GB2520018A

GB2520018A - Porous Proppants

Info

Publication number: GB2520018A
Application number: GB1319557.3A
Authority: GB
Inventors: Erling Rytter
Original assignee: Statoil Petroleum ASA
Current assignee: Equinor Energy AS
Priority date: 2013-11-06
Filing date: 2013-11-06
Publication date: 2015-05-13
Also published as: WO2015067555A2; WO2015067555A3; GB201319557D0

Abstract

A proppant particle for use in a hydrocarbon production operation is disclosed. The proppant particle has porous structure, the porous structure being formed of any of a plurality of carbon nanofibres or tubes synthesized on the surface of a core, and an alumina spinel based porous particle. The use of either of these types of material for a porous structure gives a lightweight proppant particle with adequate strength which means that less water and/or chemicals are required to disperse the proppant particles during an operation such as fracking. Also shown is a method of forming a proppant particle and a method of performing a fracking operation using the proppant.

Description

Porous proppants

TECHNICAL FIELD

The present invention relates to the field of proppants.

BACKGROUND

In order to improve the efficiency of extracting hydrocarbons from subterranean formations, it is known to inducing and/or extend existing fractures and cracks in the subterranean formation. Fractures may extend many meters and tens or even hundreds of meters from a main wellbore from which they originate.

As hydrocarbon-bearing formations are often disposed substantially horizontally, in many cases it is preferred to use horizontal drilling and fracking operations (inducing fractures in the formation) may be carried out on a single well. This may be accomplished by, for example, retracting open slots in a liner along the borehole. A common method to induce fractures is by hydraulic fracturing. In this case, a fluid is pumped into the formation via the wellbore at high pressures. The pressure can be up to around 600 bar, or in some cases even higher. The first fractures may be created by the use of explosive materials, and these are extended by the high pressure fluid. The most commonly used tracking fluid is water with added chemicals and solid particles.

Typically the solids, termed proppants, make up 5-15 volume % of the fracking fluid, chemicals make up 1-2 volume % and the remainder is water. The use of proppants is illustrated in Figure 1, in which an injection well 1 for performing a fracking operation is provided. After the fracking operation, fractures 2 appear in the formation. Proppant particles 3 (the fractures and the proppants are not shown to scale in Figure 1 as they relatively are much smaller than the wellbore) remain in the fracture 2 and help to hold the fracture 2 open when there is no more pressure from the fracking fluid.

Other fracking fluids besides water include freshwater, saltwater, nitrogen, CO2 and various types of hydrocarbons, e.g. alkanes such as propane or liquid petroleum gas (LPG), natural gas and diesel. The fracking fluid may also include substances such as hydrogen peroxide, propellants (typically monopropellants), acids, bases, surfactants, alcohols and the like.

Considering the case where the tracking fluid is LPG, in order for LPG gas to be suitable for use in tracking of wells, it is necessary to form it into a gel so that, among other properties, it may transport proppants. A gel consistency is required to maintain a suitable proppant dispersion. An advantage of this technology is the simplicity in disposal of the fracking fluid. After the tracking operation, the LPG reverts from a gel to a gas and escapes the borehole during decompression, leaving proppants in the fractures in order to hold the fractures open and prevent them from closing.

Furthermore, during the change from (gel-like) liquid to gas form, the LPG volume increases greatly, thereby increasing the pressure in the formation and further extending fractures. It is thought that recovered LPG gas is suitable for reuse.

Compared to many other methods of hydraulic fracking, the method based on LPG does not leave chemical substances in the soil, and also reduces the effect of reflux.

The chemicals added may comprise viscosifier agents and/or cross-linked polymers, often from natural vegetation like cellulose, that enhance the fracking fluid's ability to transport proppants into the reservoir and the fractures. Some chemicals also reduce the friction between the tracking fluid being pumped and the well conduits. Examples of suitable gelling agents are hydroxypropyl guars (of ionic or non-ionic type) and polyacryl imides. The fracking fluid may also be an emulsion created by mixing water with a liquid hydrocarbon. Another fracking fluid option is to form a foam, resulting from aeration of gels containing 70-80% of gas. After a fracking operation, the tracking fluid is returned, at least in part, back to the surface for reuse or disposal. This operation creates issues with handling the added chemicals and also with handling large amounts of water (where the fracking fluid is water-based). After the fracturing operation the fracking fluid normally includes bacteria and hydrogen sulphide, which need to be safely handled. Once the fracking fluids have been removed, many proppant particles remain in the fractures in the subterranean formation.

The cost of the proppant may be up to 10 % of the drilling costs. A single well may require 1,600 tons of proppant. As described above, the function of the proppant is to assist in keeping the fractures open after fracturing when the pressure from the tracking fluid is removed. Commonly used proppants are sand particles consisting mainly of silica or quartz, or ceramic particles, e.g. of titania or a-alumina made by heating loam, clay, kaolin or bauxite, the latter to temperatures above 1100°C.

Froppants may be coated particles, in which case the particles contain a thin outer layer of a polymer resin that help in reducing the drag forces during production and to make the surface hydrophobic to prevent blocking by adsorbed water. This prevents agglomeration of the proppant particles and improves dispersion in the tracking fluid.

