The present invention relates to the measurement of cement properties when cementing a well casing, and particularly to the automated measurement of oemeni viscosity without the use of laboratory equipment.

In a normal drilling process, a bore is doled into the ground using a drilling head attached to a bello dri string, Doing fluid, typically a special mud referred to as drilling mud, is pumped down t e drill string and used to cool and lubricate the drilling bit, carry the ock cuttings back to the surface, and maintain a suitable pressure in the borehole to stabilise to the borehole walls. Once the hole extends past the deepest freshwater aquifer (typically 100 to 300 metres), the drill string and drilling head are removed and replaced with a pipe, sailed a casing,

Next, cement slurry is pumped into the casing, and then drilling mud is pumped in behind the cement slurry to force the cement slurry down through the inside of the casing, out ivough a casing shoe at the bottom of the casing, and up into the annulus between the casing and the borehole wail. As it Is forced into the annulus, the cement slurry pushes the drilling mud out of the annulus and fills this space, where it sets. This cement provides a bend which fixes the casing in place and prevents any fluids moving between the casing and the borehole. This cementing process Is referred to as "primary cementing ^* .

Multiple casing sections are usually re uired to reach the desired well depth., and the \u of the casings used will depend on the geology of the area and the depth of the well. Typical casings used in oil or gas wells include: a conductor casing; a surface casing; one or more intermediate casings; and a production casing.

To install each subsequent casing, a smaller drilling head I lowered through the previous casing and a narrower bore is drilled through th cement at the bottom of the casing and Into the ground below. As above, once the hol extends to the desired depth, the drill string and drilling head are removed and replaced with the next stage of casing, which Is then cemented by the same process.

In order to ensure correct cementing, manual measurements of the properties of a test batch of cement are made in a laborator prior to the primary cement operation. Viscosity is Important property because a cement slurry having foe high a viscosity cannot be propedy pumped down th casing and up into the anftulus, but a cement having too low a viscosity can undesirably mix with the fluids In front of or behind the cement slurry as it is pumped down the casing.

During laboratory tests, s all samples are mixed to a recipe that will fee used in the cementing operation, anil then tested. However, during the cementing operation itself, there is often no possibility to measure these properties because the cement is usually pumped directly into the relevant operation after production.

In some operations, one or more samples may be taken after mixing the cement slurry and before the slurry ha been pumped into the well However, these measurements may not be representative of the mixture due to variation across the volume of the cement m ixture {such as Sue to incomplete mixing) or due to changes of the properties of the cement over time (such as due to setting of the cement).

At least the preferred embodiments of the present invention seek to solve these problems.

The present invention provides a method of monitoring one or more properties of a cement slurry during cementing of an oil o gas well, the method comprising: directing the cement slurry along a conduit to a cementing location; measuring a first pressure loss along first portion of the conduit; and calculating a viscosity of the cement slurry ased at least in p&tt on the first pressure loss.

This method enables the automated monitoring of the viscosity of the cement slurry after the final slurry has been mixed, but before it reaches the cementing operation, and without the need for samples to be tested in a laboratory. .Furthermore, the monitored value is less susceptible to inaccuracies due to changes wit time, as the viscosity Is measured only shortl before supply to the cementing operation, or across the volume of the cement slurry, as ail of the cement slurry passing through the condui is analysed.

Whilst the method may be applied to any well cementing operation, It is particularly applicable to primary cementing operations. Primary cementing is defin as the cementing required for constructing and drilling of a well. Other well cementing operations could, for example, include abandonment of a well or repairs to existing cementing of the well .

The calculation may further be based at least in part on a flow rate of the cement slurry along the conduit. For example, the cement slurry may be pumped alortg the conduit, for example using a pump. Flow rate data from the pump may be used fo the calculation. The method may further comprise; determining, based on the calculated viscosity value, the value thai would be output by rotational viscometer, and preferably a coaxial cylinder rotation viscometer testing the cement slurry; in various embodiments, the simulated viscometer may b a Ceuefte viscometer, such as a FAN ® 35 vSsconief e ,

Many industrial standards are defined in terms of measyrements output by a coaxial cylinder rotational viscometer, rather Iftart. an SI viscosity. Therefore, converting the measured viscosity into an equivalent output from a rotational viscometer (La a angle) facilitates compariso of these outputs to the existing standards.

i one embodiment the first portion of the conduit may be substantially horizontal. This configuration allows analysis of data that is independent of the density of the cement slurry , and thus facilitates the calculation of th viscosity of the slurry using only a single pressure {mea ure e t (although other measurement could stitt be used to refine the calculation).

