams/cubic centimeter may be employed as a substantially neutrally buoyant proppant in completion brine having a density of about 1.2. It will be understood that these values are exemplary only. As used herein, a "weakly gelled" carrier fluid is a carrier fluid having minimum sufficient polymer, viscosifier or friction reducer to achieve friction reduction when pumped down hole (e.g., when pumped down tubing, work string, casing, coiled tubing, drill pipe, etc.), and/or may be characterized as having a polymer or viscosifier concentration of from greater than 0 pounds of polymer per thousand gallons of base fluid to about 10 pounds of polymer per thousand gallons of base fluid, and/or as having a viscosity of from about 1 to about 10 centipoises. An ungelled carrier fluid may be characterized as containing about 0 pounds per thousand gallons of polymer per thousand gallons of base fluid. Such relatively lightweight and/or substantially neutrally buoyant materials are disclosed in U.S. Pat. No. 6,364,018, herein incorporated by reference.

Exemplary of such relatively lightweight and/or substantially neutrally buoyant fracture proppant material is a ground or crushed walnut shell material that is coated with a resin to substantially protect and water proof the shell. Such a material may have a specific gravity of from about 1.25 to about 1.35, and a bulk density of about 0.67.

Examples of other types of materials suitable for use as relatively lightweight and/or substantially neutrally buoyant proppant materials include, but are not limited to, ground or crushed shells of nuts such as walnut, pecan, almond, ivory nut, brazil nut, etc.; ground or crushed seed shells (including fruit pits) of seeds of fruits such as plum, peach, cherry, apricot, etc.; ground or crushed seed shells of other plants such as maize (e.g. corn cobs or corn kernels), etc., crushed fruit pits or processed wood materials such as those derived from woods such as oak, hickory, walnut, poplar, mahogany, etc. including such woods that have been processed by grinding, chipping, or other form of particleization. Additional information on such materials and methods for use thereof may be found in U.S. Pat. No. 6,330,916 and in U.S. Pat. No. 6,059,034, herein incorporated by reference.

Other light-weight proppants include deformable particles having a size substantially equivalent or larger than a selected regular proppant size may be employed. Such deformable particles are discussed above. For example, a deformable particulate material having a larger size than the regular proppant material may be desirable at a closure stress of about 1000 psi or less, while a deformable particulate material equal in size to the regular proppant material may be desirable at a closure stress of about 5000 psi or greater.

However, it will be understood with benefit of this disclosure that these are just optional guidelines. In one embodiment, a deformable particle is selected to be at least as big as the smallest size of regular proppant being used, and may be equivalent to the largest regular proppant grain sizes. In either case, all things being equal, it is believed that larger regular proppant and deformable particulate material is generally advantageous, but not necessary. Although deformable particulate material smaller than the regular proppant may be employed, in some cases it may tend to become wedged or lodged in the fracture pack interstitial spaces. In one embodiment, deformable particles used in the disclosed method may have a beaded shape and a size of from about 4 mesh to about 00 mesh, alternatively from about 8 mesh to about 60 mesh, alternatively from about 12 mesh to about 50 mesh, alternatively from about 16 mesh to about 40 mesh, and alternatively about 20/40 mesh. Thus, in one embodiment, deformable particles may range in size from about 1 or 2 mm to about 0.1 mm; alternatively their size will be from about 0.2 mm to about 0.8 mm, alternatively from about 0.4 mm to about 0.6 mm, and alternatively about 0.6 mm. However, sizes greater than about 2 mm and less than about 0.1 mm are possible as well.

Basically, the light-weight proppant may have any particle size or particle shape suitable for use in the methods disclosed herein. Typically, the particle sizes of the proppants employed range from about 4 mesh to about 100 mesh, alternatively from about 8 mesh to about 60 mesh, alternatively from about 12 mesh to about 50 mesh, alternatively from about 16 mesh to about 40 mesh, and alternatively about 20 to 40 mesh.

Proppant sizes may be any size suitable for use in a fracturing treatment of a subterranean formation. It is believed that the optimal size of the proppant material may be dependent, from among other things, on the size of the fracture, and on in situ closure stress.