Further, resin coated proppants may reduce proppant flow-back, lower the propensity for cracking the proppant and blocking by fines, and improve stress resistance.

Note that hydraulic fracturing is not the only means to stimulate hydrocarbon production in a subterranean reservoir. Other techniques include acid stimulation to dissolve part of the formation rock (typically carbonates like nahcolite), and steam injection in the steam assisted gravity drainage (SAGD) technique.

Hydrocarbons that can benetit from heat treatment are typically low viscosity or low mobility hydrocarbons such as bitumen, e.g. in oil sands, heavy oil, extra heavy oil, tight oil, kerogen and coal. Oils are often classified by their API gravity, and a gravity below 22.3 degrees is regarded as heavy, and below 10.00 API as extra heavy.

Bitumen is typically around 8° API.

Shale reservoirs are hydrocarbon reservoirs formed in a shale formation, often denoted as shale oil, shale gas or oil shale. It can be difficult to extract the hydrocarbons from shale reservoirs because the shale formation is of low porosity and low permeability, and so fluid hydrocarbons may not be able to tind a path through the tormation towards a production well. This means that when a well is drilled into the formation, only those fluid hydrocarbons in proximity to the well are produced, as the other hydrocarbons further away trom the well have no easy path to the well through the relatively impermeable rock formation. In order to improve hydrocarbon recovery from shale formations, the shale around the well is often hydraulically fractured. This involves propagating fractures through the shale tormation using a pressurized fluid to create conduits in the impermeable shale formation. Hydrocarbon fluids can then migrate through the conduits toward the production well. In this way, recovery of hydrocarbons from the reservoir is improved because hydrocarbons that would not previously be able to reach the well now have a path to the well and can be produced.

The term "oil shale" refers to a sedimentary rock interspersed with an organic mixture of complex chemical compounds collectively referred to as "kerogen". The oil shale consists of laminated sedimentary rock containing mainly clay minerals, quartz, calcite, dolomite, and iron compounds. Oil shale can vary in its mineral and chemical composition. When the oil shale is heated to above 260-370 °C, destructive distillation of the kerogen (a process known as pyrolysis), occurs to produce products in the form of oil, gas, and residual carbon. The hydrocarbon products resulting from the destructive distillation of the kerogen have uses that are similar to other petroleum products. Oil shale is considered to have potential to become one of the primary sources for producing liquid fuels and natural gas, to supplement and augment those fuels currently produced from other petroleum sources. It is evident that use of resin coated proppants is incompatible with the temperatures required for kerogen pyrolysis.

Known in situ processes for recovering hydrocarbon products from oil shale resources describe treating the oil shale in the ground in order to recover the hydrocarbon products. These processes involve the circulation or injection of heat and/or solvents within a subsurface oil shale. Heating methods include hot gas injection. e.g. flue gas or methane or superheated steam, hot liquid injection, electric resistive heating, dielectric heating, microwave heating, or oxidant injection to support in situ combustion.

Permeability enhancing methods are sometimes utilized including; rubblization, hydraulic fracturing, explosive fracturing, heat fracturing, steam fracturing, and/or the provision of multiple wellbores.

A typical size of the proppant particles is a diameter of around 0.5 to 2 mm. It is preferred that each particle is approximately spherical and that the size distribution of the particles is reasonably uniform to enable easy flow of the particles. The compressive strength of the particles must be very high in order for them to keep fractures open without being crushed. There may be a trade-off between the porosity and weight of a proppant particle, and its resistance to compressive strength. It will be appreciated that a proppant particle must have sufficient compressive strength to reduce the likelihood of it being crushed by a fracture attempting to close when the fracking fluid is no longer providing pressure in the fractured formation. For some applications of proppants in hydrocarbon production it is required that the proppant is resistant towards dissolution in acid environments. Notably some reservoirs contain H2S, and CO2 may be evolved or used deliberately in the production. Acids like HF and/or HCI might be added to dissolve plugs in carbonaceous reservoirs. Fuithermore, flow of hard proppants may cause erosion to pipes, production equipment and to the rock itself. In addition, the propensity to settling in the fracking fluid should be minimized, e.g. by making the proppant sufficiently light in weight. It will be appreciated that there are many requirements that apply to a proppant of high quality, and there is a need to develop novel materials that fulfil these needs. In addition it is desirable that the raw materials used should be abundant and environmentally benign, and that the production methods of low gravity proppants should be simplified.

US 200610016598 describes methods of making lightweight porous proppant particles for use in a fracking operation US 2006/0016598 teaches that sintering of porous ceramics at high temperatures causes loss of porosity due to densification, but that it has been found feasible to produce light weight strong ceramics by sintering at preferred temperatures below 850°C or, as used in example 1, at 1000°C. However, sintering of porous ceramic proppants at moderate temperatures can lead to a loss of compressive strength of the ceramic particles.

Lightweight porous and composite proppant particles have been disclosed in us 4,493,875 using a core of a conventional proppant particle and coating this with a thin layer of hollow glass microspheres imbedded in a resin.

Provision of proppants is an expensive pad of a fracking operation, requiring the use of a large amount of water and chemicals to maintain a dispersion of proppants.

