GB2545666A - A Drilling fluid monitoring system and method - Google Patents

Abstract

Drilling fluid containing drill cuttings is transported to a cuttings removal device 5 (e.g. shakers) via a conduit 7 having monitoring apparatus 9, which measures a property (e.g. density, specific weight) of the drilling fluid (mud). After cuttings removal, the fluid is returned to the monitoring conduit by way of a return conduit 13, and the property of the drilling fluid is measured again. The monitoring apparatus may include at least one measurement device, such as a Coriolis or Gamma density meter. The drilling fluid including cuttings may be transported to an inlet of the cuttings removal apparatus using one of a first conduit and a second conduit, where one of the conduits has monitoring apparatus. The return conduit may provide a path between an outlet of the cuttings removal apparatus and an inlet of the monitoring conduit.

Description

A DRILLING FLUID MONITORING SYSTEM AND METHOD FIELD OF THE INVENTION

The invention relates to monitoring of drilling fluids, and in particular to monitoring drilling fluids during bore hole drilling.

BACKGROUND OF THE INVENTION

During borehole drilling, produced drill cuttings need to be transported to the surface in order to prevent their accumulation at the bottom of the borehole. Borehole cleaning requires efficient and fast removal of the cuttings, as inadequate borehole cleaning could cause significant drilling problems such as formation fracturing, excessive torque and drag. These can result in an increase in drilling costs.

Borehole cleaning is primarily achieved by the circulation of a drilling fluid. The drilling fluid removes the drilled materials from the borehole by carrying them to the surface. It also lubricates and cools the drill bit and stabilises the borehole by maintaining hydrostatic pressure within the borehole in order to prevent formation fluid entering the borehole.

Besides cuttings retrieval, drilling mud has the following main functions: pressure control, bit lubrication and cooling, shale stability, and fluid loss control.

Due to its crucial role in the drilling process, various properties of the drilling fluid such as flow rate and annular fluid velocity, mud density and viscosity, and pressure changes have a strong effect on the efficiency of the borehole cleaning process, which is further influenced by the inclination angle of the borehole and properties of the cuttings.

By monitoring properties of the drilling fluid and the amount of cuttings brought to the surface, information can be obtained on the efficiency of the drilling and the rate of borehole cleaning.

One way to monitor the efficiency of borehole cleaning is to weigh the well cuttings that have passed through a shale shaker and estimate a volume of cuttings based on the measured weight. The volume can be compared with the theoretical volume of the borehole. A problem with the types of process described above is that a certain amount of mud necessarily adheres to the cuttings and hence contributes to the overall weight. This leads to an inaccurate estimation of the amount of cuttings removed and leads to an inaccurate estimate of borehole cleaning.

An improvement to the prior art method has been proposed by the applicant in copending application, UK Patent Application No 1417147.4 (filed 29 September 2014). This unpublished specification discloses, in the case of a drill string located in a riser, a first measurement device is located on the riser. By way of example the first measurement device may be a gamma meter. An advantage of using a gamma meter to measure the specific weight of a drilling fluid contained in the riser is that it is non-intrusive and can be retro-fitted to an existing riser, without direct contact with the drilling fluid. As the drilling fluid travels up the riser, it exits the riser at a chute. From the chute, the drilling fluid enters a cuttings removal device, for example a shaker or series of shakers. The cuttings removal device removes cuttings contained in the drilling fluid and the remaining drilling fluid exits the shaker for recycling or removal. A second measurement device is located downstream of the shaker. This is used to measure the specific weight of the drilling fluid once the cuttings have been removed. Any suitable type of device may be used. By way of example, the second measurement device may be a coriolis meter. A coriolis meter gives an accurate measurement of the specific weight of the drilling fluid and, as the cuttings have been removed, is unlikely to become blocked by cuttings. A mud pump is also provided downstream of the shaker to pump the drilling fluid. An estimate of borehole cleaning is obtained by determining the difference between the specific weight measurements provided by the first and second measurement devices over a period of time. As the first specific weight measurement is obtained before the cuttings are removed, this first measurement provides information on the total specific weight of the drilling fluid including the cuttings. Once the drilling fluid has progressed through the shaker, and the cuttings have been removed, the second measurement device obtains a second specific weight measurement. By obtaining the difference between the first and second measurements over a period of time, the rate of cuttings removal can be estimated. Essentially, the difference between the specific weights of the drilling fluid with and without cuttings is obtained to determine the specific weight of the cuttings and thus to estimate the efficiency of borehole cleaning and the rate of cuttings removal. It will be appreciated that density or specific gravity measurements may be used in place of specific weight measurements.

