WO2011044612A1

WO2011044612A1 - Proppants

Info

Publication number: WO2011044612A1
Application number: PCT/AU2010/001322
Authority: WO
Inventors: Charles Henry Rawlins
Original assignee: Eprocess Technologies Pty Ltd
Priority date: 2009-10-15
Filing date: 2010-10-06
Publication date: 2011-04-21

Abstract

A proppant formed from ceramic materials containing non-reactive substrate composed of at least one oxide selected from the group consisting of metal oxides, non-metal oxides and transition metal oxides in which the transition metal is in its highest oxidation state and reactive material composed of at least one oxide selected from the group consisting of alkali metal oxides, alkaline earth metal oxides and transition metal oxides in which the transition metal is not in its highest oxidation state, wherein the proportions of non-reactive substrate and reactive material lie between 0:100 and 99:1 on a mass:mass basis.

Description

PROPPANTS

Field of the Invention This invention relates to proppants, and in particular to reactiVe proppants capable of interacting with formation or activation fluids to be thereby activated to modify fracture flow paths in oil and gas reservoirs to, inter alia, enhance oil and gas production. Background of the Invention

Proppants are sized particles carried with fracturing fluid to hold fractures open after hydraulic fracturing treatment in oil reservoirs. In addition to naturally occurring same sand grains, man-made or especially engineered proppants, such as resin-coated sand or high-strength ceramic materials such as sintered bauxite, may also be used.

Oil reservoirs are formed by deposits of oils or gases, or mixtures of oil and gasses, of solid residues and water, enclosed in formations of rock, sand or clay. The reservoirs may be at different levels of depth and once a reservoir has been drilled and a formation has been reached, depending upon the permeability of the environment in the formation, the extraction of oil or gas begins. The oil or gas comes out of the formation, either naturally or as a result of the use of the application of external pressures, due to the permeability created by natural or artificial fractures in the formation until it reaches the surface, usually by means of metallic tubing or wellbore. Depending on the permeability, depth and the pressures on the formation, the oil or gas may not come out of the reservoir. On the other hand, even in high permeability reservoirs, from which the oil or gas comes out relatively easily, the continuous passage of the gas or oil through pores or natural fractures in the formation drags solid residues and impurities with it which gradually block the fractures or pores and consequently block the oil or gas flow.

Hydraulic fracturing is a technique developed to stimulate the oil reservoirs to allow oil and/or gas to flow properly. It consists of creating formation fractures by injecting fracturing fluids into the well bore at a rate sufficient to increase the pressure down hole to a value in excess of the fracture gradient of the formation. The pressure causes the formation rock to crack which allows the fracturing fluid to enter cracks and extend the cracks further into the formation. Once the fractures are formed, solid proppants are added to further injections of the fracturing fluid in order to keep the fracture open. The proppants, commonly a sieved rounded sand article, are introduced into the fractures. When the fracturing pressures are removed the particles present in the fractures prevent the fractures from closing. The fractures obtained and kept open with the particles provide a permeable means through which the oil and/or gas can be extracted from the reservoir. Proppant materials are carefully sorted for size and shape to provide an efficient conduit for production of fluid from the reservoir to the welled bore. The oil and gas industries spends billions of dollars on technology to increase the amount of hydrocarbons, oils and gasses, it can extract from in situ reservoirs. Currently, oil and gas companies typically recover only about one in three barrels of oil from their fields. As it is becoming increasingly difficult to access new reserves, it has become apparently clear that the oil and gas companies cannot afford to leave so much crude untapped and/or gas. The recovering of more oils from oil reservoirs has enormous implications not only for any oil and gas companies financial health but for the worlds emission supply. It is an object of the invention to provide improved proppants for use in hydraulic fracturing and the recovery of oil and gas from reservoirs.

Summary of the Invention

According to one aspect of the invention there is provided a class of reactive proppants that can be tailored to interact with formation or activation fluids to modify the fracture flow path of an oil and/or gas reservoir, thereby enhancing oil and/or gas production. The proppant is preferably made from ceramic or composite materials and is designed to hard-expand, soft-expand, partially dissolve, or fully dissolve, on activation by formation fluids or activation fluids. Upon activation, the action of the proppant is to modify the fracture flow path for a specific response. This may include increasing or reducing overall fluid flow rate, isolation of a high water producing fracture zone, or release of a specific chemical or device, such as, a nano-scale robot for data gathering.