Reducing the density of the proppants while retaining adequate strength would allow the reduction of water and/or chemicals consumption and in addition provide a means to transport a treatment chemical into the fracks containing the hydrocarbons or in proximity of the hydrocarbons to be produced.

SUMMARY

It is an object to provide an improved proppant that has a reduced weight compared to conventional proppants while maintaining sufficient strength.

According to a first aspect, there is provided a proppant particle for use in a hydrocarbon production operation, the proppant particle comprising a porous structure, the porous structure being formed of any of a plurality of carbon nanofibres or tubes synthesized on the surface of a core, and an alumina spinel based porous particle.

The use of either of these types of material for a porous structure gives a lightweight proppant particle with adequate strength. This means that less water and/or chemicals are required to disperse the proppant particles during a hydrocarbon production operation such as fracking.

As an option, the porous structure comprises a plurality of carbon nanofibres synthesized on the surface of a quartz, sand, ceramic or resin coated core particle. As a further option, the core particle comprises alumina or an alumina-based spinel or mixtures thereof. It is an advantageous option for the core particle to be porous.

Furthermore, the core particle optionally comprises a catalyst. Examples of catalysts include iion, nickel, cobalt, copper or compounds thereof.

As an option, the porous stiucture complises a magnesium spinel.

The porous structure optionally comprises a-alumina in an amount selected from any of less than 30 wt%, less than 10 wt% and less than 2 wt% as determined by XRD analysis.

As an option, the porous particle does not contain an oxide of a divalent ion as detected byXRD.

The molar ratio of divalent metal to aluminium is optionally selected from any of less than 0.5. less than 0.4 and less than 0.25.

The proppant particle optionally comprises a treatment chemical located in pores of the poious structure. This allows the treatment chemical to be later activated to maximize production. Optional examples of a treatment chemical include any of an oxidation agent, a reactant, a solvent, a diluent, a catalyst, a monopropellant, an explosive and a liquefied gas. A reactant optionally comprises a hydiogen donor.

As an option, the proppant particle has a density in a range of any of below 2.0 glcm3, below 1.6 g/cm3, below 1.2 g/cm3, and below 0.9 glcm3.

The proppant particle optionally has a porosity in a range of any of at least 25 volume %, at least 50 volume %, and at least 70 volume %.

The proppant particle optionally has a crushing strength in the range of any of at least MPa, greater than 80 MPa, and greater than 170 MPa.

The pioppant particle optionally has an ASTM attrition value in the range of any of less than 50%, less than 10%, and less than 2%.

As an option, the proppant particle has a BET pore volume in the range selected from any of at least 0.05, at least 0.15, at least 0.3, and at least 0.5 cm3/g.

As an option, the proppant particle has an incipient wetness water absorptivity in the range selected from any of at least 0.2, at least 0.5, at least 0.8, and at least 1.1 cm3tg.

The proppant particle may be one of many proppant particles, wherein the particles have a narrow particle size distribution such that 80% of the particles have a diameter within the 20% of the average particle size. Each particle may be substantially spherical in shape with an aspect ratio for at least 80% of the particles larger than 0.7.

Optional examples of hydrocarbon production operations that use the proppant particle include fracking operations, and well or near well treatment operation.

According to a second aspect, there is provided a tracking fluid for use in a tracking operation, the fracking fluid comprising a dispersion of a plurality of proppant particles as described above in the first aspect.

According to a third aspect, there is provided a method of forming a proppant particle, the method comprising forming a plurality of carbon nanofibres on a core particle, the plurality of carbon nanofibres providing a porous structure.

As an option, the method comprises depositing a transition metal on a surface of the core particle, reducing the transition metal and decomposing a carbon precursor to form carbon nanofibres. The transition metal is optionally selected from any of iron, nickel, cobalt or copper. Carbon nanofibres are optionally formed by decomposition of any of carbon monoxide, methane, ethylene, acetylene and benzene.

As an option, the method further comprises sorbing a treatment chemical located in pores of the porous structure.

According to a fourth aspect, there is provided a method of forming a proppant particle, the method comprising impregnating a v-alumina particle with a divalent metal salt, drying the impregnated particle, and calcining the impregnated particle to produce an alumina spinel based porous proppant particle.

The v-alumina particles are optionally formed by any of spray-drying, freeze-drying, oil-drop or granulation.

As an option, the metal salt is a magnesium salt selected from any of magnesium nitrate, magnesium carbonate and magnesium chloride.

As an option, the impregnation of the y-alumina particle is by incipient wetness impregnation.

The method optionally further comprises performing calcination at a temperature selected from a range of any of at least 1050°C, at least at 1100°C, and least 1150°C.

The calcination may be in at least one step using any of a stationary kiln, rotary furnace and a transport calciner.

As an option, the method further comprises sorbing a treatment chemical located in pores proppant particle.

According to a fifth aspect, there is provided a method of performing a fracking operation, the method comprising injecting a fracking fluid into a subterranean formation, the tracking fluid comprising a dispersion of proppant particles as described above in the first aspect.

