GB2517516A - Apparatus and method for sand retention testing - Google Patents

Abstract

Apparatus for sand retention testing, comprising a fluid chamber 25 with an outlet to a test apparatus, a piston 20 urging fluid contained in the chamber towards the outlet, and a mixer to mix the fluid prior to passage of the fluid through the outlet. The mixer comprises a rotating impeller 30 driven in rotation by a motor shaft 16, which passes through the piston, and is arranged coaxially with the piston. The mixer can comprise two impellers arranged to be axially slidable relative to one another to permit axial relative movement impellers during axial travel of the piston.

Description

Auparatus aid method for sand retention testing The present invention relates to apparatus and a method for sand retention testing.

In certain aspects) the invention relates to reactor vessel for use in sand retention testing.

In oil and gas wells, valuable hydrocarbons locked in an underground reservoir are recovered to surface by drilling a wellbore into the formation and flowing the production fluids containing the valuable hydrocarbons to the surface through production tubing "Production fluids" is a term used to refer to all fluids flowing from a production zone in the formation, and while production fluids flowing into the wellbore from the formation will normally contain a high proportion of usable hydrocarbons, they will usually also contain less useful components, such as particulate material comprising fine particles of sand, rock and fines etc, which may be suspended in the production fluids. Such particulate material is commonly referred to as "sand" in the art, although it will typically include many different particulates other than sand. In this specification, the term "sand" and related terms will be understood to refer to particulate material generally. In wells susceptible to sand production, it is desirable to contain the sand in the formation as far as possible, because allowing the sand into the wellbore and transporting the sand to the surface with the usable components of the production fluids adds to the cost of recovery, reducing the profitability of the well, and accumulation of the sand in the wellbore causes significant wear on the wellbore components. Sand can also settle out of suspension in the wellbore, and reduce the capacity of the conduits for recovery of usable production fluids. Also, if sand is recovered to the surface and removed only after the recovery of the production fluids, it must then be treated to reduce its environmental impact before disposal, which also adds to the cost It is known to deploy sand screens and gravel packs between the production tubing and the formation, to permit the passage of useful production fluids into the production tubing in the well, but to exclude particulate materials from the wellbore as much as possible. The sand screen must be matched to the conditions in the wellbore, as mobile sand has a widely ranging particle size distribution (PSD]. Selection of the correct sand screen can make the production of a sand-producing well viable, and can reduce the environmental impact of bringing the sand to the surface before separating it from the production fluids. Conversely, screens with a pore size that is too large or too small to suit the formation conditions can admit too much sand, or exclude valuable production fluids due to choking.

Laboratory testing can be a useful way to simulate sand production control and screen efficiency, which helps with the selection of a suitable screen for the wellbore. A suitable screen can then typically be selected based on the different parameters measured and results obtained during the laboratory experiments.

However, it is not easy to simulate realistic reservoir conditions in laboratory-based sand control tests.

According to the present invention there is provided apparatus for sand retention testing, the apparatus comprising a chamber for containing fluid to be tested, the chamber having: an outlet leading to a test apparatus; a piston configured to urge the fluid contained in the chamber towards the outlet; and a mixing device adapted to mix the fluid within the chamber prior to passage of the fluid through the outlet Typically the fluid contained with the chamber can be a multiphase fluid, comprising a suspension of solid particles in a liquid. Optionally the fluid in the chamber can be a heterogeneous mixture of liquids and solids while in the chamber, but typically the mixing device mixes the materials as they pass through the outlet whereby the fluid emerging from the outlet is substantially homogeneous, typically a substantially homogeneous suspension with substantially even flow characteristics.

Typically the outlet delivers the fluid to be tested to a sand control apparatus to be tested, such as a sand screen or a gravel pack. The particular sand control apparatus being tested is not central to the invention, and in various aspects, the invention can be used with different testing apparatus, such as core holders or pressure vessels.

Other apparatus can be used. Typically, testing of the sand control apparatus involves flowing a suspension of sand (typically 0.01 to 15% by volume, for example, 0.1% to 10% by volume and in certain embodiments 1% to 5% by volume) through the apparatus being tested, typically the sand screen, and if applicable the gravel pack. In certain examples, the sand control apparatus being tested incorporates a number of sequential screens and gravel packs to which the flow of suspended sand is tested. Differential pressure is typically measured across the system, and typically across the sand control apparatus being tested. Effluent passing through the screen or other sand control apparatus being tested is collected and typically analysed to determine the concentration of solids passing through the sand control apparatus, and the particle size distribution thereof.

