Thermoset nanocomposite particles, processing for their production, and their use in oil and natural gas drilling applications

Abstract

Use of two different methods, either each by itself or in combination, to enhance the stiffness, strength, maximum possible use temperature, and environmental resistance of thermoset polymer particles is disclosed. One method is the application of post-polymerization process steps (and especially heat treatment) to advance the curing reaction and to thus obtain a more densely crosslinked polymer network. The other method is the incorporation of nanofillers, resulting in a heterogeneous nanocomposite morphology. Nanofiller incorporation and post-polymerization heat treatment can also be combined to obtain the benefits of both methods simultaneously. The present invention relates to the development of thermoset nanocomposite particles. Optional further improvement of the heat resistance and environmental resistance of said particles via post-polymerization heat treatment; processes for the manufacture of said particles; and use of said particles in the construction, drilling, completion and/or fracture stimulation of oil and natural gas wells are described.

Claims

1. A method for treating a well penetrating a subterranean formation comprising: (a) mixing into a drilling mud formulation an effective amount of a polymeric nanocomposite spherical bead comprising: a polymer matrix; and from 0.001 to 60 volume percent of nanofiller particles possessing a length that is less than 0.5 microns in at least one principal axis direction; said nanofiller particles comprising at least one of fine particulate material, fibrous material, discoidal material, or a combination of such materials, said nanofiller particles being selected from the group consisting of natural nanoclays, synthetic nanoclays or mixtures thereof wherein said nanofiller particles are substantially dispersed throughout said polymeric nanocomposite spherical bead, wherein said polymeric nanocomposite spherical bead has a diameter ranging from 0.1 mm to 4 mm; and (b) introducing said drilling mud formulation with said effective amount of the polymeric nanocomposite spherical bead into said well.

2. The method of claim 1, wherein said nanofiller particles possess a length that is less than 0.5 microns in at least one principal axis direction and an amount from 0.1% to 15% of said polymeric nanocomposite spherical bead by volume.

3. The method of claim 1, wherein said polymer matrix comprises at least one of a thermoset epoxy, a thermoset epoxy vinyl ester, a thermoset polyester, a thermoset phenolic, a thermoset polyurethane, a thermoset polyurea, a thermoset polyimide, or mixtures thereof.

4. The method of claim 1, wherein said polymer matrix comprises a terpolymer.

5. The method of claim 4, wherein said polymer matrix is a styrene-ethylvinylbenzene-divinylbenzene terpolymer.

6. The method of claim 1, wherein said nanofiller is natural nanoclays.

7. The method of claim 1, wherein said nanofiller is synthetic nanoclays.

8. The method of claim 1, wherein said nanofiller is a mixture of natural and synthetic nanoclays.

9. The method of claim 1, wherein said polymeric nanocomposite particle is blended with other solid particles including at least one of sand, resin-coated sand, ceramic, and resin-coated ceramic.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The accompanying drawings, which are included to provide further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.

(2) FIG. 1 shows the effects of advancing the curing reaction in a series of isothermally polymerized styrene-divinylbenzene (S-DVB) copolymers containing different DVB weight fractions via heat treatment. The results of scans of S-DVB beads containing various weight fractions of DVB (w.sub.DVB), obtained by Differential Scanning calorimetry (DSC), and reported by Bicerano, et al. (1996), are compared. It is seen that the T.sub.g of typical as-polymerized S-DVB copolymers, as measured by the first DSC scan, increased only slowly with increasing w.sub.DVB, and furthermore that the rate of further increase of T.sub.g slowed down drastically for w.sub.DVB>0.08. By contrast, in the second DSC scan (performed on S-DVB specimens whose curing had been driven much closer to completion as a result of the temperature ramp that had been applied during the first scan), T.sub.g grew much more rapidly with w.sub.DVB over the entire range of up to w.sub.DVB=0.2458 that was studied.

(3) FIG. 2 provides an idealized, generic and schematic two-dimensional illustration of how a very small volume fraction of a nanofiller may be able to span and thus bridge through a vast amount of space, thus potentially enhancing the load bearing ability of the matrix polymer significantly at much smaller volume fractions than possible with conventional fillers.