As an option, the method further comprises subsequently heating the subterranean formation to activate a chemical sorbed in pores of the proppant particles.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates a cross-section of a well after a fracking operation using proppants; Figure 2 is a flow diagram showing exemplary steps of a method of manufacturing proppant particles; Figure 3 is a graph showing the effect on attrition of adding magnesium to y-alumina; Figure 4 shows XRD plots obtained at different calcination temperatures; Figure 5 is a graph showing effect on attrition of calcination temperature; Figure 6 is a micrograph showing produced y-alumina particles; Figure 7 is a graph showing the aspect ratio distribution of two different y-alumina samples and their corresponding spinel type high temperature calcined analogues.

Figure 8 is a flow diagram showing exemplary steps of a further method of manufacturing proppant particles; Figure 9 is a flow diagram showing steps of an exemplary production operation;

DETAILED DESCRIPTION

As discussed above, there is a need for lower density proppants with adequate strength. The high density of existing proppants means that the fracking fluid must be highly viscous or even gel-like in order to maintain an even dispersion of proppant particles and limit the degree of settling of proppant particles before they enter fractures where they act as spacers.

By way of example, quart has a density of 2.2 glcm3, clay 1.8-2.6 glcm3, corundum (alumina) 3.9-4.0 g/cm3 and sandstone 2.14-2.36 glcm3. Each of these materials is currently used as a proppant. For dilute suspensions of regular proppant particles in a viscous fluid, the settling velocity can be described by Stokes' law, which dictates that the velocity is proportional to the density difference between particle and the fluid and to the square of the particle radius. Due to the need for strong proppants that can withstand the compressive forces in the fractures of the formation, the present proppant materials have high densities.

Note that the definition of density or gravity of porous solid particles is not trivial, and many terms have been used. In the definitions by the ASTM (American Society for Testing and Materials) there are more than 40 definitions of density. The describing terms "specific" or "apparent" are also used ambiguously. For example, in US 200610016598 the terms "specific gravity", "apparent specific gravity" and "apparent density" are all used for proppant particles without definition and description of measurement technique. For particles that are spherical in nature, but not necessarily ideally so, the settling velocity can be estimated using the weight of the particles to the S volume of the particles including internal voids and pores, these being open or closed, but excluding any interparticle volume. For particles containing mainly pores in the micro-and meso-porous range below 100 nm, the BET method with nitrogen can be used to measure the pore volume (PV) in cm3/g. If macropores occur it is common to use Hg-intrusion to measure pore volume. If the skeletal density of the material, i.e. the density of the solid material without pores, is known, the particle density (p) can be calculated as: p=p°/(l +PV*po) (Eq.1) or pP/PV (Eq.2) where p° is the skeletal density and P the porosity, i.e. the relative portion of the total particle volume that is contained in the pores. For most particles the inaccessible internal volume by nitrogen is expected to be small and can be disregarded in the calculations, but does not limit the described principle. Another factor is the extent to which the particles during operation will be filled with the fracking fluid or not and thereby increasing the density of the particles. This will depend on many factors like the hydrophilicity of the material and the viscosity of the fluid. For the sake of argument, unless otherwise stated, it is assumed that the pores do not contribute to the weight of the particles. The relationships between some of these parameters are

shown in Table 1.

Table 1. Pore volume (PV), porosity (P) and particle density (p) of porous particles for a skeletal density of 3.9 g/cm3 (a-alumina/corundum) PV (cm3/g) P Density (g/cm3) o o 3.90 0.1 0.28 2.81 0.2 0.44 2.19 0.3 0.54 1.80 0.4 0.61 1.52 0.5 0.66 1.32 0.6 0.70 1.17 0.7 0.74 1.05 It is seen from Table 1 that even moderate pore volumes in the 0.1 to 0.3 cm3/g range give significant reduction in the particle density.

Table 2 contrasts the density, size and settling velocities of quartz, corundum and 6 theoretical particles with lower densities to illustrate how the settling velocity may be reduced by reducing the density of the proppant particles.

Table 2. Relative settling velocities, assuming density of fracking fluid to be 1.1 g/cm3 System Density (g/cm3) Size (mm) Relative settling velocity Quartz 2.2 1.5 1.0 Corundum 3.9 1.0 1.1 Material 1 1.8 1.5 0.64 Material 2 1.5 1.5 0.35 Material3 1.3 1.5 0.18 Material 4 1.3 3 0.72 Materials 1.2 5 1.0 Material 6 «= 1.1 Unlimited Floating It can be seen that by reducing the density of the particles, the settling velocity can be reduced significantly. Alternatively, the particle size can be doubled or more for light particles without compromising the settling velocity. These data suggest that reducing the density of proppant particles can give rise to a much higher efficiency in keeping fractures open. Furthermore, such proppants would allow a higher utilization ratio of the proppants/fracking fluid, and therefore require the use of a lower volume of chemicals. This has both cost and environmental benefits. It will be appreciated that as the density of the proppant particles approaches the density of the tracking fluid, the benefits increase.

By providing proppant particles with porosities in the 50-70 volume % range, it is feasible to make proppant particles with even lower densities than in Table 2. This allows them to be used with condensed gases as a fracking fluid. For example, the density of liquid propane at room temperature is 0.493 g/cm3, and the corresponding relative settling velocities are given in Table 3. Use of Liquid Petroleum Gas (LPG) or propane as a fracking gas with no (or a low level of) chemicals allows a fracking operation in which water consumption is substantially eliminated and surface handling of the return fracking fluid is easy. The propane can simply be flashed off from any reservoir water, if needed, and recycled to further use in the fracking operation.