Description of the Invention According to the invention there are provided ceramic or composite, i.e. ceramic/polymer, proppants that can achieve hard-expansion, soft- expansion, full-dissolution or partial dissolution when activated, either by response to reservoir fluids or by response to injection of a specific chemical. The effect is that the proppant becomes more than a static place holder and becomes an active participant in the production of hydrocarbon fluids.

The proppant may contain a mixture of reactive materials and inert materials. Preferably the active materials are derived from alkaline or alkaline- earth metal oxide minerals, including lithium oxide, potassium oxide, sodium oxide, magnesium oxide, calcium oxide, and barium oxide. Transition metal oxides can also be used.

The chemical composition of the proppant is a significant factor. The proppant is primarily composed of ceramic material. The ceramic material may contain a non-reactive substrate for strength primarily composed of a metal oxide (eg. A1₂0₃), or non-metal oxide (eg. Si0₂), or a transition metal oxide (eg. Ti0₂) in its highest oxidation state, or a combination of these. The ceramic material also contains reactive material primarily made of at least one alkali metal oxide (eg. Na₂0, K₂0, etc.), or at least one alkaline earth metal oxide (eg. MgO, CaO, etc.), or at least one transition metal oxide (eg. FeO, MnO, etc.) that is not in its highest oxidation state, or a combination of these. The proportions of non-reactive substrate and reactive material preferably lie between 0:100 and 99:1 on a mass:mass basis depending on what end result is desired.

Other constituents may be added to or impregnated in the ceramic material in order to facilitate specific reactions. These may include an organic catalyst, such as carbonic anhydrase, or an inorganic catalyst such as carbon, zeolite, or a non-precious transition metal. Additional constituents may include a chelating agent, a sequestering agent, or an ion exchange agent. These chemicals may be placed directly onto the ceramic material, or onto a suitable substrate such as activated carbon or chitosan, that is itself placed onto the ceramic material. The proppant is manufactured using known ceramic manufacturing and chemical processes. Powders of ceramic material plus temperature resistant chemicals are mixed in the desired proportions, optionally with a binder, and then pressed or cast into the desired shape, after which the shapes are sintered in a furnace to produce desired final mechanical and physical properties.

Chemicals that cannot resist the sintering temperature, eg. organic catalysts, are impregnated into the ceramic proppant after sintering. This can be effected through aqueous soaking or isostatic pressing.

The final proppant size is consistent with that used in the oil and gas industry, being from 4 U.S. mesh (4.75 mm) to 35 U.S. mesh (0.500 mm) diameter.

The proppant shapes preferably are consistent with those used in the oil and gas industry which is substantially semi-spherical. A sphericity of at least 0.9 is preferred. Additionally, non-traditional shapes, such as solid toroid or oblate spheroid, can be formed to meet specific flow modification requirements.

Activation of the proppants by chemical reaction may occur through contact with one or more reservoir fluids, eg. hydrocarbon fluid, fresh water, or saline water, or one or more injected reaction fluids, eg. fresh water, saline water, carbon dioxide, hydrogen peroxide, aqueous soluble acids, or aqueous soluble bases. Responses of proppants to activation include:

(a) hard-expansion (to widen fracture flow path);

(b) soft-expansion (to isolate water producing zone);

(c) full dissolution (to shut-off water producing zone or create a partial monolayer); and

(d) partial dissolution (to release tracer chemicals or micro- mechanical devices).

Isotropic proppant expansion without deformation is referred to as hard-expansion. In this response, the diameter (and volume) of the proppant granule grows uniformly with the proppant maintaining the same shape.

Proppant granule expansion pushes against the walls of the fracture. If sufficient expansion force is generated, by the sum of the proppant granules, then the fracture will expand. As the fracture expands, flow path resistance is reduced resulting in increased fluid flow from the reservoir to the wellbore, yielding a net increase in production.