(4) FIG. 3 illustrates the aggregates in which the primary particles of nanofillers such as nanoscale carbon black, fumed silica and fumed alumina commonly occur. Such aggregates may contain many very small primary particles, often arranged in a fractal pattern, resulting in aggregate principal axis dimensions that are also shorter than 0.5 microns. These aggregates (and not the individual primary particles that constitute them) are, usually, the smallest units of such nanofillers that are dispersed in a polymer matrix under normal fabrication conditions, when the forces holding the aggregates together in the much larger agglomerates are overcome successfully. This illustration was reproduced from the product literature of Cabot Corporation.

(5) FIG. 4 provides an idealized schematic illustration, in the context of the resistance of thermoset polymer particles to compression as a function of the temperature, of the most common benefits of using the methods of the present invention. In most cases, the densification of the crosslinked polymer network via post-polymerization heat treatment will have the main benefit of increasing the softening (and hence also the maximum possible use) temperature, along with improving the environmental resistance. On the other hand, in most cases, nanofiller incorporation will have the main benefits of increasing the stiffness and strength. The use of nanofiller incorporation and post-polymerization heat treatment together, as complementary methods, will thus often be able to provide all (or at least most) of these benefits simultaneously.

(6) FIG. 5 provides a process flow diagram depicting the preparation of the example. It contains four major blocks; depicting the preparation of the aqueous phase (Block A), the preparation of the organic phase (Block B), the mixing of these two phases followed by suspension polymerization (Block C), and the further process steps used after polymerization to obtain the as-polymerized and heat-treated samples of particles (Block D).

(7) FIG. 6 shows the variation of the temperature with time during polymerization.

(8) FIG. 7 shows the results of the measurement of the glass transition temperatures (T.sub.g) of the three heat-treated thermoset nanocomposite samples via differential scanning calorimetry (DSC). The samples have identical compositions. They differ only as a result of the use of different heat treatment conditions after polymerization. T.sub.g was defined as the temperature at which the curve showing the heat flow as a function of the temperature goes through its inflection point.

(9) FIG. 8 provides a schematic illustration of the configuration of the conductivity cell.

(10) FIG. 9 shows the measured liquid conductivity of a packing of particles of 14/16 U.S. mesh size (diameters ranging from 1.19 mm to 1.41 mm) from Sample 40m200C, at a coverage of 0.02 lb/ft.sup.2, under a closure stress of 4000 psi at a temperature of 190 F., as a function of time.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(11) Because the invention will be understood better after further discussion of its currently preferred embodiments, further discussion of said embodiments will now be provided. It is understood that said discussion is being provided without reducing the generality of the invention, since persons skilled in the art can readily imagine many additional embodiments that fall within the full scope of the invention as taught in the SUMMARY OF THE INVENTION section.

(12) A. Nature, Attributes and Applications of Currently Preferred Embodiments

(13) The currently preferred embodiments of the invention are lightweight thermoset nanocomposite particles possessing high stiffness, strength, temperature resistance, and resistance to aggressive environments. These attributes, occurring in combination, make said particles especially suitable for use in many challenging applications in the construction, drilling, completion and/or fracture stimulation of oil and natural gas wells. Said applications include the use of said particles as a proppant partial monolayer, a proppant pack, an integral component of a gravel pack completion, a ball bearing, a solid lubricant, a drilling mud constituent, and/or a cement additive.

(14) B. Thermoset Polymer Matrix

(15) 1. Constituents

(16) The thermoset matrix in said particles consists of a terpolymer of styrene (S, non-crosslinking), ethyvinylbenzene (EVB, also non-crosslinking), and divinylbenzene (DVB, crosslinking). The preference for such a terpolymer instead of a copolymer of S and DVB is a result of economic considerations. To summarize, DVB comes mixed with EVB in the standard product grades of DVB, and the cost of DVB increases rapidly with increasing purity in special grades of DVB. EVB is a non-crosslinking (difunctional) styrenic monomer. Its incorporation into the thermoset matrix does not result in any significant changes in the properties of the thermoset matrix or of nanocomposites containing said matrix, compared with the use of S as the sole non-crosslinking monomer. Consequently, it is far more cost-effective to use a standard (rather than purified) grade of DVB, thus resulting in a terpolymer where some of the repeat units originate from EVB.

