NO20220914A1 - Method and system for measuring a sag property of a well fluid - Google Patents

Description

FIELD OF THE INVENTION

The present invention relates to a method for determining a sag property of a well fluid. The present invention also relates to a system for determining a sag property of a well fluid. The present invention also relates to a method for verifying a well fluid before circulating the well fluid into a well. The present invention also relates to a method for performing a drilling or completion operation.

BACKGROUND OF THE INVENTION

During drilling and completion operations in an oil/gas well, a problem referred to as sag or sagging may occur. Drilling fluids, completion fluids and other well fluids typically contain a weight material suspended in the fluid. Such a weight material is typically barite, but other materials such as hematite and calcium carbonate are also used as such a weight material. In some situations, the weight material is no longer suspended equally in the fluid, as the weight material is settling, separating the weight material from the lighter constituents. In some wells, in particular inclined wells, , this can cause an influx (a kick) of formation fluid into well. Sag may also cause other problems during drilling and completion operations.

Sag is discussed in detail in “Controlling barite sag can reduce drilling problems”, by Zamora et al, published in Oil & Gas Journal, 14 February 1994. It is further described a method for obtaining a sag index, by using a Fann viscometer.

EP 0417 885 describes a method and apparatus for analyzing sag phenomena in well fluids wherein an elongate container containing a sample of a fluid to be tested is mounted at an angle with respect to vertical on a force responsive device which provides a measurable, variable indication of the center of mass of the container. The angle is chosen to correspond to that of a well deviation angle for which testing is to be done, and the sample may be subjected to heat and pressure to further simulate downhole conditions. The aforementioned indication provided by the force responsive device is repeatedly measured and functionally related to time. This method and apparatus require manual work for filling/emptying the elongated container.

US 6,330,826 describes an apparatus and a method are described for measuring the sag properties of a drilling fluid by using a conically or frustoconically shaped inner body and an outer body having an opening with contours closely matching those of inner body such that in conjunction inner and outer body are separated by a narrow gap defining a conically or frustoconically volume with a vertex; a motor drive for rotating the inner body with respect to the outer body; and a sampling access to determine the density of said drilling fluid within a localized part of said volume, wherein said sampling is permanently located in vicinity on said vertex. Also this method and apparatus require manual work for filling/emptying the equipment for analyzing the results.

One object of the present invention is to provide a more efficient method and system for determining sag-related properties of a fluid. More particularly, the object is to provide a more efficient method and system for predicting the risk of sag of a fluid.

SUMMARY OF THE INVENTION

The present invention relates to a method for determining a sag property of a well fluid, wherein the method comprises the steps of:

- filling the well fluid into a pipe section;

- adjusting an inclination of the pipe section relative to a horizontal plane;

- measuring a first parameter representative of a first fluid pressure of the well fluid at a first height within the pipe section;

- measuring a second parameter representative of a second fluid pressure of the well fluid at a second height within the pipe section, wherein the first height is different from the second height;

- calculating the sag property as a difference between the first parameter and the second parameter.

By measuring parameters representative of fluid pressures, the cause of the sag challenge is measured directly.

In one aspect, the sag property is calculated as an absolute value of the difference between the first parameter and the second parameter.

In one aspect, the step of adjusting the inclination of the pipe section comprises the step of adjusting an angle of the inclination of the pipe section between 5º - 90º, preferably between 30º – 90º relative to the horizontal plane.

In one aspect, the method further comprises the step of:

- waiting a predetermined period of time after filling of the pipe section before performing the steps of measuring the first parameter and the second parameter.

In one aspect, the predetermined period of time of waiting is 10 seconds to 60 minutes.

In one aspect, the method further comprises the step of:

- measuring the first parameter and the second parameter as a function of time.

In one aspect, the step of filling the well fluid into the pipe section comprises:

- flushing or circulating the well fluid through the pipe section for a predetermined period of time;

- stopping the flushing or circulation of the well fluid through the pipe section after the predetermined period of time.