In a second embodiment described in the above co-pending UK patent application, the first device is located on the riser and is arranged to measure a property of a drilling fluid and to determine a first time at which a variation in the property is detected. It will be appreciated that any type of device can be used in order to measure the property of the fluid contained in the riser. By way of example a gamma meter is referred to. A second device is located downstream of the first device at a distance d from the first device. This second device is used to measure the property of the drilling fluid and to determine a second time at which a variation in the property is detected. Any suitable type of device may be used. By way of example, the second device may also be a gamma meter. The velocity of the drilling fluid is calculated as follows. A property of the drilling fluid is measured by the first device and the first time at which a variation in the property of the drilling fluid is detected by the first device is determined. The property of the drilling fluid is measured by the second device and the second time at which a variation in the property of the drilling fluid is detected by the second device is also determined. As velocity is given by the relationship elapsed time over distance, using the difference between the first and second times, and the known distance d between the locations of the first and second devices, the velocity of the drilling fluid can be calculated. By way of example, the property of the drilling fluid that is measured could be specific weight. The first device measures the specific weight of the fluid and also determines the time at which variation in specific weight occurs. Variation in specific weight of a drilling fluid could be the result of, for example, change in the formation lithology, or degradation of the properties of the drill bit. The second device also measures the specific weight of the fluid and determines the time at which variation in specific weight occurs. By comparing the first and second times at which the variation was detected and using the distance between the first and second devices, the velocity of the drilling fluid can be calculated. An advantage of the method described above is that it does not interfere with other operations, it provides a continuous monitoring capability of the drilling fluid velocity, and it can be implemented by utilising a nonintrusive apparatus, which has no direct contact with the drilling fluid. Note that in addition to measuring the flow velocity, other information can be derived. For example, a sudden increase in flow velocity could indicate a kick caused by, for example, gas entering a liquid hydrocarbon flow. Furthermore, changes in velocity can provide information about cuttings settling or a loss of drilling fluid. In a third embodiment described in the above referenced co-pending patent application, only a single gamma meter (or other measuring device) is provided. In this case the property (e.g. specific weight or specific gravity) is continuously or periodically measured. Variations in the measured value will occur regularly, but if the variation is over a predetermined amount and within a predetermined time (in other words, a large and sudden variation) then this is indicative of a kick in the fluid flow.

One problem with having the measuring equipment on the riser is that the risers may need to be changed more or less frequently. If the measurement device or devices are integrated with the riser then extra work is required each time the riser is changed. Furthermore, since Gamma sensors are not a direct measurement of the density, they are considered to be less accurate or at least difficult to calibrate. The results from measurement at the riser are generally in respect to a reference, or a calculated expected value of the density which introduces estimation errors. GB-A-2363847 discloses a drilling fluid analyzing apparatus has at least two sensors is used to analyze drilling fluid that contains cuttings. The sensors are mounted vertically spaced in a well riser and in communication with well return fluid. The sensors convert the pressure exerted by the return fluid to signals that are then conveyed to a processor. The processor determines the density of the return fluid, and the fluid density is indicative of borehole cleaning efficiency. Two additional sensors may be added to the drilling fluid input mud pipe to sense the pressure exerted by the drilling mud before it is contaminated with cuttings. GB-A-2363847 more generally discloses a method and system for continuously measuring the efficiency of drilling fluid. More specifically, the invention relates to a system and method for detecting cuttings accumulation and washout in wellbore during drilling operations by analyzing the return fluid containing drilling mud and cuttings exiting the wellbore.