(17) 2. Proportions

(18) The amount of DVB in said terpolymer ranges from 3% to 35% by weight of the starting mixture of the three reactive monomers (S, EVB and DVB) because different applications require different maximum possible use temperatures. Even when purchased in standard product grades where it is mixed with a large weight fraction of EVB, DVB is more expensive than S. It is, hence, useful to develop different product grades where the maximum possible use temperature increases with increasing weight fraction of DVB. Customers can then purchase the grades of said particles that meet their specific application needs as cost-effectively as possible.

(19) C. Nanofiller

(20) 1. Constituents

(21) The Monarch 280 product grade of nanoscale carbon black supplied by Cabot Corporation is being used as the nanofiller in said particles. The reason is that it has a relatively low specific surface area, high structure, and a fluffy product form; rendering it especially easy to disperse.

(22) 2. Proportions

(23) The use of too low a volume fraction of carbon black results in ineffective reinforcement. The use of too high a volume fraction of carbon black may result in difficulties in dispersing the nanofiller, dispersion viscosities that are too high to allow further processing with available equipment, and detrimental interference in polymerization and network formation. The amount of carbon black ranges from 0.1% to 15% by volume of said particles because different applications require different levels of reinforcement. Carbon black is more expensive than the monomers (S, EVB and DVB) currently being used in the synthesis of the thermoset matrix. It is, therefore, useful to develop different product grades where the extent of reinforcement increases with increasing volume fraction of carbon black. Customers can then purchase the grades of said particles that meet their specific application needs as cost-effectively as possible.

(24) D. Polymerization

(25) Suspension polymerization is performed via rapid rate polymerization, as taught by Albright (U.S. Pat. No. 6,248,838) which is incorporated herein by reference, for the fabrication of said particles. Rapid rate polymerization has the advantage, relative to conventional isothermal polymerization, of producing more physical entanglements in thermoset polymers (in addition to the covalent crosslinks). Suspension polymerization involves the preparation of an the aqueous phase and an organic phase prior to the commencement of the polymerization process. The Monarch 280 carbon black particles are dispersed in the organic phase prior to polymerization. The most important additional formulation component (besides the reactive monomers and the nanofiller particles) that is used during polymerization is the initiator. The initiator may consist of one type molecule or a mixture of two or more types of molecules that have the ability to function as initiators. Additional formulation components, such as catalysts, inhibitors, dispersants, stabilizers, rheology modifiers, buffers, antioxidants, defoamers, impact modifiers, plasticizers, pigments, flame retardants, smoke retardants, or mixtures thereof, may also be used when needed. Some of the additional formulation component(s) may become either partially or completely incorporated into the particles in some embodiments of the invention.

(26) E. Attainable Particle Sizes

(27) Suspension polymerization produces substantially spherical polymer particles. (While it is a goal of this invention to create spherical particles, it is understood that it is exceedingly difficult as well as unnecessary to obtain perfectly spherical particles. Therefore, particles with minor deviations from a perfectly spherical shape are considered perfectly spherical for the purposes of this disclosure.) Said particles can be varied in size by means of a number of mechanical and/or chemical methods that are well-known and well-practiced in the art of suspension polymerization. Particle diameters attainable by such means range from submicron values up to several millimeters. Hence said particles may be selectively manufactured over the entire range of sizes that are of present interest and/or that may be of future interest for applications in the oil and natural gas industry.

(28) F. Optional Further Selection of Particles by Size

(29) Optionally, after the completion of suspension polymerization, said particles can be separated into fractions having narrower diameter ranges by means of methods (such as, but not limited to, sieving techniques) that are well-known and well-practiced in the art of particle separations. Said narrower diameter ranges include, but are not limited to, nearly monodisperse distributions. Optionally, assemblies of particles possessing bimodal or other types of special distributions, as well as assemblies of particles whose diameter distributions follow statistical distributions such as gaussian or log-normal, can also be prepared.