In one aspect, the step of circulating the well fluid through the pipe section comprises:

- circulating the well fluid between the pipe section and a tank in which the well fluid is stored.

According to these steps of flushing or circulating the well fluid, it is achieved that any previously settled constituents of the well fluid becomes removed or flushed away before the pipe section is filled.

In one aspect, method comprises the following step after the step of c irculating the well fluid through the pipe section:

- preventing the well fluid from returning to the tank before performing the steps of measuring the first parameter and the second parameter.

In one aspect, the step of preventing the well fluid from returning to the tank comprises the step of:

- controlling a first valve provided in the lower end of the pipe section or in a first fluid line between the lower end of the pipe section and the tank to be in its closed state.

In one aspect, the method further comprises the steps of:

- measuring a third parameter representative of a third fluid pressure of the well fluid at a third height within the pipe section; wherein the third height is different from the first height and the second height;

- calculating a sag property as a difference between the third parameter and the first parameter and/or as a difference between the third parameter and the second parameter.

In one aspect, the step of adjusting the angle of the pipe section is performed before the step of filling well fluid into the pipe section. Alternatively, the step of filling well fluid into the pipe section is performed before or simultaneously as the step of adjusting the angle of the pipe section.

In one aspect, the step of adjusting the angle comprises the step of adjusting the angle of the pipe section to be similar or equal to an inclination of the well in which the well fluid is being used.

The inclination of the well may vary depending on the depth of the well. Hence, the step of adjusting the angle may comprise adjusting the angle of the pipe section to be similar or equal to an inclination of the well at a predetermined depth of the well in which the well fluid is being used.

In one aspect, the method further comprises the step of heating the well fluid to a temperature similar or equal to the temperature of the well in in which the well fluid is being used.

The temperature of the well may vary depending on the depth of the well. Hence, the step of heating the well fluid may comprise heating the well fluid to a temperature similar or equal to the temperature at a predetermined depth of the well in which the well fluid is being used.

In one aspect, the method further comprises the step of pressurizing the well fluid to a pressure similar or equal to the pressure of the well in which the well fluid is being used.

The pressure of the well may vary depending on the depth of the well. Hence, the step of pressurizing the well fluid may comprise pressurizing the well fluid to a pressure similar or equal to the pressure at a predetermined depth of the well in which the well fluid is being used.

The present invention also relates to a system for determining a sag property of a well fluid, wherein the system comprises:

- a pipe section;

- an angle adjuster for adjusting an inclination of the pipe section relative to a horizontal plane;

- a first sensor for measuring a first parameter representative of a first fluid pressure of the well fluid at a first height within the pipe section;

- a second sensor for measuring a second parameter representative of a second fluid pressure of the well fluid at a second height within the pipe section, wherein the first height is different from the second height;

- a signal processing unit connected to the first sensor and the second sensor, wherein the signal processing unit is configured to calculate the sag property as a difference between the first parameter and the second parameter.

In one aspect, an angle of the inclination of the pipe section is adjustable between 5º - 90º, preferably between 30º – 90º relative to the horizontal plane.

In one aspect, the system comprises:

- a first fluid line having a first end connected to the pipe section and a second end configured to be provided in fluid communication with a tank containing the well fluid.

In one aspect, the system comprises a pump for filling the pipe section with well fluid from the tank via the first fluid line.

In one aspect, the second end may be configured to be provided in fluid communication with the tank by connecting the second end of the first fluid line to the tank or by inserting the second end of the first fluid line into the tank.

In one aspect, the system comprises:

- a second fluid line having a first end connectable to the pipe section and a second end configured to be provided in fluid communication with the tank, wherein the pump is configured to circulate the well fluid in the tank through the pipe section via the first fluid line and the second fluid line.

In one aspect, the second end may be configured to be provided in fluid communication with the tank by connecting the second end of the second fluid line to the tank, by inserting the second end of the second fluid line into the tank or by locating the second end of the second fluid line above the tank.

In one aspect, the system comprises a first valve for preventing the well fluid from flowing out from the pipe section via the first fluid line in its closed state.