Table 3. Relative settling velocities, assuming a tracking fluid density of 0.493 g/cm3 System Density (glcm3) Size (mm) Relative settling velocity Quartz 2.2 1.5 1.0 Corundum 3.9 1.0 1.1 Materiall 1.8 1.5 1.5 Material 2 1.5 1.5 0.92 Material 3 1.3 1.5 0.73 Materiall 1.1 1.5 0.55 MaterialS 0.9 1.5 0.37 Material9 0.7 1.5 0.19 Note that hypothetical Materials ito 3 are the same as in Tables 1 and 2.

There is a clear advantage in providing porous proppant particles. A suitable candidate class materials is porous ceramics. Examples of ceramics that can be produced in porous form are aluminium oxide, zirconium toughened alumina, silicon (oxy) carbide, silicon nitride, and stabilized zirconia with porosities of 10-55% or higher with compressive strengths up to 130 MPa.

Strong porous materials are used for catalysis. Catalyst supports can be designed for operation in turbulent flow in a slurry reactor. One class of catalyst supports that appears particularly promising for use as a proppant is composed of aluminate spinel, in particular when treated at high temperatures above 1050°C. Figure 2 is a flow diagram showing exemplary steps for preparing alumina spinel proppant particles. The following numbering corresponds to that of Figure 2: 51. -alumina particles of suitable size and shape are impregnated (for example, using incipient wetness impregnation) using an aqueous solution of a divalent metal salt. Any suitable salt can be used, such as magnesium nitrate, magnesium carbonate, magnesium sulphate and magnesium chloride. Corresponding nickel salts such as Ni(N03)2 can also be used.

52. The impregnated y-alumina particles are dried to remove excess water.

S3. The impregnated particles are calcined above 1050°C (temperature above 1150°C have been found to be most suitable). Calcination can take place by any known method including stationary kilns, rotary furnaces, transport calciners and the like.

Where the metal salt used is Mg(NO3)2, the resultant particles comprise MgAI2O4 spinel, and where the metal salt used is Ni(N03)2, the resultant particles comprise NiAI2O4 spinel. The spinel can be in essentially pure form if the starting ratio between v-alumina and divalent metal corresponds to the stoichiometric ratio in the spinel.

However, for other stoichiometric ratios there will be some excess cx-alumina on one hand or oxide of the divalent salt used on the other hand. It has been found that excess MgO or NiO is unsuitable for use as proppants as these materials are more easily dissolved in water based solutions and also weakens the product. However, an excess of a-alumina is much to be preferred as presence of a-alumina together with the spinel toughens the material considerably during calcination and are highly inert under acidic and basic conditions.

An example of the effect of adding magnesium to v-alumina is shown in Figure 3 when S3 above is carried out in a laboratory furnace at 1140 °C for 16 h starting in 51 with a water solution of Mg(N03)2 and impregnation of spray-dried y-alumina by incipient wetness. Attrition measurements were carried out by ASTM D5757 and were found to correlate well with hardness micro-indentation measurements by the Vickers method.

In case that no divalent metal is added, the transformation to pure a-alumina results in a very weak material. Adding 5 and especially 10 wt% Mg as the nitrate solution, give up to ca. 50 fold improvement in attrition resistance. A 10 wt% Mg sample corresponds to 0.318 mol% of MgO and the rest being A1203. Evidently, pure spinel corresponds to mol% MgO and alumina, and the 10 wt% Mg sample therefore upon calcination contains MgAI2O4 spinel and surplus a-alumina.

Typical X-Ray Diffraction (XRD) plots of evolution of the different phases are given in Figure 4. It can be observed that first the spinel phase is evolving before a-alumina becomes pronounced at the highest temperatures. The temperature scales between XRD and calcination in a furnace may be off-set as the thermocouple is placed differently in the XRD. The effect of temperature is illustrated clearly in Figure 5 on the sample with 10 wt% added Mg as nitrate. Temperatures above 1050 °C are needed, preferably above 1100 °C.

The resultant particles typically have a pore volume in the range 0.1 to 0.5 cm3/g.

Surface areas vary from 10 to 50 m2/g material. The 10 wt% material in Figure 5 calcined to 1140 °C has a BET surface area of 29 m2/g, a pore diameter of 19 nm and a pore volume of 0.18 cm3/g. As can be seen from Table 1, a pore volume around 0.2 gives a significant reduction in the particle density. However, measurement of water absorptivity by the incipient wetness tapping method gives a value of 0.51 crn3/g. This higher value is partly due to a water layer on the outer periphery of the particles, but may also in part result from macropores not detected by BET. Corresponding data for calcination of pure v-alumina at the same temperature gives 7-10 m2/g, 15-18 nm, 0.03-0.05 cm3/g, high attrition (see Figure 3) and a water absorptivity of 0.71-0.74 cm3/g, clearly unsuitable as a proppant. Clearly, macro-pores are formed between poorly linked crystal grains.