Deformable particles having any density suitable for fracturing a subterranean formation may be employed in the practice of the disclosed method. In another specific embodiment, a particular divinylbenzene crosslinked polystyrene particle may have a bulk density of from about 0.4 to about 0.65, and alternatively of about 0.6. In another specific exemplary embodiment, a particular divinylbenzene crosslinked polystyrene particle may have a specific gravity of about .055. However, other specific gravities are possible.

Advantageously, in one embodiment when deformable particles having a density less than that of a selected regular proppant material are employed, reduced treating pressures and concentration levels of potentially formation-damaging gelled or viscous fluids may be employed. This may allow higher treating rates and/or result in higher formation productivity.

Other light-weight proppants are described in International Patent Application No. WO 2013/036350, which is incorporated by reference.

Those of skill in the art will understand that selection of suitable proppants will depend, in part, on the density of the fracturing fluid and on whether it is desired that the selected proppant particle be relatively lightweight or substantially neutrally buoyant in the selected fluid, and/or whether or not it is desired that the fluid be non-gelled or non- viscosified.

When introducing elements disclosed herein, the articles "a", "an", "the", and "the" are intended to mean that there are one or more of the elements.

The above disclosure generally describes the present invention. A more complete understanding can be obtained by reference to the following specific example. This example is described solely for purposes of illustration and is not intended to limit the scope of the invention. Changes in form and substitution of equivalents are contemplated as circumstances may suggest or render expedient. Although specific terms have been employed herein, such terms are intended in a descriptive sense and not for purposes of limitation. Example 1

Using the system 10 shown in Figure 2, the proppant storage tanks 12 contained conventional 30/50 US mesh sand and the fluid tanks 24 contained slick water. The sand and slick water were transferred to the blender unit 16 via the flow lines 20 and 28. The resultant sand and slick water slurry was then pumped (through the high pressure pumps 32 via the flow lines 36) through the flow lines 42 to the wellhead 40, following a conventional treatment schedule. The proppant storage tank 14 contained light-weight proppant (SG is 1.05 g/cc, 16/30 LWP™ HT material from the 3M Company) and the fluid tanks 26 contained slick water. The slick water was pumped from fluid tanks 26 to the blender unit 18 via the flow lines 30 (through the high pressure pumps 34 via the flow lines 38) through the flow lines 44 to the wellhead 40, which was then followed by the light-weight proppant being pumped from the proppant storage tank 14 to the blender unit 18 via the flow line 22 to form a slurry with the slick water in the blender unit 18. The slurry of light-weight proppant and slick water was then pumped (through the high pressure pumps 34 via the flow lines 38) through the flow lines 44 to the wellhead 40 at about 50 kg/m ³ . Each proppant slurry was introduced, approximately, simultaneously into the wellbore at about the wellhead 40, where the proppant slurries mix together at the wellhead 40 and are pumped into the formation as a combined slurry.

The conventional sand was introduced at a concentration from about 50 to about 300 kg/m ³ , while the light-weight proppant was injected at about 50 kg/m ³ .

All concentrations are referenced at the wellhead, with the fully mixed slurries. For example, to obtain 100 kg/m ³ at the wellhead 40, and as long as each blender unit 16 and 18 is pumping half of the total flow rate, the blender unit 16 injected the slurry of sand at a concentration of about 200 kg/m ³ , which would then be diluted by the slurry of light-weight proppant injected from the blender unit 18 at the wellhead 40, and the resulting slurry mixture would then have a concentration of 100 kg/m ³ of the conventional sand.

Comparison data is shown in Figures 3 and 4 for wells treated conventionally (e.g. with sand alone) and a well treated using the method as described in this Example 1 with LWP™. Figure 3 shows hydrocarbon recovered is about 18% greater than the average of three conventionally treated wells. Figure 4 shows that water recovered from treated wells using the method described herein is about 50% of the amount of water recovered from conventionally treated wells.

The foregoing disclosure and description of the invention is illustrative and explanatory thereof, and various changes in the size, shape, and materials, as well as in the details of illustrative construction and assembly, may be made without departing from the spirit of the invention.