In one aspect, the first valve is provided in the lower end of the pipe section.

Alternatively, the first valve is provided in the first fluid line. In yet an alternative, the fluid is prevented from flowing out from the pipe section by means of the pump, i.e. when the pump is stopped, fluid is prevented from returning from the pipe section to the tank.

In one aspect, the system comprises a second valve for preventing the well fluid from flowing out from the pipe section via the second fluid line in its closed state.

In one aspect, the second valve is provided in the upper end of the pipe section. Alternatively, the second valve is provided in the second fluid line.

In one aspect, the pump is configured to pressurize the well fluid located within the pipe section to a pressure similar or equal to the pressure of a predetermined depth of the well.

In one aspect, the system comprises a control system for controlling the pump and/or the first valve and/or the second valve and/or the angle adjuster.

In one aspect, the control system is provided in communication with the signal processing unit and wherein the control system is configured to:

- fill the well fluid into the pipe section by means of the pump;

- adjust an angle of the inclination of the pipe section to be between 5º - 90º, preferably between 30º - 90º, relative to a horizontal plane;

- send a signal to the signal processing unit to start calculating the sag property; - receive the sag property from the signal processing unit;

- circulate well fluid through the pipe section;

repeating the above steps periodically.

According to the above, it is achieved an automatic system for determining the sag property of the well fluid. The control system may be provided in communication with a well operation management system, wherein personnel responsible for the well operation may receive periodical status reports regarding the sag property from the system.

In one aspect, the system comprises a third sensor for measuring a third parameter representative of a third fluid pressure of the well fluid at a third height within the pipe section; wherein the third height is different from the first height and the second height;

wherein the signal processing unit is configured to calculate the sag property as a difference between the third parameter and the first parameter and/or as a difference between the third parameter and the second parameter.

In one aspect, the system comprises a heater for heating the well fluid to a predetermined temperature.

In one aspect, the predetermined temperature is similar or equal to the temperature of the well in which the well fluid is being used.

In one aspect, the pipe section may have a height of at least 2 m when oriented vertically or close to vertically. In one aspect, the pipe section may have an inner diameter of 20-200 mm.

In one aspect, the system is skid-mounted.

In one aspect, the system comprises further sensors for measuring further properties of the well fluid. Such properties may be density and/or rheology.

In one aspect, the signal processing unit is integrated with the control system.

It should be noted that the system is connectable to the tank, i.e. the tank itself is not a part of the system. Alternatively, the system may comprise the tank. The tank may be the tank from which well fluid is retrieved from during a well operation.

The present invention also relates to a method for verifying a well fluid before circulating the well fluid into a well, wherein the method comprises the steps of: - performing the method above, wherein the method further comprises the step of: - determining that the well fluid is acceptable if the absolute value of the sag property is equal to or lower than a threshold value.

The present invention also relates to a method for performing a drilling or completion operation, wherein the method comprises the steps of:

- pumping a well fluid from a tank down through a well string;

- receiving the well fluid into the tank from the well bore;

- determining a sag property of the well fluid according to the method above, wherein the pipe section is connected to the tank;

- continuing pumping of the well fluid from the tank if the absolute value of the sag property is equal to or lower than a threshold value;

- changing the sag property of the well fluid if the absolute value of the sag property is higher than a threshold value.

In one aspect, the step of receiving the well fluid into the tank from the well bore comprises the step of:

- removing cuttings from the well fluid before returning the well fluid into the tank.

DETAILED DESCRIPTION

Embodiments of the invention will be described in detail with reference to the enclosed drawings, wherein:

Fig. 1 illustrates a side view of a first embodiment of the measuring system schematically;

Fig. 2 illustrates a second embodiment of the measuring system schematically; Fig. 3 illustrates a third embodiment of the measuring system schematically;

Fig. 4 illustrates a fourth embodiment of the measuring system schematically;

Fig. 5 illustrates a system for performing a drilling or completion operation.