Further proppants based on spinel were prepared by incipient wetness with a water solution of Mg(N03), followed by drying at 110 °C and calcination. The alumina was obtained from Sasol GmbH and was in the form of regularly shaped spheres of 1.8 mm diameter and a pore volume of 0.75 cm3/g made by the oil-drop method. Different loadings of magnesium and calcination temperatures were investigated with results in line with those of Figures 3 to 5. The solid spinel phase has a density of 3.58 g/cm3 and with a pore volume of 0.3 cm3/g; this gives a particle density of 1.73 g/cm3, down from 3.9 for solid corundum particles. The settling velocity of such a spinel based proppant will be only 22.5 % of corundum particles of the same size, ref the calculation procedure in Table 2.

It is known to be an advantage that the proppants are reasonably uniform in size and that the shape is close to spherical. With the method starting with y-alumina the size distribution and shape of the calcined proppants to a large extent will be dictated by the properties of the y-alumina starting material. Figure 6 shows rather uniform y-alumina particles, whereas Figure 7 discloses the aspect ratio distribution of two different y-alumina lots and their corresponding spinel type high temperature calcined analogues.

The aspect ratio on the abscissa is the ratio between the breadth and length of the particles, or b/I, whereas the ordinate shows the accumulated number of particles in per cent. First, two nearly parallel sets of curves can be noted. Each of these sets represents the parent y-alumina particles and their spinel analogue. It can be observed that the difference between the corresponding alumina and spinel samples is minimal.

Further, there can be rather large variations in sphericity, but both classes of materials shown here are useful as proppants. However, the material illustrated by the two curves shifted to the right in the diagram evidently exhibits particles with more ideal spheres.

Another type of material that may be used is based on carbon nanofibres (or carbon nanotubes), CNF. Carbon nanofibres are cylindrical graphitic nanostructures with graphene layers stacked on top of each other in a regular fashion. The stacking has been described as platelet, fishbone, cups or cones. If the fibres are hollow they are called carbon nanotubes. There are many variations of these materials including multi-walled and doped varieties. Synthesis is by decomposition of CO, methane, olefins or other hydrocarbons in the gas phase on a transition metal catalyst, most commonly iron, nickel, cobalt or copper, or by growth from a carbon source that is vaporized.

Many synthesis techniques exist, including simple decomposition of the carbon precursor at high pressure, chemical vapour decomposition, arc discharge and using a plasma torch. The fibres have diameters in the range of 1-100 nm and can have a length of several hundred pm and beyond. The fibre is light, flexible, has high surface area and a claimed strength that exceeds steel. The main challenge towards applications as light strong particles, e.g. as proppants, is forming the material into a suitable shape that can be stored, transported, mixed with the fracking fluid and exhibits preferable flow properties.

The term carbon nanofiber (CNF) is used herein to collectively encompass carbon nanotubes. CNFs provide simple preparation, low density (carbon is a very light material), high porosity and unique strength. In addition, the flexibility of the CNFs will reduce any issues with erosive attacks and also disperse the forces imposed to the proppant by the rock walls over a larger area thereby making the proppant less prone toward cracking. There is also a potential for the CNF coated proppants to link together in the fractures through the surface fibres and thereby prevent the proppants from leaving the fractures as the fracking fluid is removed. Further, we have found that it is possible to retain the porosity and light weight of the ceramic proppant by calcination at high temperatures, notably above 1050 °C, and at the same time improve the strength significantly.

It is possible to grow carbon nanofibres on the surface of existing proppant particles, including porous proppant particles. The carbon nanofibre surface provides a low density, high porosity surface that reduces the overall density of the proppant particles.

Figure 8 is a flow diagram showing exemplary steps in preparing carbon nanofibre proppant particles. The following numbering corresponds to that of Figure 8: S4. Deposit transition metal compound on surface of core particle.

S5. Reduce transition metal in a reducing atmosphere to form transition metal catalyst particles on surface of core particle.

S6. Provide atmosphere of a carbon-containing gas such as carbon monoxide, methane, ethylene, acetylene and benzene optionally with an inert gas to form carbon nanofibres.

It should be understood that both the reduction step S5 and the CNF growth step S6 are to be carried out at conditions known in the art, specifically at adequate temperature, pressure and gas composition.

The provision of strong and porous particles to be used as proppants leads to a further surprising beneficial application: the proppant may be used as a carrier for chemicals into the reservoir and the hydrocarbon source. By sorbing treatment chemicals into the pores of proppant particles, the chemicals can be injected directly into the fractures.

This is particularly useful where, for example! the chemicals would not be activated until after the fracking fluid has been removed. If the chemicals were carried in the fracking fluid, they would no longer be present after the fracking operation. For example, a fracking operation may be performed at ambient reservoir temperatures.

The reservoir may be subsequently heated. In this case, chemicals contained in pores of the proppant particles may be activated at a certain temperature and so be substantially inert during the fracking operation itselt, but subsequently active during a heating phase.

The provision of a hydrogen donating fluid as a treatment chemical can be advantageous as higher yield of hydrocarbons can be obtained at a given temperature, or the process temperature can be reduced, thereby saving in energy use. Treatment chemicals that provide oxidation reactions can also be suitable for enhanced extraction of the hydrocarbons, by providing heat and breaking down the complex structure of heavy hydrocarbons.

A solvent can also be introduced by sorbing it in the pores of the proppant, thereby facilitating transport of hydrocarbons to the surface. The solvent can be of any convenient form, including water, hydrocarbons, oxygenates and mixtures thereof.