Preferably, the flcst port ion of the conduit is at an angle to the horizontal of less than S ^v , and preferably less than 2*, and most preferabl less than V,

The method preferably further comprises: measuring a second pressure loss along a second portion of the ..conduit, the first portion of the conduit being at a first angle with respect to horiKontat and the second portion of the conduit being at second, different angle wit respect to horizontal, whe ein the viscosity of the cement slurry Is calculated based on the first pressure loss and the second pressure loss.

The measurement of a second pressure loss at a different angle allow the system to separate the effects of density from those of viscosity, thus enabling viscosity to be calculated without requiring additional Inputs, although additional data from other sources may again still be used to refine the calculation.

The secon portion of the conduit is preferably at an angle of at least 45" from the horizontal, and is preferably substantially vertical. In various

embodiments, the second portion of the conduit is at an angle to the vertical of less tha S", and preferably less than 2\ and most preferably less than i°.

The method may further comprise: calculating a density of the cement slurry based on the first pressur loss and the second pressure loss. The use of two pressure losses allows the effects of density and viscosity to be separated. The ^■ density.' of the cement slurry ma be another useful factor for determining abnormal cement properties.

The method preferably comprises; comparing the calculated viscosity value to a pre-determined viscosity value; and takin an action when a difference between the calculated viscosit value and the pre-deter ined viscosit value exceeds- a threshold,

.Similarly, the method may comprise: . comparing the calculated density value to a pre-determined density value; and taking an action when a difference between the calculated density value and the pre-determined density value exceeds a threshold.

That is to say, if an abnormal or unexpected property of the cement is defected, then suitable action may be taken. For example, the flow rate of the cement slurry may be decreased in order to reduce the shear rate of the cement slurry. In extreme cases,, the action ma he to stop the cementing operation. For smaller abnormalities, the action may include recording details of the abnormality for later analysis.

The method may further comprise: changing the flow rate of the cement slurry pumped along th conduit; and determining a second viscosity of the cement slurr at the new How rate.

The cement viscosity varies with respect to its shear rate. Therefore, fe changing the flow rate of the cement through the conduit, ft is possible to measure the viscosity of the cement at different shear rates. This provides furthe

Information for detecting abnormal properties of the cement.

in some embodiments, the method may further comprise measuring the temperature of the cement slurry, prefe aoy within the conduit. Viscosity varies significantly with temperature, and therefore a viscosity measurement is preferably accompanied by a corres onding temperature measurement.

In further embodiments, the method may further comprise: adjusting the calculated viscosity bas d on the measured temperature of the cement slurry. For example, the calculated viscosity may be adjusted to give an equivalent viscosity for a reference temperature different from the measured tem erature.

Typically, the variou cement standards will define the acceptable viscosity of the cement slurry at a particular reference tem erature. Using Known techniques and assumptions regarding viscosity variation with temperature, It is possible to use the measured temperature to determine what the equivalent viscosity of the cement siu fry ould be at the reference temperature; whic can then be corrspared to the relevant standard.

Viewed: from ano her aspect, the invention ca also be seen to ovide system configured to perform the method d scried abov . The present invention therefore also provides a system for monitoring one or more properties of a cement slurry, the system comprising: a source of cement slurry; conduit connecting the source of cement slurry to a cementing location; a first pressure sensor configured to measure a first pressure toss along a first portion of the conduit; and a processing device configured to -calculate a viscosity of the eernent slurry based at least in part on the first pressure toss.

The system may comprise a pump configured to pump the cemen slurry along the conduit. The calculation performed by the processing device may further b based at least in part on a flow rate of the cement slurry along the conduit. The pump may oe configured to supply data representative of the flow rate of the cement slurry to the processing device.

The system may be configured to change a flow rate of the cement slurry pumped along me conduit by the pump; and the processing device may be configured to determine a second viscosit of the cement slurry- -at the new flow rate.