(30) The optional preparation of assemblies of particles having diameter distributions of interest from any given as polymerized assembly of particles can be performed before or after any optional heat treatment of said particles. Without reducing the generality of the invention, in the currently most preferred embodiments of the invention, any optional preparation of assemblies of particles having diameter distributions of interest from the product of a run of the pilot plant or production plant reactor is performed after the completion of any optional heat treatment of said particles.

(31) The particle diameters of current practical interest for various uses in the construction, drilling, completion and/or fracture stimulation of oil and natural gas wells range from 0.1 to 4 millimeters. The specific diameter distribution that would be most effective under given circumstances depends on the details of the subterranean environment in addition to depending on the type of application. The diameter distribution that would be most effective under given circumstances may be narrow or broad, monomodal or bimodal, and may also have other special features (such as following a certain statistical distribution function) depending on both the details of the subterranean environment and the type of application.

(32) G. Optional Heat Treatment

(33) Said particles are left in the reactor fluid that remains after suspension polymerization if optional heat treatment is to be used. Said reactor fluid thus serves as the heat treatment medium. This approach works especially well (without adverse effects such as degradation and/or swelling) in enhancing the curing of said particles where the polymer matrix consists of a terpolymer of S, EVB and DVB. Since the reactor fluid that remains after the completion of suspension polymerization is aqueous while these terpolymers are very hydrophobic, the reactor fluid serves as an excellent heat transfer medium which does not swell the particles. The use of the reactor fluid as the medium for the optional heat treatment also has the advantage of simplicity since the particles would have needed to be removed from the reactor fluid and placed in another fluid as an extra step before heat treatment if an alternative fluid had been required.

(34) Detailed and realistic simulations based on the solution of the heat transfer equations are often used optionally to optimize the heat exposure schedule if optional heat treatment is to be used. It has been found that such simulations become increasingly useful with increasing quantity of particles that will be heat treated simultaneously. The reason is the finite rate of heat transfer. Said finite rate results in slower and more difficult equilibration with increasing quantity of particles and hence makes it especially important to be able to predict how to cure most of the particles further uniformly and sufficiently without overexposing many of the particles to heat.

EXAMPLE

(35) The currently preferred embodiments of the invention will be understood better in the context of a specific example. It is to be understood that said example is being provided without reducing the generality of the invention. Persons skilled in the art can readily imagine many additional examples that fall within the scope of the currently preferred embodiments as taught in the DETAILED DESCRIPTION OF THE INVENTION section. Persons skilled in the art can, furthermore, also readily imagine many alternative embodiments that fall within the full scope of the invention as taught in the SUMMARY OF THE INVENTION section.

(36) A. Summary

(37) The thermoset matrix was prepared from a formulation containing 10% DVB by weight of the starting monomer mixture. The DVB had been purchased as a mixture where only 63% by weight consisted of DVB. The actual polymerizable monomer mixture used in preparing the thermoset matrix consisted of roughly 84.365% S, 5.635% EVB and 10% DVB by weight.

(38) Carbon black (Monarch 280) was incorporated into the particles, at 0.5% by weight, via dispersion in the organic phase of the formulation prior to polymerization. Since the specific gravity of carbon black is roughly 1.8 while the specific gravity of the polymer is roughly 1.04, the amount of carbon black incorporated into the particles was roughly 0.29% by volume.

(39) Suspension polymerization was performed in a pilot plant reactor, via rapid rate polymerization as taught by Albright (U.S. Pat. No. 6,248,838) which is incorporated herein by reference. In applying this method, the dual initiator approach, wherein two initiators with different thermal stabilities are used to help drive the reaction of DVB further towards completion, was utilized.

(40) The required tests only require a small quantity of particles. The use of a liquid medium (such as the reactor fluid) is unnecessary for the heat treatment of a small sample. Roughly 500 grams of particles were hence removed from the slurry, washed, spread very thin on a tray, heat-treated for ten minutes at 200 C. in an oven in an inert gas environment, and submitted for testing.

(41) The glass transition temperature of these heat-treated particles, and the liquid conductivity of packings thereof, were then measured by independent testing laboratories (Impact Analytical in Midland, Mich., and FracTech Laboratories in Surrey, United Kingdom, respectively).