Typically, the chamber is formed of metal, and has seals capable of withstanding high pressures and temperatures similar to those encountered at the formation of the well. Typically the fluid being used to suspend the sand for testing of the sand control apparatus can have high viscosity, similar to the rheological properties of the formation fluids prevailing in the well. Typically flow rates through the apparatus are also similar to the flow rates encountered in the well, and can be relatively high, typically in the order of litres/mm e.g. 0.05 I/rn -5 or 10 l/m, for example, 2-5 I/rn. However, the flow rate is typically set at a value that is similar to the flow rate prevailing in the wellbore conditions. This in situ flow rate can vary greatly, so different embodiments of the invention can function equally well in different flow rate regimes which reflect different wellbore conditions. For example, in certain cases, the flow rate can optionally be greater than S l/m, for example up to 10 l/m or more. In many cases the power and capacity of the pump can limit the maximum flow rate and hence with higher capacity pumps certain embodiments of the invention can optionally function at different higher flow rates reflective of the actual wellbore conditions in situ.

Typically the apparatus incorporates sensors configured to measure and optionally record flow rate, pressure, temperature and/or other parameters in the chamber, and in the outlet. Optionally similar sensors can be incorporated in the sand control apparatus, so that variations in flow rate, pressure, temperature and other parameters can be measured across different components of the apparatus, allowing the calculation of differentials across the sand control apparatus for

example.

Typically the mixing device comprises at least one rotating impeller. Optionally, the mixing device is located at or adjacent the outlet of the chamber, typically at an end cap assembly, which typically also includes the outlet Typically the outlet comprises a fluid passage formed through the end cap assembly. Typically the outlet has a valve which is selectively operable, to admit fluids through the outlet when open.

Typically the valves controlling fluid flow through the conduit avoid radial incursions into the flow path when open, thereby retaining a consistent area of flow path along the conduits and reducing the tendencies for pressure drops as a result of bottle necks or the like.

Typically the outlet is located at the top of the chamber, and the reactor vessel is operated on end so that the fluids are urged towards the outlet by the piston in an upward direction.

Typically the mixing device is driven in rotation by a motor shaft, which typically passes through the piston, and is optionally coaxial with the piston. Typically the motor shaft is coaxial with the axis of the chamber. Typically the motor shaft is connected to an axle, which supports the mixing device. Typically the mixing device rotates relative to a baffle, which can optionally be mounted on a fixed portion of the chamber for example on an end cap or on a wall of the chamber, and can optionally remain stationary relative to the rotating impeller.

Typically the mixing device can comprise 1st and impeller devices. The 1st and 2nd impeller devices can optionally be arranged to be axially slidable relative to one another to permit axial relative movement of the P and 2" impeller devices during axial travel of the piston. Optionally the 1st and 2" impeller devices can be rotationally connected, to transmit torque from one to the other. Optionally the Pt and 2 impeller devices can be rotationally connected by a sliding carriage.

Optionally the mixer device can maintain rotational movement generate thrust in

S

the fluid within the chamber along substantially the entire length of the chamber while the piston is moving axially to displace fluid from the outlet.

Optionally, the fluid can be a suspension of particulate matter in a liquid. The particulate matter can comprise a suspension of sand from an oil or gas well formation, and the fluid can comprise a reservoir fluid, which can optionally be recovered from an oil or gas well, and which may contain hydrocarbons. Optionally the fluids can be a simulated or artificial reservoir fluid.

Optionally, surfaces of the apparatus that are exposed to the fluids are provided with a hard facing to resist erosion. Typically the hard facing can be a coating applied to the surface, or a treatment. A suitable hard facing or coating can comprise tungsten carbide or a similar coating.

The invention also provides a method for delivering fluid to a testing apparatus for sand retention testing, the method comprising containing the fluid in a chamber, the chamber having: an outlet leading to a test apparatus; a piston configured to urge fluid contained in the chamber towards the outlet; and a mixing device adapted to mix the fluid within the chamber prior to passage of the fluid through the outlet, and wherein the method includes: activating the piston to urge the fluid through the outlet; and mixing the fluid as it passes into the outlet.