The processing device may be further configured to determining, based on the calculated viscosity value, the value that would be output by a rotational viscometer, and preferably a coaxial cylinder rotation viscometer testing the cement slurry, in vahous embodiments, the simulated viscometer may be a Couette viscometer, such as a FA ® 35 viscometer.

The first portio of the conduit may be substantially horizontal Preferably, the first portion of the conduit is at an angle to th horizontal of less than S ^e , and preferably less than 2°, and most preferably less than 1*

The system may further comprise a second pressure sensor configured to measure a second pressure loss along a second portion of the conduit, th first portion of the conduit being at a first angle with respect to horizontal and the second portion of the conduit being at a second, different angle with respect to horizontal, wherein the viscosity of the cement slurry is caioulaled based at least in part on the first pressure loss and the second pressure loss.

The second portion of the conduit is preferably at an angle of at least 45* from t e horizontal, and Is preferably substantially vertical, In various embodiments, the second portion of the conduit is at an angle to tne vertical of Jess than and preferably less than .2 ^* , and most preferably less t an 1 ^*

The processing de ce m y foe configured to calculate a density of the cement slurry based on the first pressure loss and the second pressure loss.

S The processing device may be con igured to compare the calculated

viscosity value to a pre-determtned viscosity value; and taking a action when a difference between the calculated viscosity value and the redetermined viscosity value exceeds a threshold.

Similarly, the processing device may be configured to compare the

0 calculated density value to a predetermined density value; and taking an action w en a difference between the calculated density value and the predetermined density value exceeds a threshold.

The system ma further comprise a tern peratu re sensor configured to measure the temperature of the cement slurry, preferably whilst i Is within the S conduit, The processing ma be configured to adjust the calculate viscosity based on the measured temperature of the cement slurry. For example, the calculated viscosity may be adjusted to give an equivalent viscosity for a reference temperature different from the measured temperature.

Oertain preferred embodiments of the invention will now be described in0 greater detail, by way of example only and with reference to the accompanying drawings, in which the sole figure, Figure 1 , Illustrates a portion of an apparatus used for a primary cementing operation for an oil or gas well.

In Figure 1 ₍ an apparatus 10 is shown being used to perform a primar cementing operation for a casing 18 that has been positioned within a bore 20 of an oil or gas well 12.

A cement slurry is prepared to a pre~se!ected recipe and stored as a cement supply 13- From the cement supply 13, the cement slurry is then supp-lied to a pump 14. The pump 1 pumps the cement slurry along a conduit 1 connecting th pump 14 to the casing 18, The diameter of the conduit 8 will typicall he equal to the diameter of the casing 18, but larger and smaller diameters can be used.

Disposed along the conduit are a number of sensors 22. 24. 26 for continuously monitoring properties of the cement slurry during the primary cementing operation. In this embodiment, the sensors include a firs differential pressure sensor 22, a temperature sensor 24, and a second differential pressure sensor 28, The first dPer ttfai pressure sensor 22 measures the pressure drop along a first portion 28 of the conduit S, and the second differential pressure sens 26 measures th pressure drop along a second portion 30 of the conduit 18. The length of the portions.28, 30 can ary, but will typically be between i and 30 meters in length.

The angles, with respect to horizontal, of the first and second portions 28, 30 may be anywhere between 0 degrees and 180 degrees, but should be at least at different angles to one another, and these portions 28, 30 are preferabl

substantially horizontal and substantially vertical, respectively; In f ig re 1, the first portion 28 is oriented in an approximately horizontal direction, white the second portion 3D is orien ed i an approximately vertical direetion.

The ^: data from each of the sensors 22. 24, 26, as well as data from the pump 14 are transmitted to a processing device 32. Based on at least the

measurements from two differential pressure sensors 22, 8, the processing device 32 determines the density and viscosit of the cement slurry. Additional subordinate measured values may also include flow velocity {determined by the pump 14 or a flow meter) and temperature (determined by the temperature sensor 24).