Drilling fluids are employed when drilling boreholes into subterranean formations. The drilling fluid "mud" consists of mixture of liquids and solids to provide special properties to better perform several primary functions in a drilling well. Drilling fluids lift the formation cuttings to the surface, control subsurface pressure, lubricate the drill string and bit, aid bottom-hole cleaning, aid formation evaluation, and provide protection to formation productivity.

One of the primary functions of the drilling fluid is the control of the formation pressure. The hydrostatic pressure exerted by the mud column, which is controlled by the density of the drilling fluid, is maintained above the pressure of the formation.

If the formation pressure exceeds the pressure exerted by the mud column, formation fluid may enter the wellbore, causing a kick, which is any unscheduled entry of formation fluid into the wellbore; this results in a gain in the flow rate of the returning fluid. Additionally, the drilling fluid may incur losses due to the presence of a fracture in the formation. Fractures can result in loss of the drilling fluid, which results in a loss of the fluid flow rate at the surface. It is important to continuously monitor for the pressure of kicks and the fracture during drilling of wellbores. There are several methods and systems well known in the art that measure flow rate directly with various sensors.

Another primary purpose of the drilling fluid is to lift cuttings from the wellbore. The drilling mud is circulated down the drill string, through the bit, and returns to the surface through the annular space between the drill string and the wellbore wall. The mud returning to the surface is known as return fluid comprising drilling mud, formation particles called cuttings, and possibly some formation fluids. The drilled cuttings are picked up at the bit and returned to the surface for separation from the mud and for disposal. This removal of the drilled solids from the mud stream is critical to the subsequent reconditioning of the mud for recirculation in the well.

To control and improve drilling performance, evaluation of wellbore condition is important. Keeping the hole clean, especially in extended reach wells, is a key issue as cuttings accumulation in the annulus can contribute to, if not directly cause, pipe sticking and twist-offs; this is a concern when drilling a deviated well since a bed of cuttings is almost always formed on the lower side of the drill pipe. By measuring the cuttings discharge at the surface, the buildup of cuttings in the well can be detected early and remedial action taken to prevent a catastrophic failure.

Another obstacle encountered in drilling operations is washout. Washout is excessive borehole enlargement caused by solvent and erosion action by the drilling fluid. Washout can cause severe damage to the formation, contaminate the connate formation fluids, and waste costly drilling mud. Early detection through the measurement of cuttings exiting the wellbore can also help the mitigation of this problem.

In typical cuttings evaluation, the cuttings from the well are discharged over one or more shale shaker screens to separate them from the drilling mud, and all cuttings coming from the shakers are weighed. With expected cuttings density known by the user, the expected volume of the cuttings is calculated and the volume removed is compared to the volume calculated Thus hole-cleaning efficiency is evaluated. However, this process of weighing after the shaker or cuttings removal Weighs also drilling fluid and is not accurate

Currently the main types of mud-out weight-sensors used are a strain gauge and suspended heavy weight system, systems using differential pressure plates in the mud pit, and radioactive source sensors Some of the mud adheres to the cuttings and is carried over with the cuttings discharged from the shale shaker This portion of mud is lost to the mud system, which has been reported to be as high as two barrels of mud for every barrel of cuttings The mud lost in the cuttings causes lo accuracy problems with the first two sensor types The third system, although more accurate, is costly and requires certification and approval The first two systems are not accurate enough for the cuttings removal performance application because of the settlement of the cuttings in the pits.