According to the invention, the apparatus allows agitation and suspension of larger volumes of sand in fluid than in previous testing, whilst operating at high pressures (e.g. typically up to 3000psi and optionally above) and high temperatures (typically up to 120°C and optionally above). The mixer delivers a suspension of sand to the flow-path that can then be passed through the screen at a selected rate. Certain examples of the invention provide a continuously stirred reactor which allows performance of laboratory sand control test at high pressure, high flow rates and high temperatures.

The various aspects of the present invention can be practiced alone or in combination with one or more of the other aspects, as will be appreciated by those skilled in the relevant arts. The various aspects of the invention can optionally be provided in combination with one or more of the optional features of the other aspects of the invention. Also, optional features described in relation to one aspect can typically be combined alone or together with other features in different aspects of the invention.

Various aspects of the invention will now be described in detail with reference to the accompanying figures. Still other aspects, features, and advantages of the present invention are readily apparent from the entire description thereof, including the figures, which illustrates a number of exemplary aspects and implementations. The invention is also capable of other and different examples and aspects, and its several details can be modified in various respects, all without departing from the spirit and scope of the present invention. Accordingly, the drawings and descriptions are to be regarded as illustrative in nature, and not as restrictive. Furthermore, the terminology and phraseology used herein is solely used for descriptive purposes and should not be construed as limiting in scope.

Language such as "including," "comprising" "having" "containing," or "involving," and variations thereof, is intended to be broad and encompass the subject matter listed thereafter, equivalents, and additional subject matter not recited, and is not intended to exclude other additives, components, integers or steps. Likewise, the term "comprising" is considered synonymous with the terms "including" or "containing" for applicable legal purposes.

Any discussion of documents, acts, materials, devices, articles and the like is included in the specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention.

In this disclosure, whenever a composition, an element or a group of elements is preceded with the transitional phrase "comprising", it is understood that we also contemplate the same composition, element or group of elements with transitional phrases "consisting essentially of", "consisting", "selected from the group of consisting of", "including", or is" preceding the recitation of the composition, element or group of elements and vice versa.

All numerical values in this disclosure are understood as being modified by "about".

All singular forms of elements, or any other components described herein are understood to include plural forms thereof and vice versa. References to directional and positional descriptions such as upper and lower and directions e.g. "up", "down" etc. are to be interpreted by a skilled reader in the context of the examples described and are not to be interpreted as necessarily limiting the invention to the literal interpretation of the term, but instead should be as understood by the skilled addressee.

BRIEF DESCRIPTION OF THE DRAWINGS

A detailed description of certain examples of the present invention follows, with reference to the attached drawings, wherein: Figures 1 schematically illustrates a first sand retention testing apparatus in a first configuration with the piston retracted; Figure 2 schematically illustrates the figure 1 apparatus in a 2nd configuration with the piston extended; Figure 3 schematically illustrates a plan view from beneath of the piston in the apparatus shown in figures 1 and 2; Figure 4 schematically illustrates a plan view from above of the base plate and mixing device in the apparatus shown in figures 1 and 2; Figure 5 schematically illustrates a second sand retention testing apparatus in a 1st configuration with the piston retracted;

B

Figure 6 schematically illustrates the apparatus of fig 5 in a 2nd configuration with the piston extended; Figure 7 schematically illustrates a 3rd sand retention testing apparatus in a 1st configuration with the piston retracted; and Figure 8 schematically illustrates the fig 7 apparatus in a 2nd configuration with the piston extended.

DETAILED DESCRIPTION

Referring now to the drawings, figures 1-4 show a 1st example of a testing apparatus for sand retention testing. In the 1st example, the apparatus comprises a reactor vessel 1 having a chamber 25 with a capacity of around 5-101 formed by a generally cylindrical casing 5 with a hollow bore being closed at each end, and having an overall capacity of around 10-201. The reader will appreciate that these values are exemplary and not binding. Typically the reactor vessel 1 is operated in a vertical orientation, with the axis of the bore generally parallel to the vertical axis.