Example 1

It is now referred to fig. 1. Here it is shown a system 1 for determining a sag property SP of a well fluid. The well fluid is here located within a tank 2, wherein a filling level FL indicates the volume of the well fluid within the tank 2.

The system 1 comprises a pipe section 10 and an angle adjuster 12 for adjusting an inclination of the pipe section 10 relative to a horizontal plane HP. The inclination is indicated in fig. 1 as an angle α. The angle α of the inclination of the pipe section 10 is in the present embodiment adjustable between 30º – 90º relative to the horizontal plane HP. However, in other embodiments, the angle α may be adjusted between 5º - 90º.

The system 1 further comprises a first fluid line 3a having a first end connected to the lower end of the pipe section 10 and a second end provided in fluid communication with the tank 2. The angle adjuster 12 is here connected between the lower end of the pipe section 10 and the first end of the first fluid line 3a. Hence, the angle adjuster 12 is adjusting the angle between the first fluid line 3a and the pipe section 10.

The system 1 further comprises a second fluid line 3b having a first end connected to the upper end of the pipe section 10 and a second end configured to be provided in fluid communication with the tank 2.

The first fluid line 3a, the pipe section 10 and the second fluid line 3b form a fluid path, where well fluid may be circulated from the tank 2 via the first fluid line 3a, the pipe section 10 and the second fluid line 3b back to the tank 2 again.

In the present embodiment, the first fluid line 3a and the second fluid line 3b are rigid pipes. In this case, the second fluid line 3b should be arranged such that the well fluid can be returned to the tank 2 for any allowable angle of the angle adjuster 12. In fig. 1, it is shown that the second end of the second fluid line 3b is located above the tank 2 and that even if the second fluid line 3b will move together with the pipe section 10 when the angle adjuster is operated, well fluid will be circulated back to the tank 2.

However, it should be noted that in an alternative embodiment, at least a section of the second fluid line 3b may be provided as a flexible hose.

In fig. 1 it is further shown that the system 1 comprises a pump 4 for circulating the well fluid in the tank 2 through the pipe section 10 via the first fluid line 3a and the second fluid line 3b.

The system 1 further comprises a number of sensors 20. Three of these sensors are provided with a reference number in fig. 1, namely a first sensor 20A for measuring a first parameter P1 representative of a first fluid pressure of the well fluid at a first height H20A within the pipe section 10, a second sensor 20B for measuring a second parameter P2 representative of a second fluid pressure of the well fluid at a second height H20B within the pipe section 10, and a third sensor 20C for measuring a third parameter P3 representative of a third fluid pressure of the well fluid at a third height H20C within the pipe section 10.

The first height H20A is lower than the second height H20B and the third height H20C for all inclinations of the pipe section 10. The third height H20C is higher than the first height H20A and lower than the second height H20C for all inclinations of the pipe section 10.

As shown in fig. 3, the system 1 comprises a signal processing unit 51 connected to the sensors 20A, 20B and 20C. It is also shown that the system 1 comprises a control system 50 for controlling the pump 4. The control system 50 may also be used to control the angle adjuster 12.

The operation 1 of the system 1 will now be described.

First, well fluid is pumped from the tank 2 into the pipe section 10 by means of the pump 4. The pump 4 is then stopped, thereby preventing well fluid from returning from the pipe section 10 to the tank 2. The inclination of the pipe section 10 is adjusted to a desired angle before or after the pipe section 10 has been filled with well fluid. Typically, the angle α of the pipe section 10 is adjusted to be equal to an inclination at a depth of the well in which the well fluid is going to be used.

The signal processing unit 51 will receive pressure measurements from the sensors 20A, 20B and 20C. A sag property SP of the well fluid is calculated by the signal processing unit 51 as a difference in the pressure measurements by the sensors 20A, 20B and 20C. More specifically, the sag property SP can be calculated as the difference between the first parameter P1 and the second parameter P2, as the difference between the third parameter P3 and the first parameter P1 and/or as the difference between the third parameter P3 and the second parameter P2. Typically, the absolute values of the above differences are calculated.