Thus in GB-A-2363847 two pressure sensors are installed vertically displaced in a well riser to sense the pressure exerted by the return fluid including drilling mud and cuttings. A processor is provided for receiving signals from the sensors and for processing the data to determine hole cleaning performance. The advantage in measuring the return fluid is that the flow out including the cuttings is homogenous in the riser and no settlements occur. GB-A-2363847 also discloses another embodiment, where two additional sensors are provided to measure the drilling mud as it enters the well With two sensors measuring return fluid pressure and two sensors measuring the pressure of drilling fluid entering the well, a processor can calculate efficiency based on more measured parameters The processed data is an indication of well cleaning efficiency that can allow for early detection of washout or cuttings accumulation. GB-A-2363847 also discloses another embodiment where sensors provided in a riser during tripping operations are used in conjunction with other sensors, such as flow rate sensors, to detect washouts and kick through the measurement of mud parameters entering the wellbore.

SUMMARY it is an object of the present invention to provide a new technique for monitoring the bore hole cleaning process. More specifically, It is desirable to improve the accuracy of the monitoring. It is also desirable to provide calibration of the monitoring means. It is furthermore desirable to accurately determine the end of the cleaning procedure and to reduce circulating time of the drilling fluid.

The invention provides a method of monitoring a drilling fluid in a drilling fluid system having a cuttings removal apparatus, the method comprising receiving drilling fluid including cuttings, measuring a property of the drilling fluid including cuttings using monitoring apparatus, removing cuttings from the drilling fluid, returning the drilling fluid after cuttings removal to the monitoring apparatus used in said measuring a property of the drilling fluid including cuttings, measuring a property of the drilling fluid after cuttings removal using said monitoring apparatus.

The monitoring apparatus may include at least one measurement device for taking a measurement relating to at least one property of the drilling fluid.

The method may comprise transporting the received drilling fluid including cuttings to an inlet of the cuttings removal apparatus using at least one of a first conduit and a second conduit, one of said first and second conduits having said monitoring apparatus associated therewith and being designated a monitoring conduit.

The method may comprise monitoring the drilling fluid including cuttings transported by the monitoring conduit using said monitoring apparatus associated with said monitoring conduit.

The method may comprise transporting drilling fluid between an outlet of the cuttings removal apparatus and an inlet of the monitoring conduit using a return conduit.

The method may comprise monitoring drilling fluid received from the return conduit using the said monitoring apparatus associated with the monitoring conduit.

The invention also provides a drilling fluid monitoring system for monitoring drilling fluid comprising a cuttings removal apparatus, a monitoring conduit in a path for transporting received drilling fluid containing cuttings to the cuttings removal apparatus, monitoring apparatus associated with said conduit for measuring a property of the drilling fluid, a return conduit for returning the drilling fluid after cuttings removal to the monitoring apparatus associated with said first conduit, whereby a property of drilling fluid after cuttings removal can be measured.

The monitoring apparatus may include at least one measurement device for taking a measurement relating to at least one property of the drilling fluid.

The drilling fluid system may comprise a further conduit selectively communicating with the first conduit, for transporting drilling fluid containing cuttings to the cuttings removal apparatus.

The return conduit may provide a path for the drilling fluid between an outlet of the cuttings removal apparatus and an inlet of the monitoring conduit.

Furthermore, none of the prior art takes Into account gas lost in the cuttings removal process; as a consequence in prior techniques calculations based on density, or specific weight etc. make an assumptions in relation to density of the drilling fluid before and after cuttings removal that do not take into account loss of entrained or dissolved gases that might be lost during cuttings removal. These assumptions may lead to Inaccuracies in the calculations, it would be desirable to be able to measure the amount of gas lost and to calibrate the system accordingly. Alternative to pressuring it is also possible to provide a gas measurement device in the measuring apparatus, for example a bubble detector.

Accordingly, it Is proposed to take out a small part of the return flow of drilling fluids before the chute (gutter) and to direct it through a specific weight meter that measures the total net weight of both cuttings and drilling fluid. Periodically shutting out the flow of cuttings and opening for a small portion of the drilling fluid after the shaker which is pumped through the same specific weight meter. This measurement is used as a zero adjustment of the system. Drill cuttings have a specific weight about 2.6 kg / L and drilling fluids around 1.5 kg / I. The difference in specific weight with and without cuttings will always provide the amount of cuttings returned. To avoid gas issues, valves may be provided to allow the pressure to be increased during weight measurement.