The data can be analysed automatically and provide immediate warning when the cement properties deviate from the expected properties. This may indicate, for example, that the eeenenf sfuny has been Insuffici ntly mixed or mixed to the wrong recipe, or that the cement slurry has beg un to set, A decision may then be taken, either automatically or b a human supervisor, to stop the cementing operation before the anomalous cement slurry is pumped info the casing.

Cement slurry displays non-Newtonian properties, in that its viscosity varies with respect to shear rate. The various standards therefore define acceptable propertie at multiple shear rates. Thus,, whilst the apparatus 10 cou¾ fee operated so as to analyse viscosity at only a single shear rate (which would stllf provide a useful safety ehec the apparatus 10 could also be operated to analyse viscosity at multiple shear rate®, i.e, the pump 14 can be configured to change the flow rate of the cement slurry to facilitate examination of the viscosity at multiple shear rates. In one example, the pump 14 ma periodically operate at one or more different flow rates to enabl viscosity measurements to be made, before returning the flow rate to normal operating conditions. The most commonly use laboratory testing apparatuses for cement are coaxial cy finder rotational viscometers, and indeed many industry standards are defined in terms of coaxial cylinder rotat ionai viscometer measurements. The rocessi g device 32 is, In at least one rn ode of operaiion, therefore adapted to simulate a coaxial cylinder rotational viscometer add to output viscosity

measurements i a format corresponding to those that woul have been output foy an equivalent test of the cement slurry using a rotational viscometer. Thi facilitates the comparison of the output from the processing device 32 with th respective standards;

Coaxial ^■■ .cylinder rotational viscometers are broadly classified as "Couetfe" or "Searie" systems. The most common rotational viscometer is the A N®- 35 viscometer, whic is a Couette coaxial cylinder rotationai viscometer. I a Couette system such as the FAUU® 35 viscometer, to perform a viscosity test, a test fluid sample i contained in an annular space form d between two cylinders. The outer cylinder, or rotor, is rotated at k own velocities through f a ing, and the viscous drag exerted by the fluid generates a torque on the inner cylinder, of bob.

The bob is supported by a tension spring and the torque generated causes a rotational deflection of the bob, which Is measured and then related to the test conditions and instrument constants, Depending on th material being tested, various rotor-bob combinations and/or torsion springs can be substituted to extend the torque measuring range of to increase the sensitivity of the torque

measurement. A Searie system operates in a similar manner, except that the food is rotated instead of the outer cylinder.

Viscosity varies significantl with temperature, and therefor cemen viscosity standards are usually defined at a specific reference temperature. In practice, however, the cement being tested dy the method described above will rarel be. at that reference temperature, and it Is therefor© necessary to correct the temperature before comparison against the relevant standard.

The temperature sensor 24 is located along the conduit 18 and is configured to measur the temperature of the cement slurry within the conduit 16, This measured temperature is supplied to the processing device 32

The processing device 32 is then configured correct the calculated viscosity that Is determined based on the pressure losses measured by the differential pressure sensors 22, 26 fo account for the temperature of the cement slurry, i.e. to give an equivalent viscosity at a reference temperature of the relevant standard. The eQuivaleni viscosity can then be easily compared to the viscosity values given in the standard.

The correction of the viscosity cm e carried out fey the processing devic 32 using known techniques and assumptions regarding viscosity vari tion wit temperature.

In. the above embodiments, two differential pressure sensors 22, 28 are used. Whils t e me of two differential pressure sensor's 22, 28 is preferred, the viscosity of the cement slurry can fee determined using only a single differential pressure sensor- For example, using the firs! differential r ssure sensor 22 when the first portion 28 of the conduit 16 is substantially horizontal, the pressure loss is largely independent of gravity effects, and so the pressure loss is dominated by viscosity losses, .Alternatively, the viscosity ©an be calculated using only the second differential pressure sensor § when the second portion 30 of the conduit 16 is not horizontal, but where the density is known by othe means {such as based on the composition of the cement slurry or from laboratory tests, or by stopping the pimp 14 suc that the pressure dro is based only on density).

Furthermore, whilst the embodiment shown in Figure 1 shows the sensors 22, 24, 28 as monitoririg the main conduit 18 supplying cement slurry to the eil 12, in other embodiments, the sensors 22, 24. 6 may monitor a smaller, sub-conduit carrying only a portion of the cement slurry.