If sagging is not occurring, the sensors 20A, 20B and 20C will measure a fluid pressure P1, P2, P3 being almost the same along the entire length of the pipe section. Hence, if the signal processing unit 51 is calculating that the absolute value of the difference between any two of the fluid pressures P1, P2, P3 measured by the sensors 20A, 20B and 20C is below a predetermined threshold value, it can be concluded that sagging is not occurring.

However, if the signal processing unit 51 is calculating that the absolute value of the difference between any two of the fluid pressures P1, P2, P3 measured by the sensors 20A, 20B and 20C is above a predetermined threshold value, it can be concluded that sagging is occurring.

If sagging is occurring, the operator may choose to change properties of the well fluid in the tank 2 and then repeat the above measurement until sagging is not occurring.

It should be noted that the measurements and calculation of the sag property may be performed a period of time, for example ten seconds to several minutes after the well fluid has been supplied to the pipe section 10.

In a final step, the well fluid is removed from the pipe section by starting the pump 4 again and replacing the well fluid with new well fluid from the tank 2. If sagging was detected, it is preferred that the pump is circulating well fluid through the pipe section 10 for a period of time in order to flush or clean the pipe section 10. Hence, it is avoided that settled constituents which were formed during the previous measurement is affecting the next measurement.

Example 2

Example 2 has many of the features of example 1 above. Hence, only differences between example 2 and example 1 will be described in detail herein.

As indicated by the dashed rectangle 1 in fig. 2, the pump 4 is here not considered to be part of the system 1. Instead, a pump 4 already mounted to the tank 2 is utilized for the purpose of pumping well fluid from the tank 2 to the pipe section 10.

In addition, the system 1 comprises a heater 40 for heating the well fluid to a predetermined temperature. The heater 40 is mounted to the pipe section 10.

Preferably, the heater 40 is controllable by the control system 50 to heat the fluid within the pipe section 10 to a desired temperature. The desired temperature is similar or equal to the temperature of the depth of the well in which the well fluid is going to be used.

In fig. 2, it is further shown that the second end of the second fluid line 3b is inserted into the tank 2. The third fluid line 3b is here at least partially a flexible hose.

It is further shown in fig. 2 that the system 1 comprises a first valve 31 provided in the lower end of the pipe section 10 for the purpose of preventing well fluid from flowing out from the pipe section 10 when the first valve 31 is in its closed state. Moreover, the system 1 comprises a second valve 32 provided in the upper end of the pipe section 10, for the purpose of preventing the well fluid from flowing out from the pipe section 10 when the second valve 32 is in its closed state.

In the second example, the pump 4 may pressurize the well fluid located within the pipe section 10 to a pressure similar or equal to the pressure of the well at the depth in which the well fluid is going to be used. This is performed by filling the pipe section, then closing the second valve 32 and then increase the pressure within the pipe section 10 until the desire pressure is achieved. Finally, the first valve 31 is closed, before the pressure measurements may start, similar to the first example above.

Preferably, also the opening and closing of the first valve 31 and the second valve 32 are controlled by the control system 50 (the control system 50 is indicated in fig.

3).

Example 3

Example 3 has many of the features of example 1 and example 2 above. Hence, only differences between example 3 the other examples will be described in detail herein.

It is now referred to fig. 3. Here it is shown that the second end of the second fluid line 3b is connected to the tank 2. Also here, the second fluid line 3b is at least partially a flexible hose to enable the angle adjuster 12 to move the pipe section 10.

Example 4

Example 4 is an alternative to the second example above. Here, the first valve 31 is located within the first fluid line 3a and the second valve 32 is located within the second fluid line 3b.

Example 5

It is now referred to fig. 5. Here a drilling system or completion system is indicated with reference number 100. The system 100 comprises a drill string 101 provided in a wellbore WB and a pump 102 for circulating the well fluid from the tank 2 via the drill string 101 into the well bore WB. The well fluid is received from the well bore WB and sent to a shale shaker 103, in which cuttings are removed from the well fluid before the well fluid is returned into the tank 2 again.