BRIEF DESCRIPTION OF DRAWINGS

Figure 1 shows schematically an exemplary arrangement according to an embodiment of the present invention

DETAILED DESCRIPTION

The following description refers to measuring specific weight and/or specific gravity. It will be appreciated that any related property derivable from mass may be used, such as mass, weight, specific weight, specific gravity and density. Typically density is measured and the required property of the fluid determined from the density.

Figure 1 shows schematically an exemplary arrangement according to the present invention. The spatial arrangements of the features shown in Figure 1 are not intended to be representative of an actual system, but merely schematically represent how the system is arranged.

To drill a well bore, a drilling fluid is pumped from a source pit into tubing 1 of a drill string having a drill bit (not shown) at its bottom end. The drilling fluid discharges at the drill bit bottom and returns to the surface via an annular space carrying the ‘cuttings’. The return fluid including the cuttings passes into a riser 2, and then into a generally horizontal outflow or return line 3. The return line may be of substantial cross section so as to allow installation of sensors above the fluid flow. In the present embodiment the return line includes a conduit such as a chute 4 in the form of a gutter or half pipe.

In any case the return line 3 is generally of sufficient cross-section to prevent return fluid filling up the return line 3.

The chute 4 transports the return fluid to the cuttings removal device 5, in this embodiment a series of shakers 5. The cuttings are separated from the drilling fluid which is returned via outlet pipe 6 to a source pit (not shown) used as the source of new drilling fluid, which is pumped back to the drill string.

In the present embodiment, in addition to, or instead of the sensors described in the prior art, for example GB-A-2363847, there is provided a sampling passage 7 (or monitoring conduit) connected to the return line 3 or chute 4, and a valve 8 or other means for diverting a portion of the return fluid through measurement apparatus 9. The chute 4 and sampling passage 7 constituting (a part of) a path for transporting received drilling fluid containing cuttings to the cuttings removal apparatus

The measurement apparatus 9 may include a single device, for example a Coriolis or Gamma meter (Gamma densimeter), or a suite of measurement devices. The Coriolis or Gamma meter can be used for determining the specific weight of the sample for example. It will be appreciated that any type of specific weight measurement device can be used in order to measure a specific weight of a fluid, for example, any suitable mass or volume flow meter. Since the density of the fluid may change with various factors, including temperature and pressure, the measurement apparatus 9 can conveniently include temperature and pressure sensors. The presence of temperature and pressure sensors allows the measurements to be made at the same pressures and temperatures or to allow for changes in these parameters.

Since drill cuttings generally have a specific weight of about 2.6 kg/l and drilling fluids around 1.5 kg/l, the key measurement apparatus should include at least one measurement device from which a measure related to the specific weight of the sample can be determined, conveniently mass flow meters and densimeters can be used to provide the specific weight measurement.

Whilst in an embodiment a mass flow meter Is used such as a coriolis, or a densimeter such as a Gamma meter. Other density meters could be used. Other sensors could be used in addition to, or instead of, these sensors and the aforementioned pressure and temperature sensors, for example, Rheometers (Rheological properties ensure the fluid formulation exhibits the flow required for optimum lubrication) volumetric flow meters, ultrasound devices, particulate monitors (e.g. colorimetry meters, turbidity meters and spectrophotometers). Clearly the suite of devices Included in the measurement apparatus 9 can include and sensing equipment that might provide useful information in relation to the sample of drilling fluid. Whilst the measurement apparatus are represented by a single unit 9, the various devices can be contained in separate units along the sample passage 7.

Clearly, additional measurement apparatus can be installed on the main chute 4 as well as In the riser and anywhere in the system that would provide useful information with regards to the drilling fluid.