Soft-expansion takes place anisotropically, where the volume of the proppant granule increases but the morphology is not maintained. Proppant expansion has insufficient force to overcome the rock fracture stress, thus will only expand into open areas of the fracture. The net result is reduction in permeability in the fracture path, with associated lowering of fluid flow from the reservoir to the wellbore. Proppant with a design adapted to soft-expand can be placed in zones where water breakthrough is probable. When water breakthrough occurs the proppant will expand to reduce flow from or isolate that specific zone to reduce net water production. The proppant can be designed to provide a reduction in flow (not total flow shut-off) from a particular zone, thus the drawdown pressure will pull less fluid from that zone giving time for oil to migrate to the formation area around that zone and drawing in less water. The idea is not to shut off the zone completely, but have a lower net fluid flux from the potential water producing areas and produce only hydrocarbon fluid. Expansion of the proppant can be activated by water from the formation, or from an injected chemical. The fracture maintains its original shape and width. Thus re-fracturing would start from that baseline.

Full dissolution response is a straightforward reaction in which the proppant in place completely dissolves. The primary use for full dissolution proppant is in development of a partial monolayer in the frac path. A combination of inert and dissolvable proppant is injected into the fracture zone, the injection pressure is removed allowing the fracture to close, and then the reactive proppant is dissolved. The net result is a more open network of inert proppant material giving higher permeability in the frac path. Another use is similar to soft-expansion proppant where all the proppant is dissolved allowing the fracture to close. In this way, closure of the fracture may offer complete isolation of a particular zone in order to reduce water production. Fracture closure may allow relaxation of the formation rock back into its original state, thus future re-fracturing may require similar pressures as the original frac job.

Partial dissolution response is designed to customize the proppant for a specific response. Dissolution can take place uniformly, where the proppant itself becomes permeable over time, or variably where regions of the proppant granule dissolve at different rates or in different amounts. Partial dissolution response may result in parts of the granule dissolving at specific times to release a tracer chemical, or a section of the granule dissolving to release a micro-mechanical monitoring device similar to Saudi Aramco's Nano Resbot.

Ceramic or composite (ceramic/polymer) proppants according to the invention are intended to achieve one or more of the foregoing responses discussed and can be formulated to do so. For instance, proppants containing no non-reactive substrate can be used to achieve a full dissolution response. These responses can be activated either automatically by exposure to the reservoir fluids, or by injection of a specific chemical which acts as an activating agent. In this manner proppant becomes more than a static placeholder, and becomes an active participant in the production of hydrocarbon fluids.

The theoretical basis for the invention is as follows.

Fracture proppant is commonly made from natural materials (i.e. river sand) or artificial materials (i.e. sintered ceramics). Both the natural and artificial materials are oxides of transition metals (i.e. Fe or Ti) or metalloids (i.e. Si or Al). River sand is primarily Si0₂, while the artificial ceramics are formed from mixtures of oxides, such as Si0₂, A1₂0₃, Ti0₂, Fe₂0₃, etc, with alumino-silicates dominating the composition. These oxides are chemically stable in aqueous, hydrocarbon, and carbon dioxide environments, form inter- phase solid solutions, have favourable mechanical properties such as high hardness, high crush strength, and medium density, and are relatively inexpensive. The favourable factors of these oxide phases has led to their use as fracture proppants. As these oxides are only reactive to hydrofluoric acid or concentrated caustic solutions (i.e. pH>13), they are considered chemically stable under ambient and reservoir conditions.

Solid-state reactions involving alkali metal, or alkaline-earth metal oxide mineral phases form the basis for investigating reactive proppant.

Common alkali metal oxide mineral phases are Li₂0, K₂0, and Na₂0, while common alkaline-earth metal oxide mineral phases are MgO, CaO, and BaO. All of these oxide phases react with water to form hydroxides and carbon dioxide to form carbonates. Table 1 illustrates, inter alia, the stepwise reaction of an alkali metal oxide and an alkaline-earth metal oxide from the base oxide through hydroxide to carbonate. All elements in the same chemical group will show the same type of reaction, and K₂0 and CaO are identified by way of example only.