At the upper end, the bore of the casing 5 is typically closed by art top plate 7, which is sealed within the bore by means of an annular seal such as an 0-ring or similar, and is held in place against an internal shoulder extending radially into the bore and facing the upper end by an end cap 6, the end cap 6 having an internal thread that cooperates with an external thread on the outer surface of the upper end of the casing 5. The end cap 6 screws onto the casingS, holding the top plate 7 in place against the shoulder, and sealing the upper end of the reactor vessel 1. The top plate 7 has a central aperture that is coaxia' with the bore, and which is adapted to receive and support an axle 15 (which will be described below) within a bearing 9.

The top plate 7 also has an outlet aperture 8 that is offset from the central aperture, and which permits flow of fluids out of the chamber 25. Typically the aperture typically discharges fluids to a sand screen or other sand control device, typically at a defined flow rate, and with a defined particle size distribution of sand dispersed in the fluid. According'y, the sand screen being tested can be subjected to defined conditions determined by the conditions in the chamber 2S.

At the lower end of the reactor vessel 1, the bore is closed by a base plate 11 held in place by a lower end cap 10 against an internal shoulder, and sealed in the bore in the same manner as the end cap 6 seals the top plate 7 at the upper end. The base plate 11 has a central bore that is coaxial with the aperture in the top plate 7, and coaxial with the bore of the casingS, and which optionally accommodates a bearing plate 12 providing an internal bearing surface for a motor shaft 16 arranged coaxially with the casing 5, the base plate 11, and the top plate 7. The motor shaft 16 extends from a motor assembly (not shown] arranged below the reactor vessel 1, and its upper end engages the lower end of the axle 15, with which it is coaxial, and which optionally extends through a set of bearings in the top plate 7. Accordingly, rotation of the motor shaft 16 driven by the motor assembly rotates the axle lb within the chamber 25. Typically, the lower end of the axle ibis supported in a set of bearings 17 on an inner end of the base plate 11, which typically has a top hat structure extending axially into the bore of the casing 5, to accommodate the interface between the motor shaft 16 and the axle 16 and the bearings 17 to support the lower end of the axle 15.

The axle 15 has, at its upper end, adjacent to the top plate 7, a mixing device in the form of an arrangement of 4 impeller plates 30 each having one inner end connected to the outer surface of the axle 15, and one free outer end. The plates 30 are typically in generally rectangular form, and rotate with the axle 15, being driven by the motor shaft 16 and motor assembly from outside the reactor vessel 1. The impeller plates 30 agitate the fluid within the chamber 25 in the region of the outlet 8, just as the fluid is passing into and through the outletS from the chamber 25.

Typically, there are 4 impeller plates 30, but optionally, other examples can incorporate a different number of plates 30, or the impellers can take different forms.

In this example, the reactor vessel 1 has a number of baffles 32 that are fixed in position within the chamber 25, and typically take the form of extending from the inner surface of the wall of the casing S radially inwards towards the axle 15. In this example, the baffles 32 are typically fixed, for example by welding or by other methods, to a fillet ring 4 which is optionally fixed to the top plate 7, but in other examples, the baffles 32 can be fixed, for example by welding, to the inner surface of the wall of the casing 5. The baffles 32 typically remain fixed in position within the chamber, as the impellers 30 rotate with the axle 15, thereby mixing the fluids within the chamber 25. Typically three baffles 30 are shown in this example, but different examples can dispense with baffles altogether, or can incorporate different numbers or arrangements of baffles.

The reactor vessel 1 incorporates a piston 20 which is axially movable within the bore of the casingS, and which is shown in fig 1 at the lower end of the reactor vessel 1. The volume of the chamber 25 formed between the upper end of the piston 20 and the top plate 7 is at its maximum when the piston 20 is in the lower position shown in figure 1. As the piston 20 moves axially upwards within the bore of the casing 5, the volume of the chamber 25 decreases to its minimum vcilume as shown in figure 2, when the upper end of the piston 20 has travelled its maximum axial distance within the bore of the casing 5. The piston 20 has at least one, and typically 3 piston legs 21 which extend axially from the upper end of the piston 20, towards the upper end of the reactor vess& 1. The piston egs 21 are circumferentially spaced apart in a generally even manner around the upper end of the piston, and are radially spaced from their central axis, as best shown in figure 3.