(42) FIG. 5 provides a process flow diagram depicting the preparation of the example. It contains four major blocks; depicting the preparation of the aqueous phase (Block A), the preparation of the organic phase (Block B), the mixing of these two phases followed by suspension polymerization (Block C), and the further process steps used after polymerization to obtain the as-polymerized and heat-treated samples of particles (Block D).

(43) The following subsections will provide further details on the formulation, preparation and testing of this working example, to enable persons who are skilled in the art to reproduce the example.

(44) B. Formulation

(45) An aqueous phase and an organic phase must be prepared prior to suspension polymerization. The aqueous phase and the organic phase, which were prepared in separate beakers and then used in the suspension polymerization of the particles of this example, are described below.

(46) 1. Aqueous Phase

(47) The aqueous phase used in the suspension polymerization of the particles of this example, as well as the procedure used to prepare said aqueous phase, are summarized in TABLE 1. TABLE 1. The aqueous phase was prepared by adding Natrosol Plus 330 and gelatin (Bloom strength 250) to water, heating to 65 C. to disperse the Natrosol Plus 330 and the gelatin in the water, and then adding sodium nitrite and sodium carbonate. Its composition is listed below.

(48) TABLE-US-00001 INGREDIENT WEIGHT (g) % Water 1493.04 98.55 Natrosol Plus 330 (hydroxyethylcellulose) 7.03 0.46 Gelatin (Bloom strength 250) 3.51 0.23 Sodium Nitrite (NaNO.sub.2) 4.39 0.29 Sodium Carbonate (Na.sub.2CO.sub.3) 7.03 0.46 Total Weight in Grams 1515.00 100.00

(49) 2. Organic Phase

(50) The organic phase used in the suspension polymerization of the particles of this example, as well as the procedure used to prepare said organic phase, are summarized in TABLE 2. Note that the nanofiller (carbon black) was added to the organic phase in this particular example. TABLE 2. The organic phase was prepared by placing the monomers, benzoyl peroxide (an initiator), t-amyl peroxy(2-ethylhexyl)monocarbonate (TAEC, also an initiator), Disperbyk-161 and carbon black together and agitating the resulting mixture for at least 15 minutes to disperse carbon black in the mixture. Its composition is listed below. After taking the other components of the 63% DVB mixture into account, the polymerizable monomer mixture actually consisted of roughly 84.365% S, 5.635% EVB and 10% DVB by weight. The total polymerizable monomer weight of was 1356.7 grams. The resulting thermoset nanocomposite particles thus contained [1006.8/(1356.7+6.8)]=0.5% by weight of carbon black.

(51) TABLE-US-00002 INGREDIENT WEIGHT (g) % Styrene (pure) 1144.58 82.67 Divinylbenzene (63% DVB, 98.5% 215.35 15.56 polymerizable monomers) Carbon black (Monarch 280) 6.8 0.49 Benzoyl peroxide 13.567 0.98 t-Amyl peroxy(2-ethylhexyl)monocarbonate 4.07 0.29 (TAEC) Disperbyk-161 0.068 0.0049 Total Weight in Grams 1384.435 100
C. Preparation of Particles from Formulation

(52) Once the formulation is prepared, its aqueous and organic phases are mixed, polymerization is performed, and as-polymerized and heat-treated particles are obtained, as described below.

(53) 1. Mixing

(54) The aqueous phase was added to the reactor at 65 C. The organic phase was then introduced over roughly 5 minutes with agitation at the rate of 90 rpm. The mixture was held at 65 C. with stirring at the rate of 90 rpm for at least 15 minutes or until proper dispersion had taken place as manifested by the equilibration of the droplet size distribution.

(55) 2. Polymerization

(56) The temperature was ramped from 65 C. to 78 C. in 10 minutes. It was then further ramped from 78 C. to 90 C. at the rate of 0.1 C. per minute in 120 minutes. It was then held at 90 C. for 90 minutes to provide most of the conversion of monomer to polymer, with benzoyl peroxide (half life of one hour at 92 C.) as the effective initiator. It was then further ramped to 115 C. in 30 minutes and held at 115 C. for 180 minutes to advance the curing with TAEC (half life of one hour at 117 C.) as the effective initiator. The particles were thus obtained in an aqueous slurry. FIG. 6 shows the variation of the temperature with time during polymerization.