Table 1. Hydroxide and Carbonate Reactions with Metal Oxides

Some transition metals are multivalent which results in multiple oxidation states. The lower oxide form (i.e. FeO vs. Fe₂0₃, MnO vs. Mn0₂, Cu₂0 vs. CuO, etc) can react with carbon dioxide to form a carbonate phase as shown in Table 1. The formation of a transition metal oxide carbonate is also associated with the release of heat and increase in molar volume (i.e. solid state expansion). The difference between alkali metal or alkaline-earth metal oxide carbonates and transition metal oxide carbonates is that the former are at the lowest energy state. For example, calcium carbonate (CaC0₃) is the most stable (i.e. lowest energy state) form of this oxide, however iron carbonate (FeC0₃) can oxidize to a lower energy state in the form of Fe₂0₃.

The reactions in Table 1 are characterized as:

• Type 1 : "Dry Carbonation" in which the carbonate is formed directly from the oxide

Type 2: "Hydroxide" in which the hydroxide is formed from the oxide

Type 3: "Wet Carbonation" in which the hydroxide converts to a carbonate.

Type 2 reactions are incomplete in that exposure of the hydroxide to C0₂ will lead to further reaction. Type 1 reactions are, in effect, a summation of Type 2 and Type 3. However, the Type 1 reaction is a gas-solid reaction which is mass transport limited compared to a liquid-solid reaction therefore the rate of conversion of an oxide to a carbonate will be enhanced using an aqueous intermediary (Type 2 + Type 3). The conversion from base oxide to hydroxide or carbonate is characterized by release of heat and increase in molar volume. The change in volume as water or carbon dioxide is reacted leads to solid state expansion of the oxide. Table 2 lists several of the common alkali metal, alkaline-earth metal, and transition metal oxides, and the energy and volume change upon their conversion to the hydroxide and carbonate states.

Table 2. Thermodynamic Properties of Oxide-Hydroxide-Carbonate

Conversion

All of the reactions have a negative enthalpy change (ΔΗ°_Γ ), indicating the reactions are spontaneous (e.g. self-driven) and will proceed based on thermodynamics. Reactions involving the alkali metal oxide (e.g. K₂0 and Na₂0) with carbon dioxide are the strongest as shown by the magnitude of the enthalpy change, while those of the transition metal oxides are the weakest. The enthalpy change of a Type 1 reaction equals the sum of the respective Type 2 and 3 reactions. The increase in volume (Δν) is calculated from the molar volume values. The largest volume increase, based on carbon dioxide reaction, comes from alkaline-earth metal oxides and transition metal oxides. Specific volume increases can be achieved by mixing two or more materials in the proper proportion. A second type of reaction, aside from volume increase, that may occur between proppant material and reservoir fluids is dissolution. The oxide, hydroxide, and carbonate forms of each metal have different aqueous dissolution rates. Table 3 shows the relative aqueous dissolution rates for many of the common materials.

Table 3. Aqueous Dissolution Rates of Ceramic Metal Oxides at pH=7

At neutral pH the alkali metal oxides are very soluble, the alkaline- earth metal oxides are slightly soluble, and the transition metal oxides are insoluble. This trend is similar for the hydroxide forms, but conversion to carbonate makes most of these oxides insoluble. The carbonate forms are confirmed to be insoluble in hydrocarbon fluids, and the oxide and hydroxide forms should be following dissolution chemistry. The differential dissolution amounts between the various metal oxides and forms, allows the design of a specific dissolution rate to be achieved by mixing two or more oxides in appropriate proportions. The data presented in Table 2 on volume increase (e.g. expansion) and Table 3 on solubility are based on thermodynamic evaluation. However, the extent and rate of expansion are also based on kinetics, thus the physical design of proppant granules, in addition to the chemical composition, is important to achieve the desired properties. Variables such as density, total porosity, pore size and structure, particle permeability, surface area, and activation fluid concentration all can be adjusted to achieve a specific response. Additionally, activation chemicals such as liquid-based enzymes or solid-based catalysts can be added to the chemical composition to control the reaction rate.