The piston legs 21 control the spacing between the piston 20 and the end plate 7 when the piston 20 is in its uppermost position shown in figure 2. The upper ends of the piston legs 21 are set at a wider radial spacing than the outer ends of the impeller plates 30, but at a narrower radial spacing than the baffles 32. Thus, in the fig 2 position, the legs 21 slot into the radial space between the impeller plates 30, and the baffles 32 to engage the top plate 7 and space the piston 20 away from the top plate 7, to permit the continued rotation of the impeller plates 30. Since the legs 21 are radially spaced from the plates 30 and the baffles 32, the legs 21 engaging the top plate 7 does not affect the rotation of the plates 30, which can typically continue to rotate unimpeded by the piston legs 21 while the piston 20 is in the figure 2 position.

The piston 20 typically has a top hat structure with a central axial bore. The lower end of the bore is typically provided with an axially extending recess on the inner surface to receive the upper end of the base plate 11. The upper end of the bore of the piston 20 is typically narrower, to receive and support the axle 15. Typically the axle 15 is driven in rotation within the central bore of the piston 20, which freely moves axially along the axle 15. Typically the piston 20 is provided with at least one and optionally two radial flanges which extend radially outwards and engage the inner surface of the casing 5. Typically the radial flanges are sealed in at least one position against the inner surface of the casing 5. Typically the lower radial flange of the piston 20 is sealed against the inner surface of the casing 5.

The piston 20 is urged axially within the reactor vessel 1 by hydraulic force, which is typically provided by hydraulic fluid supplied from a pump or [not shown] which is typically mounted below the reactor vessel 1. The hydraulic fluid [which can be liquid or gas) is supplied through a port up within the base plate 11 into a chamber at the lower end of the reactor vessel 1, extending axially between the base plate 11 and the sealed radial flange of the piston 20. A pressure differential is generated across the piston 20 by the supply of the hydraulic fluid into this chamber, which urges the movement of the piston axially from the fig 1 position at the lower end of the reactor vessel 1, to the fig 2 position at the upper end of the reactor vessel 1.

Equalisation ports are typically provided in the motor shaft casing provided by the top hat structure of the base plate 11 in order to reduce the tendency for hydraulic lock. Typically the equalisation ports can be provided with a valve such as a check valve. Similar equalisation ports can be provided on the axle 15 and motor shaft 16 in order to reduce the tendency of pressure differentials to affect the rotational movement of the motor shaft 16.

Coolant is typically injected into the casing of the motor shaft 16 through a coolant port 13, which typically circulates coolant through the motor shaft 16 and the axle (which is typically hollow for this purpose) via ports.

In use, a suspension of sand in fluid is loaded into the chamber 25. The sand is typically maintained in suspension in the fluid by the rotation of the impellers 30, driven by the motor shaft 16 and motor assembly via the axle 15. The suspension of sand in the fluid is typically delivered in a homogenous state through the outletS to the sand control apparatus being tested. This is typically achieved by the supply of hydraulic fluid under pressure through the port Up to the back of the piston 20, which drives the piston 20 axially within the reactor vessel 1, discharging the fluid within the chamber 25 through the outlet 8, as the mixing device maintain homogeneity of the suspension during discharge from the chamber 25. Typically sensor packs 8s within the chamber 25 (or in communication with the interior of the chamber 25] can report and optionally record the temperature, pressure, particle size distribution, and/or flow rate of the fluid suspension prevailing within the chamber 25. Typically the fluid is displaced through the outlet 8 at a relatively high pressure, for example 3000 to 5000 psi. The discharge of fluid from the chamber through the outletS is typically rapid, and the fluid suspension typically reaches the sand control apparatus being tested in a relatively homogenous state, without substantial settling of the sand suspended in the fluid. Therefore, the pressure and flow rate, and optionally temperature and other parameters of the sandy fluid being supplied to the sand control apparatus being tested is typically relatively consistent, allowing different conditions to be stimulated in the laboratory for testing of the sand control apparatus.

The motor shaft 16 is positioned across the reactor 1 and drives the rotation of the axle 15 and impellers 30 to keep the solid particles in suspension. The shaft is typically driven by a variable speed motor.

The conduits connecting the outletS with the sand control apparatus being tested are typically provided with high pressure and high temperature seals, and typically avoid unnecessary diversions of the axial flow of fluid through the conduit, thereby helping to retain the sand in suspension within the fluid, and avoiding decelerations in the fluid which may tend to induce settlement of the sand out of suspension before reaching the sand control apparatus being tested.