The sample is then returned to the chute 4 in advance of the inlet to the cuttings removal apparatus 5.

In the embodiment of figure 1 the sample passage is provided with a fluid level sensor 10. A stirrer (not shown) may be added in advance of the measurement apparatus 9. Furthermore, an open and close valve 12 Is provided for closing the sample passage 7 for regulating the fluid pressure as discussed below.

So far the apparatus of the embodiment provides a convenient alternative to providing the necessary measurement devices either on the riser 2 or on the chute 4. The measurements taken of the sampled drilling mud can be used to detect any changes in the properties of the drilling mud that might indicate, problems in the drilling process, or an end point of the bore hole cleaning process. The presence of the sample passage 7 does not preclude sensors or other measuring equipment being present anywhere on the return path from the riser 2 to the cuttings removal device 5.

In the present embodiment the outlet pipe 6 from the cuttings removal device 5 includes a return conduit or path 13 back to the sample passage 7. The return path is normally closed, in this embodiment by a valve 14, during measurements taken in relation to the sample of the return fluid including the cuttings. When the valve 14 is shut the fluid passes through outlet pipe to a source pit for recirculation in the drilling process. Return path 13 includes a level meter 15 before mud pump 16. Conveniently, a drainage valve 17 is provided ίο allow the system to be emptied periodically, for example, between cleaning cycles.

Finally, an air vent is provided communicating with the sample passage 7 to allow venting of gases from the sample of drilling fluid in the sample passage.

Consequently the system provides a means of sampling the return drilling fluid under controlled conditions to monitor the cleaning process, whilst also being able to perform monitoring of the drilling fluid after cuttings removal using the same measurement system so as to reduce errors and also to allow calibration of the monitoring system. A controller, riot shown is typically provided so that the various components, such as pumps, mixers, valves, etc. can be controlled based on the system status as reported by the fluid level meters.

In a typical process, the system may be run as follows. Once drilling has begun and return fluid enters the cuttings removal device 5 via the chute 4, valves 4 and 8 are closed, the drilling fluid leaves the system to be returned (after optional processing) to a source pit to be recirculated in the drilling process. To take a sample of the drilling fluid, with cuttings, valve 8 is opened, either manually or more conveniently under control of a controller using an actuation system that can be easily implemented by the skilled person. Initially, when valve 8 Is opened, valve 12 in the sample passage may be closed to allow the fluid level to build in the sample passage. Fluid level sensor 1G can provide an indication that the level Is sufficient to allow flow of fluid In the sample passage 7. For example the fluid level might need to be sufficient to run a stirrer, or to ensure a consistent fluid pressure for measurements. The fluid level may need to be sufficient to prevent further entrainment of gases. When the fluid level is sufficient, the valve 12 is opened to allow fluid to flow in the sample passage 7 and through measurement apparatus 9 where the required measurements are taken in relation to the sample including the cuttings.

From these measurements alone the return fluid can be monitored for any changes, for example, in the density or specific weight, in order to monitor the progress of the hole cleaning process.

Initially, if valves 12 and 14 are closed the drilling fluid is compressed. By opening valve 12 the gas concentration may be measured. This allows the pressure to be regulated to be constant for each measurement.

The present invention allows the same measurement apparatus to be used to test the drilling fluid before and after cuttings removal. In order to do this the valve 8 is closed so that all return drilling fluid from the riser 2 is transported directly to the cuttings removal apparatus 5. In practise it may be more convenient to close the valve 8 only when there is sufficient circulation from the return path 13, thus the valve 8 may be closed gradually whilst the drill fluid without cuttings replaces the drill fluid with cuttings. Valve 14 in return path 13 is opened allowing a portion of the drilling fluid exiting the cuttings removal apparatus to be diverted along return path 13. Again, initially, valve 12 may be closed (once the previous sample of drilling fluid with cuttings has passed through sample passage 7) to allow build-up of drilling fluid (without cuttings) in the sample passage. Level meter 15 allows pump 16 to be controlled so as to pump drilling fluid to the sample passage 7 at the appropriate pressure. Once the fluid level (as determined by fluid level sensor 10) or pressure is sufficient the valve 12 can be opened to allow flow of fluid through the sample passage 7 and through measurement apparatus 9.