(57) 3. As-Polymerized Particles

(58) The aqueous slurry was cooled to 40 C. It was then poured onto a 60 mesh (250 micron) sieve to remove the aqueous reactor fluid as well as any undesirable small particles that may have formed during polymerization. The as-polymerized beads of larger than 250 micron diameter obtained in this manner were then washed three times with warm (40 C. to 50 C.) water

(59) 4. Heat-Treated Particles

(60) Three sets of heat-treated particles, which were imposed to different thermal histories during the post-polymerization heat treatment, were prepared from the as-polymerized particles. In preparing each of these heat-treated samples, washed beads were removed from the 60 mesh sieve, spread very thin on a tray, placed in an oven under an inert gas (nitrogen) blanket, and subjected to the desired heat exposure. Sample 10m200C was prepared with isothermal annealing for 10 minutes at 200 C. Sample 40m200C was prepared with isothermal annealing for 40 minutes at 200 C. to explore the effects of extending the duration of isothermal annealing at 200 C. Sample 10m220C was prepared with isothermal annealing for 10 minutes at 220 C. to explore the effects of increasing the temperature at which isothermal annealing is performed for a duration of 10 minutes. In each case, the oven was heated to 100 C., the sample was placed in the oven and covered with a nitrogen blanket; and the temperature was then increased to its target value at a rate of 2 C. per minute, held at the target temperature for the desired length of time, and finally allowed to cool to room temperature by turning off the heat in the oven. Some particles from each sample were sent to Impact Analytical for the measurement of T.sub.g via DSC.

(61) Particles of 14/16 U.S. mesh size were isolated from Sample 40m200C by some additional sieving. This is a very narrow size distribution, with the particle diameters ranging from 1.19 mm to 1.41 mm. This nearly monodisperse assembly of particles was sent to FracTech Laboratories for the measurement of the liquid conductivity of its packings.

(62) D. Reference Sample

(63) A Reference Sample was also prepared, to provide a baseline against which the data obtained for the particles of the invention can be compared.

(64) The formulation and the fabrication process conditions used in the preparation of the Reference Sample differed from those used in the preparation of the examples of the particles of the invention in two key aspects. Firstly, carbon black was not used in the preparation of the Reference Sample. Secondly, post-polymerization heat treatment was not performed in the preparation of the Reference Sample. Consequently, while the examples of the particles of the invention consisted of a heat-treated and carbon black reinforced thermoset nanocomposite, the particles of the Reference Sample consisted of an unfilled and as-polymerized thermoset polymer that has the same composition as the thermoset matrix of the particles of the invention.

(65) Some particles from the Reference Sample were sent to Impact Analytical for the measurement of T.sub.g via DSC. In addition, particles of 14/16 U.S. mesh size were isolated from the Reference Sample by sieving and sent to FracTech Laboratories for the measurement of the liquid conductivity of their packings.

(66) E. Differential Scanning Calorimetry

(67) DSC experiments (ASTM E1356-03) were carried out by using a TA Instruments Q100 DSC with nitrogen flow of 50 mL/min through the sample compartment. Roughly nine milligrams of each sample were weighed into an aluminum sample pan, the lid was crimped onto the pan, and the sample was then placed in the DSC instrument. The sample was then scanned from 5 C. to 225 C. at a rate of 10 C. per minute. The instrument calibration was checked with NIST SRM 2232 indium. Data analysis was performed by using the TA Universal Analysis V4.1 software.

(68) DSC data for the heat-treated samples are shown in FIG. 7. T.sub.g was defined as the temperature at which the curve for the heat flow as a function of the temperature went through its inflection point. The results are summarized in TABLE 3. It is seen that the extent of polymer curing in Sample 10m220C is comparable to that in Sample 40m200C, and that the extent of polymer curing in both of these samples has advanced significantly further than that in Sample 10m200C whose T.sub.g was only slightly higher than that of the Reference Sample. TABLE 3. Glass transitions temperatures (T.sub.g) of the three heat-treated samples and of the Reference Sample, in C. In addition to being an as-polymerized (rather than a heat-treated) sample, the Reference Sample also differs from the other three samples since it is an unfilled sample while the other three samples each contain 0.5% by weight carbon black.