Optionally, surfaces of conduit, valves and other components of the reactor vessel 1 exposed to the test fluids are provided with a hard facing to resist erosion. Seals are typically recessed. Typically bore diameters of conduits are kept relatively constant to resist pressure drops resulting from changes in diameter and volume of conduits.

Typically the distances between the outlet of the chamber and the inlet of the sand control apparatus being tested are minimised in order to reduce the tendency of the suspension of sand to settle during transit between the reactor vesse' 1 and the sand control apparatus being tested. Typically the fluid is used for suspension of the sand in the chamber 25 can be water) but optionally other more viscous fluids can be used in order to simulate highly viscous production fluids prevailing in the wellbore.

A typica' sand control apparatus being tested is shown in figure 9. The suspension of sand in fluid discharged from the outletS typically passes through conduits to the inlet 40i of a sand control test apparatus 40 in the form of a combination sand screen 41 and gravel pack 42. The pressurised suspension of sand in fluid enters the test chamber 43, and a sensor pack 44 typically senses and optionally records the temperature, pressure, particle size distribution, and/or flow rate of the fluid suspension. The fluid suspension is then passed through the gravel pack 42, and then through the sand screen 41. The effluent discharged from the downstream end of the sand screen 41 is analysed by the sensor pack 45, which typically senses and optionally records the temperature, pressure, particle size distribution and/or flow rate of the fluid suspension as it leaves the outlet 40o of the test apparatus 40.

Differentials in the parameters being sensed can then be measured by comparison of the data gathered by the sensor packs 44, 45, allowing assessment of the suitability of the sand screen and gravel pack for the particular conditions prevailing in the chamber 25. Optionally additional sensor packs can typically be provided in other locations, for example between the gravel pack and the sand screen, to provide data for calculation of differentials between intermediate locations.

Referring now to figures 5 and 6, a 2nd example of a reactor vessel 101 is shown similar to the 1st reactor vessel 1, and having a casing 105 top plate 107, base plate 111 with port lip and bearing plate 112, end caps 106, 110, an axle 115, impellers 130, baffles 132, fillet 104 and bearings 109 and 117. These components of the reactor vessel 101 are generally similar to the corresponding components of the reactor vessel 1 described above, and have similar reference numbering increased by 100. The reader is referred to the above description of these components for a fuller understanding of the structure and function, which will not be repeated here for the sake of brevity.

The 2nd example shown in figures 5 and 6 differs from the 1st example shown in figures 1 to 4 in that the motor shaft 116 engages the axle 115 at the upper end of the reactor vessel 101 rather than at the lower end. Equalisation ports and coolant injection ports 113 are provided at the upper end of the axle 115, to cool the axle and resist the effects of hydraulic lock as a result of pressure differentials within the reactor vessel 101. In other respects, the structure and function of the 2nd example of the reactor vessel 101 is as described above in relation to the 1st example of the reactor vessel 1.

Arranging the impeller blades 130, 30 immediately adjacent to the outlet 108, 8 mixes the fluid just as it passes into the outlet and leaves the chamber, which allows certain examples of the invention to deliver a more homogenous mixture of fluid suspension, which is typically less prone to settling of sand during transit between the reactor vessel and the apparatus being tested. This is particularly effective in conjunction with providing the outlet 108 at the upper end of the reactor vessel 101, as there is little or no tendency for the sand to settle under gravity as it leaves the reaction chamber. Mounting the motor shaft 116 on the upper surface of the reaction chamber to engage with the axle 115 through the top plate facilitates the vertical mounting of the reaction vessel in this manner, and leads to more consistency in the particle size distribution of the fluid passing through the outlet 108. It will be appreciated that all examples described herein can be operated in different orientations, with the outlet at the top or the bottom of the vessel, and in each case with the motor a the top of the bottom of the vessel, without departing from the scope of the invention.

Referring now to figures 7 and 8, a 3rd example of a reactor vessel 201 is shown similar to the 1st reactor vessel 1, and having a casing 205 top plate 207, base plate 211 with port 211p and optional bearing plate) end caps 206) 210) an axle 215, and bearings 209 and 217. These components of the reactor vessel 201 are generally similar to the corresponding components of the reactor vessel 1 described above) and have similar reference numbering increased by 100. The reader is referred to the above description of these components for a fuller understanding of the structure and function) which will not be repeated here for the sake of brevity.