As shown in fig. 5, the above system 1 is connected to the tank 2, for determining the sag property SP of the well fluid in the tank 2.

Hence, it is achieved an automatic system for determining the sag property SP of the well fluid. The control system 50 may be provided in communication with a well operation management system. Hence, personnel responsible for the well operation may receive periodical status reports regarding the sag property from the system 1.

It is then possible to stop the well operation if the absolute value of the sag property SP is higher than a threshold value and then change the properties of the well fluid until the absolute value of the sag property SP of the well fluid is below the threshold value before the well operation is continued.

It is further possible to prepare a second tank (not shown) with well fluid, and measure the sag property of the well fluid within this second tank, before well fluid is circulated from this second tank via the drill string.

The system 1 shown in fig. 5 is preferably mounted to a skid. When the well operation is in a phase in which sag property measurements are required, the sys tem 1 is moved to an area near the tank 2 and is connected to the tank 2. However, when the well operation is in a phase in which sag property measurements are not required, the system 1 is moved away from the tank 2. On the same skid, further sensors for measuring further properties of the well fluid can be mounted. Such further properties may be density and/or rheology.

Other alternative embodiments

It should further be noted that the second fluid line 3b is not an essential feature in at least some of the above embodiments. In some cases, it is possible to discharge the well fluid after the test. Such an alternative is indicated with a dashed outline in fig. 3, where fluid is discharged via a third fluid line 3c and possibly via a third valve 33 instead of being returned to the tank 2 via the second fluid line 3b.

Claims (19)

1. A method for determining a sag property (SP) of a well fluid, wherein the method comprises the steps of:

- filling the well fluid into a pipe section (10);

- adjusting an inclination of the pipe section (10) relative to a horizontal plane (HP); - measuring a first parameter (P1) representative of a first fluid pressure of the well fluid at a first height (H20A) within the pipe section (10);

- measuring a second parameter (P2) representative of a second fluid pressure of the well fluid at a second height (H20B) within the pipe section (10), wherein the first height (H20A) is different from the second height (H20B);

- calculating the sag property (SP) as a difference between the first parameter (P1) and the second parameter (P2).

2. The method according to claim 1, wherein the method further comprises the step of:

- waiting a predetermined period of time after filling of the pipe section (10) before performing the steps of measuring the first parameter (P1) and the second parameter (P2).

3. The method according to claim 1 or 2, wherein the method further comprises the step of:

- measuring the first parameter (P1) and the second parameter (P2) as a function of time.

4. The method according to any one of the above claims, wherein the step of filling the well fluid into the pipe section (10) comprises:

- flushing or circulating the well fluid through the pipe section (10) for a predetermined period of time;

- stopping the flushing or circulation of the well fluid through the pipe section (10) after the predetermined period of time.

5. The method according to claim 4, wherein the step of circulating the well fluid through the pipe section (10) comprises:

- circulating the well fluid between the pipe section (10) and a tank (2) in which the well fluid is stored.

6. The method according to claim 5, wherein the method comprises the following step after the step of circulating the well fluid through the pipe section (10):

- preventing the well fluid from returning to the tank (2) before performing the steps of measuring the first parameter (P1) and the second parameter (P2).

7. The method according to any one of the above claims, wherein the method further comprises the steps of:

- measuring a third parameter (P3) representative of a third fluid pressure of the well fluid at a third height (H20C) within the pipe section (10); wherein the third height (H20c) is different from the first height (H20A) and the second height (H20B);

- calculating a sag property as a difference between the third parameter (P3) and the first parameter (P1) and/or as a difference between the third parameter (P3) and the second parameter (P2).