(69) TABLE-US-00003 ISOTHERMAL HEAT SAMPLE TREATMENT IN NITROGEN T.sub.g ( C.) Reference Sample None 117.17 10m200C For 10 minutes at a temperature of 200 C. 122.18 10m220C For 10 minutes at a temperature of 220 C. 131.13 40m200C For 40 minutes at a temperature of 200 C. 131.41
F. Liquid Conductivity Measurement

(70) A fracture conductivity cell allows a particle packing to be subjected to desired combinations of compressive stress (simulating the closure stress on a fracture in a downhole environment) and elevated temperature over extended durations, while the flow of a fluid through the packing is measured. The flow capacity can be determined from differential pressure measurements. The experimental setup is illustrated in FIG. 8.

(71) Ohio sandstone, which has roughly a compressive elastic modulus of 4 Mpsi and a permeability of 0.1 mD, was used as a representative type of outcrop rock. Wafers of thickness 9.5 mm were machined to 0.05 mm precision and one rock was placed in the cell. The sample was split to ensure that a representative sample is achieved in terms of its particle size distribution and then weighed. The particles were placed in the cell and leveled. The top rock was then inserted. Heated steel platens were used to provide the correct temperature simulation for the test. A thermocouple inserted in the middle port of the cell wall recorded the temperature of the pack. A servo-controlled loading ram provided the closure stress. The conductivity of deoxygenated silica-saturated 2% potassium chloride (KCl) brine of pH 7 through the pack was measured.

(72) The conductivity measurements were performed by using the following procedure: 1. A 70 mbar full range differential pressure transducer was activated by closing the bypass valve and opening the low pressure line valve. 2. When the differential pressure appeared to be stable, a tared volumetric cylinder was placed at the outlet and a stopwatch was started. 3. The output of the differential pressure transducer was fed to a data logger 5-digit resolution multimeter which logs the output every second during the measurement. 4. Fluid was collected for 5 to 10 minutes, after which time the flow rate was determined by weighing the collected effluent. The mean value of the differential pressure was retrieved from the multimeter together with the peak high and low values. If the difference between the high and low values was greater than the 5% of the mean, the data point was disregarded. 5. The temperature was recorded from the inline thermocouple at the start and at the end of the flow test period. If the temperature variation was greater than 0.5 C., the test was disregarded. The viscosity of the fluid was obtained from the measured temperature by using viscosity tables. No pressure correction is made for brine at 100 psi. The density of brine at elevated temperature was obtained from these tables. 6. At least three permeability determinations were made at each stage. The standard deviation of the determined permeabilities was required to be less than 1% of the mean value for the test sequence to be considered acceptable. 7. At the end of the permeability testing, the widths of each of the four corners of the cell were determined to 0.01 mm resolution by using vernier calipers.

(73) The test results are summarized in TABLE 4.

(74) TABLE 4. Measurements on packings of 14/16 U.S. mesh size of Sample 40m200C and of the Reference Sample at a coverage of 0.02 lb/ft.sup.2. The conductivity (mDft) of deoxygenated silica-saturated 2% potassium chloride (KCl) brine of pH 7 through each sample was measured at a temperature of 190 F. (87.8 C.) under a compressive stress of 4000 psi (27.579 MPa).

(75) TABLE-US-00004 Time Reference Sample Time Sample 40m200C (hours) Conductivity (mDft) (hours) Conductivity (mDft) 27 1179 45 1329 49 1040 85 1259 72 977 109 1219 97 903 133 1199 120 820 157 1172 145 772 181 1151 168 736 205 1126 192 728 233 1110 218 715 260 720

(76) These results are shown in FIG. 9. They demonstrate clearly the advantage of the particles of the invention in terms of the enhanced retention of liquid conductivity under a compressive stress of 4000 psi at a temperature of 190 F.