A treatment chemical that evaporates at the sort of temperatures experienced during a heating operation (for example, around 350°C) expands during heating as it evaporates, and so enhances the size of the fractures and provides additional pressure to push produced hydrocarbons towards the production well. It may assist in creating further fractures. An even larger effect might be possible using a mono-propellant or an explosive.

Figure 9 is a flow diagram showing an exemplary production operation. The following numbering corresponds to that of Figure 9: S7. Porous proppants are provided. These may be made by any of the techniques described above.

S8. Treatment chemicals (examples of which are described above) are sorbed into the pores of the porous proppant particles. Note that more than one type of treatment chemical may be used. For example, where a first and a second type of treatment chemical are used, they may both be sorbed into the pores of the particles.

Alternatively, the first type of treatment chemical may be sorbed into the pores of a first batch of proppant particles, and the second type of treatment chemical may be sorbed into a second batch a proppant particles.

59. The proppant particles are dispersed in the fracking fluid.

510. The fracking operation is performed using the fracking fluid, leaving proppant particles in fractures in the subterranean formation.

Si 1. Subsequent operations (such as heat treatment, injection of steam etc.) activate the treatment chemicals, which treat the formation or the hydrocarbons in various ways as described above.

S12. Hydrocarbons are produced from the formation.

EXAMPLE

y-alumina was impregnated by a water solution of Mg(N03) for 1 h, followed by drying at 110 °C and calcination. The impregnation gave a nominal concentration of magnesium in the particle of 5 wt%. The alumina was obtained from Sasol GmbH and was in the form of regularly shaped spheres of 1.8 mm diameter and a pore volume of 0.75 cm3/g made by an oil-drop method. Different loadings of magnesium and calcination temperatures (1000, 1050 and 1100 °C) were investigated, and the porosity and strength of the final material measured.

In Table 4 some properties of five known proppant types have been compared with four proppants as described herein that are either alumina spheres made by the oil-drop method, or these spheres impregnated with a magnesium solution, dried and calcined.

A huge difference in properties can be observed between the two sets of materials.

For example, the commercial proppants have very much lower surface areas, pore volumes and water absorbtivities, the first two properties being measured by nitrogen physisorption on Micromeretics Tristar. The densities are also much lower for two of the proppants that are described herein. All nine materials are very strong as measured by the attrition test described in ASTM D5757 giving less than 2 % fines after 5 h. Further, they are reasonably spherical with a spherisity obtained by the particle size instrument Camsizer XT between 0.92 and 0.99, the light weight gravel being lowest at 0.923 and the alumina spheres being very ideal spheres with a spherisity of 0.991.

Table 4. Some properties of proppants System Apparent Water Size Surface Pore bulk density absorptivity D50 area volume ____________________ (g/cm3) (ml/g) (kim) (m2/g) (cm3/g) Known proppants Resin coated ceramic 1.643 0.13 1094 <0.1 <0.05 Light weight gravel 1.156 0.25 826 0.15 C 0.05 Ac Pack 1.658 0.18 620 <0.1 <0.05 Sand 20/40 mesh 1.723 0.21 614 0.12 <0.05 Carbolite 20/40 mesh 1.698 0.23 781 <0.1 <0.05 Proppants described herein Alumina spheres 0.544 1.15 1735 211 0.53 Spinel 5%Mg; 1000°C 0.86 1537 86 0.24 Spinel 5%Mg; 1050°C 1.26 1346 12 0.07 Spinel 5%Mg; 1100°C 1.29 1331 7 0.05 The techniques described above may be applied to various types of subterranean formation, including low permeability subterranean formations such as shales or formation containing more than 50% shale by volume. Furthermore, they may be applied to the production of different types of hydrocarbon, such as oil, gas, shale oil, kerogens, coal and so on. It should be understood that the term hydrocarbon" present in the subterranean formation is now used in a broad meaning of the term, i.e. not only covering material and compounds that strictly is composed of hydrogen and carbon atoms, but also to a larger or smaller extent contains heteroatoms that typically are oxygen, sulphur or nitrogen, but also minor amounts of phosphorous, mercury, vanadium, nickel, iron or other elements can be present. In situ catalytic reactions and/or heat treatment may modify the composition of the hydrocarbon product. The term "hydrocarbon product" is also used in a broad sense to cover products that contain heteroatoms, in particular oxygen. This hydrocarbon product will often be further treated in one or more processing steps to give a secondary or final product, e.g. to be shipped to a refinery or sold to a consumer. The hydrocarbon product may contain alcohols, in particular phenols or other aromatic compounds with attached alcohol groups.

It will be appreciated by the person of skill in the art that various modifications may be made to the above-described embodiments without departing from the scope of the piesent invention.