In the 3rd example, the axle 215 can optionally be rotated from the lower end of the reactor vesse' 201 as described in relation to the 1st example, or from the upper end of the reactor vessel 201, as described in relation to the 2nd example.

The 3rd example described herein differs from the 1st and 2nd examples in respect of the mixing device. In the 3rd example, the reactor vessel is provided with a mixing device in the form of at least 2 axially overlapping impeller devices which are typically arranged concentrically on the same axis, typically each at a different radial spacing away from the axis of the axle, and typically able to axially overlap so that with the piston in the lower configuration shown in figure 7 the degree of axial overlap between the two impeller devices 230, 232 is relatively small, and is less than when the piston is in the upper configuration shown in figure 8, where the axial overlap between the impeller devices is greater. Typically, the axially overlapping arrangement of impeller devices permits increased consistency of mixing of the fluid within the chamber all along the axith length of the chamber between the piston 220 and the top plate 207, and leads to a higher consistency of fluids that leave the chamber through the outlet for passage through the testing apparatus.

Typically the 1st impeller device 230 is rotationally connected to the axle 215.

Typically the 1st impeller device 230 comprises a number (for example 3] of impeller blades having a generally rectangular configuration and arranged generally parallel to the axle 215. Typically the impeller blades are circumferentially spaced around the axle) and are typicafly fixed to the axle 215 at end blocks via fixings such as screws, boils etc. As the axle 215 rotates under the force of the motor shaft, the impeller blades of the 1st impeller device 230 are driven in rotation around the axis of the axle 215 at the same speed and in the same direction as the axle 215. The impeller blades of the 1st impeller device 230 typically retain their axial position within the chamber during the movement of the piston between the 1st and 2nd configurations shown respectively in figures 7 and 8.

Typically the 2nd impeller device 232 is in the form of a number (for example 3) of impeller blades. The blades of the 2" impeller device are typically mounted on slides that are connected to the radially outermost edge of the 1t impeller device, so that the Pt and 21K1 impeller devices are rotationally connected together (whereby torque applied to the 1 impeller device is transferred to the 2 impeller device), but the two impeller devices are axially slidable relative to one another. Therefore, while the two impeller devices are rotating in the chamber, the 2nd impeller device 232 can also move axially within the chamber 225 with the piston 220, as it slides axially relative to the Pt impeller device, which remains axially static within the chamber. The 2'' impeller device can therefore also typically move rotationally relative to the piston 220, which typically travels axially within the chamber without rotation. Typically, the 2nd impeller device 232 is radially spaced further away from the axis of the axle 215 than the 1st impeller device 230. In other words, the 1st and 2nd impeller devices are typically arranged in a concentric manner, with the 1st impeller device arranged concentrically inside the radial space within the 2nd impeller device) and able to slide axially therein. Thus, since the 2nd impeller device 232 is axially slidable on the 1st impeller device 230, and both impellers are mounted on the rotating ax'e 215, as the piston 220 moves axially through the bore of the reactor vess& 201, the two impefler devices 230, 232 collapse axiafly while rotating and thereby maintaining the rotational movement of the mixing device to gcncrae thrust in the fluid within the chamber 225 along the entire length of the chamber 225 between the piston 220 and the top plate 207, while the piston is moving axially to displace fluid from the outlet 208. Therefore, since mixing is taking place all along the axial length of the chamber while the piston 220 is moving) the 3rd example can optionally deliver a more consistent particle size distribution of sand suspended in fluid through the outlet 208 for the duration of the test. The increased consistency in the particle size distribution in the fluid in particular improves the test results, because the operator of the test equipment can be more certain that any variables observed in the measured parameters between the inlet and outlet of the sand control device are less likely to be artefacts resulting from inconsistencies in the particle size distribution or other parameters of the fluid leaving the reactor vessel through the outlet thereof. As before, although the 3' example has been described in a vertical orientation with the outlet at the top and the motor at the bottom, it can be operated in different orientations, with the outlet at the top or the bottom of the vessel, and in each case with the motor at the top or the bottom of the vessel, without departing from the scope of the invention.