Significant features of the invention are believed to be:

• The creation of solid-state ceramic based proppant using ceramic material that is reactive with water, carbon dioxide, or other chemicals, in addition to providing mechanical proppant action;

• The addition of catalysts, chelating, sequestering, or ion exchange agents into the proppant to elicit a specific chemical response; · Activation of a chemical response in the proppant based on interaction with reservoir or artificially introduced fluids;

• Tailored response mechanism of the proppant including expansion or dissolution responses;

• Expansion responses including hard-expansion to widen fracture paths or soft-expansion to fill fracture paths;

• Dissolution response including full-dissolution to allow fracture path to close or create a partial monolayer, or partial dissolution to release a production stimulation or modification chemical or to release a nano- scale device such as a monitoring robot or tracer chemical;

• Enhanced hydrocarbon production achieved by increased overall fluid flow rate (i.e. hard expansion to widen fracture or full dissolution of some proppant to create a partial monolayer), reduction in water production (i.e. partial expansion or full dissolution to isolate water breakthrough zone), improved fluid properties (i.e. partial dissolution to release surfactant or stimulation chemical), or from improved formation/reservoir knowledge (i.e. partial dissolution to release nano-scale device to gather data); and

• Non-spherical designs of the proppant including solid toroids or oblate spheroids. The reference to any prior art in this specification is not, and should not, be taken as an acknowledgment or any form or suggestion that the prior art forms part of the common general knowledge in Australia.

While the foregoing describes the concept and certain applications of the invention, it is to be understood that variations can be made without departing from the spirit and scope of the disclosure.

Claims

1. Proppants formed from non-reactive substrate primarily composed of at least one metal oxide, or at least one non-metal oxide, or at least one transition metal oxide wherein the transition metal is in its highest oxidation state or a combination of these, in admixture with a reactive material composed primarily of at least one alkaline metal oxide, or at least one alkaline earth metal oxide, or at least one transition metal oxide wherein the transition metal is not in its highest oxidation state, or a combination of these.

2. A proppant as claimed in claim 1 wherein the proportions of non-reactive substrate and reactive material lie between 1:99 and 99:1 on a mass:mass basis.

3. Proppants formed from a reactive material composed primarily of at least one alkaline metal oxide, or at least one alkaline earth metal oxide, or at least one transition metal oxide wherein the transition metal is not in its highest oxidation state, or a combination of these.

4. A proppant as claimed in any one of claims 1 to 3 wherein is added a catalyst being selected from organic catalysts or inorganic catalysts.

5. A proppant as claimed in any one of claims 1 to 4 having a shape which is either substantially semi-spherical, solid toroidal or oblate spheroidal.

6. A proppant formed from ceramic materials containing non- reactive substrate composed of at least one oxide selected from the group consisting of metal oxides, non-metal oxides and transition metal oxides in which the transition metal is in its highest oxidation state and reactive material composed of at least one oxide selected from the group consisting of alkali metal oxides, alkaline earth metal oxides and transition metal oxides in which the transition metal is not in its highest oxidation state, wherein the proportions of non-reactive substrate and reactive material lie between 0:100 and 99:1 on a mass:mass basis.

7. A proppant as claimed in claim 6 wherein the substrate also contains polymers.

8. A proppant as claimed in claim 6 wherein an organic catalyst or an inorganic catalyst is added thereto or impregnated therein.

9. A proppant as claimed in claim 6 wherein at least one agent selected from the group consisting of chelating agents, sequestering agents or in exchange agents is added thereto.

10. A proppant as claimed in claim 10 wherein the said at least agent is added to an activated carbon or chitosan prior to application of the resulting combination to the proppant.

11. A proppant as claimed in claim 6 wherein said proppant is in the form of particles which are substantially semi-spherical in shape.

12. A proppant as claimed in claim 6 wherein said proppant is in the form of particles which are substantially toroidal or oblate spheroidal in shape.

13. A proppant as claimed in claim 6 wherein said proppant is in the form of particles being from 4.75 mm to 0.500 mm diameter.