8. A system (1) for determining a sag property (SP) of a well fluid, wherein the system comprises:

- a pipe section (10);

- an angle adjuster (12) for adjusting an inclination of the pipe section (10) relative to a horizontal plane (HP);

- a first sensor (20A) for measuring a first parameter (P1) representative of a first fluid pressure of the well fluid at a first height (H20A) within the pipe section (10); - a second sensor (20B) for measuring a second parameter (P2) representative of a second fluid pressure of the well fluid at a second height (H20B) within the pipe section (10), wherein the first height (H20A) is different from the second height (H20B);

- a signal processing unit (51) connected to the first sensor (20A) and the second sensor (20B), wherein the signal processing unit (51) is configured to calculate the sag property (SP) as a difference between the first parameter (P1) and the second parameter (P2).

9. The system (1) according to claim 8, wherein the system (1) comprises:

- a first fluid line (3a) having a first end connected to the pipe section (10) and a second end configured to be provided in fluid communication with a tank (2) containing the well fluid.

10. The system (1) according to claim 9, wherein the system (1) comprises

- a pump (4) for filling the pipe section (10) with well fluid from the tank (2) via the first fluid line (3a).

11. The system (1) according to claim 9 or 10, wherein the system (1) comprises: - a second fluid line (3b) having a first end connectable to the pipe section (10) and a second end configured to be provided in fluid communication with the tank (2), wherein the pump (4) is configured to circulate the well fluid in the tank (2) through the pipe section (10) via the first fluid line (3a) and the second fluid line (3b).

12. The system (1) according to any one of claims 8 - 11, wherein the system (1) comprises a first valve (31) for preventing the well fluid from flowing out from the pipe section (10) via the first fluid line (3a) in its closed state.

13. The system (1) according to claim 12, wherein the system (1) comprises a second valve (32) for preventing the well fluid from flowing out from the pipe section (10) via the second fluid line (3b) in its closed state.

14. The system (1) according to claim 13, wherein the pump (4) is configured to pressurize the well fluid located within the pipe section (10) to a pressure similar or equal to the pressure of a predetermined depth of the well.

15. The system (1) according to any one of claims 8 – 14, wherein the system comprises a control system (50) for controlling the pump (4) and/or the first valve (31) and/or the second valve (32) and/or the angle adjuster (12).

16. The system (1) according to claim 15, wherein the control system (50) is provided in communication with the signal processing unit (51) and wherein the control system (50) is configured to:

- fill the well fluid into the pipe section (10) by means of the pump (4);

- adjust an angle (α) of the inclination of the pipe section (10) to be between 5º -90º, preferably between 30º - 90º, relative to a horizontal plane (HP);

- send a signal to the signal processing unit (51) to start calculating the sag property (SP);

- receive the sag property (SP) from the signal processing unit (51);

- circulate well fluid through the pipe section (10);

repeating the above steps periodically.

17. The system (1) according to any one of claims 8 – 16, wherein the system comprises a third sensor (20C) for measuring a third parameter (P3) representative of a third fluid pressure of the well fluid at a third height (H20C) within the pipe section (10); wherein the third height (H20C) is different from the first height (H20A) and the second height (H20B);

wherein the signal processing unit (51) is configured to calculate the sag property (SP) as a difference between the third parameter (P3) and the first parameter (P1) and/or as a difference between the third parameter (P3) and the second parameter (P2).

18. A method for verifying a well fluid before circulating the well fluid into a well, wherein the method comprises the steps of:

- performing the method according to any one of claims 1 – 7, wherein the method further comprises the step of:

- determining that the well fluid is acceptable if the absolute value of the sag property (SP) is equal to or lower than a threshold value.

19. A method for performing a drilling or completion operation, wherein the method comprises the steps of:

- pumping a well fluid from a tank (2) down through a well string (101);

- receiving the well fluid into the tank (2) from the well bore (WB);

- determining a sag property (SP) of the well fluid according to the method of any one of claims 1 – 7, wherein the pipe section (10) is connected to the tank (2); - continuing pumping of the well fluid (2) from the tank (2) if the absolute value of the sag property (SP) is equal to or lower than a threshold value;

- changing the sag property (SP) of the well fluid if the absolute value of the sag property (SP) is higher than a threshold value.