After a predetermined length of time, valve 14 can be shut and pump 16 shut off to continue sampling the drilling fluid from the riser as discussed above.

Valves 12 and 8 may be closed to allow the drilling fluid after cuttings to be compressed allowing the gas concentration to be measured. By subtracting from the measurement made in relation to the drilling fluid prior to cuttings removal it is possible to determine how much gas is removed from the drilling fluid by the cuttings removal operation thus allowing the accuracy of the cuttings removal calculation to be improved by taking into account this factor. In other words, the calculations of density can be corrected by taking into account the loss of gas from the fluid during cuttings removal.

Since the measurements on the drilling fluid samples before and after cuttings removal are both taken with the same equipment, and the estimation of cuttings removal is calculated by subtracting the measured density after cuttings removal from the measured density before cuttings removal, the system is self-calibrating.

In view of the improved determination of cuttings removal, a more accurate determination of the hole cleaning can be made allowing reduced circulation times saving cost in terms of running costs, processing costs (for example of the recirculated drilling fluid), material costs and time savings.

The system could also be used to determine whether the cuttings removal apparatus is working correctly.

Claims (10)

CLAIMS:

1. A method of monitoring a drilling fluid in a drilling fluid system having a cuttings removal apparatus, the method comprising: receiving drilling fluid including cuttings; measuring a property of the drilling fluid including cuttings using monitoring apparatus; removing cuttings from the drilling fluid; returning the drilling fluid after cuttings removal to the monitoring apparatus used in said measuring a property of the drilling fluid including cuttings; measuring a property of the drilling fluid after cuttings removal using said monitoring apparatus.

2. The method of monitoring as claimed in claim 1, wherein said monitoring apparatus includes at least one measurement device for taking a measurement relating to at least one property of the drilling fluid.

3. The method of monitoring as claimed in claim 1 or 2, and comprising transporting the received drilling fluid including cuttings to an inlet of the cuttings removal apparatus using at least one of a first conduit and a second conduit, one of said first and second conduits having said monitoring apparatus associated therewith and being designated a monitoring conduit.

4. The method of monitoring as claimed in claim 3, comprising monitoring the drilling fluid including cuttings transported by the monitoring conduit using said monitoring apparatus associated with said monitoring conduit.

5. The method of monitoring as claimed in claim 4, comprising transporting drilling fluid between an outlet of the cuttings removal apparatus and an inlet of the monitoring conduit using a return conduit.

6. The method of monitoring as claimed in claim 5, comprising monitoring drilling fluid received from the return conduit using the said monitoring apparatus associated with the monitoring conduit.

7. A drilling fluid monitoring system for monitoring drilling fluid comprising: a cuttings removal apparatus, a monitoring conduit in a path for transporting received drilling fluid containing cuttings to the cuttings removal apparatus, monitoring apparatus associated with said conduit for measuring a property of the drilling fluid, a return conduit for returning the drilling fluid after cuttings removal to the monitoring apparatus associated with said first conduit, whereby a property of drilling fluid after cuttings removal can be measured.

8. The drilling fluid system as claimed in claim 7, wherein said monitoring apparatus includes at least one measurement device for taking a measurement relating to at least one property of the drilling fluid.

9. The drilling fluid system as claimed in claim 7 or 8, and comprising a further conduit selectively communicating with the first conduit, for transporting drilling fluid containing cuttings to the cuttings removal apparatus.

10. The drilling fluid system as claimed in claim 7, 8 or 9, wherein the return conduit provides a path for the drilling fluid between an outlet of the cuttings removal apparatus and an inlet of the monitoring conduit.