Claims

CLAIMS: 1. A proppant particle for use in a subterranean hydrocarbon production operation, the proppant particle comprising a porous structure, the porous structure being formed of any of a plurality of carbon nanofibres synthesized on the surface of a core, and an alumina spinel based porous particle.
2. The proppant particle according to claim 1, wherein the porous structure comprises a plurality of carbon nanofibres synthesized on the surface of a quartz, sand, ceramic or resin coated core particle.
3. The proppant particle according to claim 2, wherein the core particle comprises alumina or an alumina-based spinel or mixtures thereof.
4 The proppant particle according to claim 2 or 3, wherein the core particle is porous.
5. The proppant article according to any of claims 2, 3 or 4, wherein the core particle comprises a catalyst
6. The proppant particle according to claim 5, wherein the catalyst is selected from any of iron, nickel, cobalt, copper or compounds thereof.
7. The proppant particle according to claim 1, wherein the porous structure comprises a magnesium spinel.
8. The proppant particle according to claim 1 or 7, wherein the porous structure comprises a-alumina in an amount selected from any of less than 30 wt%, less than 10 wt% and less than 2 wt% as determined by XRD analysis.
9. The proppant particle according to any of claims 1, 7 or 8, wherein the porous particle does not contain an oxide of a divalent ion as detected by XRD.The proppant particle according to any of claims 1, 7 or 8, wherein the molar ratio of divalent metal to aluminium is selected from any of less than 0.5, less than 0.4 and less than 0.25.
10. The proppant particle according to any one of claims ito 9, further comprising a treatment chemical located in pores of the porous structure.
11. The proppant particle according to claim 10 wherein the treatment chemical comprises any of an oxidation agent, a reactant, a solvent, a diluent, a catalyst, a monopropellant, an explosive and a liquefied gas.
12. The proppant particle according to claim 11, wherein the reactant comprises a hydrogen donor.
13. The proppant particle according to any of claims 1 to 12, wherein the proppant particle has a density in a range of any of below 2.0 g/cm3, below 1.6 g/cm3, below 1.2 gtcm3, and below 0.9 g/cm3.
14. The proppant particle according to any of claims 1 to i3, wherein the proppant particle has a porosity in a range of any of at least 25 volume %, at least 50 volume %, and at least 70 volume %.
15. The proppant particle according to any of claims 1 to 14 having a crushing strength in the range of any of at least 30 MPa, greater than 80 MPa, and greater than MPa.
16. The proppant particle according to any of claims 1 to 15, the proppant particle having an ASTM attrition value in the range of any of less than 50%, less than 10%, and less than 2%.
17. The proppant particle according to any of claims 1 to 16, the proppant particle having a BET pore volume in the range selected from any of at least 0.05, at least 0.15, at least 0.3, and at least 0.5 cm3/g.
18. The proppant particle according to any of claims 1 to 17, the proppant particle having an incipient wetness water absorptivity in the range selected from any of at least 0.2, at least 0.5, at least 0.8, and at least 1.1 cm3lg.
19. The proppant particle according to any of claims 1 to 18 having a narrow particle size distribution such that 80% of the particles have a diameter within the 20% of the average particle size.
20. The proppant particle according to any of claims 1 to 19, the particle being substantially spherical in shape with an aspect ratio for at least 80% of the particles larger than 0.7.
21. The proppant particle according to claim 1, wherein the hydrocarbon production operation is a fracking operation.
22. A fracking fluid for use in a fracking operation, the fracking fluid comprising a dispersion of a plurality of proppant particles according to claim 21 particles.
23. The proppant particle according to claim 1, wherein the hydrocarbon production operation is a well or near well treatment operation.
24. A method of forming a proppant particle, the method comprising forming a plurality of carbon nanofibres on a core particle, the plurality of carbon nanofibres providing a porous structure.
25. The method according to claim 24, further comprising depositing a transition metal on a surface of the core particle, reducing the transition metal and decomposing a carbon precursor to form carbon nanofibres.
26. The method according to claim 25, wherein the transition metal is selected from any of iron, nickel, cobalt or copper.
27. The method according to any of claims 24, 25 or 26, further comprising forming carbon nanofibres by decomposition of any of carbon monoxide, methane, ethylene, acetylene and benzene.
28. The method according to any of claims 24 to 26, further comprising sorbing a treatment chemical located in pores of the porous structure.
29. A method of forming a proppant particle, the method comprising: impregnating a v-alumina particle with a divalent metal salt; drying the impregnated particle; calcining the impregnated particle to produce an alumina spinel based porous proppant particle.
30. The method according to claim 28, comprising forming the y-alumina particles by any of spray-drying, freeze-drying, oil-drop or granulation.
31. The method according to claim 29 or 30, wherein the metal salt is a magnesium salt selected from any of magnesium nitrate, magnesium carbonate, magnesium sulphate and magnesium chloride.
32. The method according to any of claims 29. 30 or 31, wherein the impregnation of the y-alumina particle is by incipient wetness impregnation.
33. The method according to any of claims 29 to 32, comprising performing calcination at a temperature selected from a range of any of at least 1050°C, at least at 1100°C. and least 1150°C.
34. The method according to any of claims 29 to 33, wherein the calcination is in at least one steps using any of a stationary kiln, rotary furnace and a transport calciner.
35. The method according to any of claims 29 to 34, further comprising sorbing a treatment chemical located in pores proppant particle.
36. A method of performing a fracking operation, the method comprising: injecting a fracking fluid into a subterranean formation, the fracking fluid comprising a dispersion of proppant particles according to any of claim 21.
37. The method according to claim 36, further comprising subsequently heating the subterranean formation to activate a chemical sorbed in pores of the proppant particles.