Determination of rheology of fluid in an oil or gas well

Abstract

The invention relates to the measurement of the rheology of drilling fluid down a hydrocarbon well in real time during operations. A sensor device comprising a pipe rheometer with multiple diameters is installed in a bottom hole assembly tool, such that a portion of the total flow of drilling fluid passes through it. In this way the rheological properties of the drilling fluid can be determined under the high pressures and elevated temperatures encountered downhole.

Claims

1. A method of determining rheology characteristics of a non-Newtonian drilling fluid at a point down an oil or gas well in real time while drilling, the method including the steps of: a) installing in a drillstring a test apparatus comprising a housing having either (i) a bore having different diameter bore sections along its length or (ii) more than one bore, each bore having a different diameter; b) arranging for a known flow of the drilling fluid to pass through the or each bore while drilling; c) for each different diameter bore or bore section, while drilling, sensing the pressure difference between two points along the bore or bore section; and d) calculating from the bore or bore section diameter, flow rates, sensed pressure differences and distance between pressure sensing points a viscosity value for each diameter bore or bore section.

2. The method of claim 1 wherein the flow through the test apparatus is only a portion of the total flow of the fluid in the well.

3. The method of claim 1, further including providing a volume flow sensor for sensing the volume flow rate of fluid passing through the test apparatus.

4. The method of claim 1 wherein the test apparatus is mounted in the drillstring such that the test apparatus is substantially prevented from rotating with the drillstring.

5. A method of determining rheology characteristics of a non-Newtonian drilling fluid at a point down an oil or gas well in real time while drilling, the method including the steps of: a) installing in a bottom hole assembly on a drillstring a test apparatus comprising a housing having either (i) a bore having different diameter bore sections along its length or (ii) more than one bore, each bore having a different diameter; b) arranging for a known flow of the drilling fluid to pass through the or each bore while drilling; c) for each different diameter bore or bore section, while drilling, sensing the pressure difference between two points along the bore or bore section; and d) calculating from the bore or bore section diameter, flow rates, sensed pressure differences and distance between pressure sensing points a viscosity value for each diameter bore or bore section.

6. The method of claim 5 wherein the flow through the test apparatus is only a portion of the total flow of the fluid in the well.

7. The method of claim 5, further including providing a volume flow sensor for sensing the volume flow rate of fluid passing through the test apparatus.

8. The method of claim 5 wherein the test apparatus is mounted in the bottom hole assembly such that the test apparatus is substantially prevented from rotating with the drillstring.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) A more complete understanding of the present invention and benefits thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings in which:

(2) FIG. 1 is a schematic representation of a first embodiment of a downhole rheometer in accordance with the invention;

(3) FIG. 2 is a schematic representation of a second embodiment of a downhole rheometer in accordance with the invention;

(4) FIGS. 3a and 3b are a schematic representations of a third embodiment of a downhole rheometer in accordance with the invention;

(5) FIG. 4 is a schematic representation of a fourth embodiment of a downhole rheometer in accordance with the invention; and

(6) FIG. 5 is a schematic representation of the mounting of a tool in accordance with the invention in the inner bore of a bottom hole assembly tool.

DETAILED DESCRIPTION

(7) Turning now to the detailed description of the preferred arrangement or arrangements of the present invention, it should be understood that the inventive features and concepts may be manifested in other arrangements and that the scope of the invention is not limited to the embodiments described or illustrated. The scope of the invention is intended only to be limited by the scope of the claims that follow.

(8) As shown in FIG. 1, a first embodiment of sensor apparatus is shown. The apparatus 1 is designed to measure parameters related to the rheology of drilling mud comprises a tube 2 having a number of sections 3 each with a different internal diameter R.sub.1, R.sub.2, etc. Pressure sensors P.sub.1 to P.sub.12 are located along the length of the tube, with two sensors in each tube section 3 which are spaced apart along the length of the tube section.

(9) The sections 3 are of decreasing diameter in the intended direction of flow of fluid (see arrow Q.sub.1). Between the sections 3 are transition regions 4 of gradually changing diameter. Each section 3 has a length L.sub.12, L.sub.22, L.sub.32, etc, whilst the distance between the sensor pairs in each section 3 is referenced L.sub.11, L.sub.21, L.sub.31, etc. The exact dimensions of the device have not yet been determined but the overall length is likely to be a few metres in length, e.g. 2 to 10 metres.

(10) The device is designed to be installed in the internal bore of a drillstring, through which drilling mud is circulated, or more likely in a tool forming part of the bottom hole assembly (BHA) of a drillstring. The exact dimensions of the apparatus have not yet been finally established by the inventor(s), but its largest diameter will be less that the internal diameter of the drillstring if installed within the drillstring, so that drilling fluid can bypass the device. If the device is installed in a tool forming part of a BHA, the tool is preferably also be designed such that the majority of the circulating drilling fluid bypasses the sensor apparatus, although in one embodiment the entire flow passes through the device (see FIG. 4).

(11) The diameters R1, R2, etc of each section 3 and the lengths of each section may be determined so as to result in a known flow or a known proportion of the flow of drilling fluid flowing through the device.

(12) Alternatively, apparatus such as a flow diverter is provided to regulate the volume flow entering the device may be provided at the upstream end, for example in a way similar to a demand valve on a scuba tank. In this way the valve allows only what is needed from the main flow to be taken to do the test; furthermore it will only work once the main flow is above a certain threshold so that it can supply enough flow to do the test.

(13) Alternatively or in addition, a volume flow rate sensor may be provided on the device.

(14) The pressure drop, Δp, between each pair of sensors in each section, together with the volume flow rate, can be used according to well-known formulas to provide viscosity values at a certain shear stress. The formulas may be found, for example, in the paper by Vajargah cited above.

(15) For a given flow, the diameters and lengths of each section will be determined according to well-known formulas to give equivalent viscosity measurements to the rotating drum type viscometers/rheometers, e.g. the Fann 75 or Grace 7500 mentioned above. Conventionally, the Fann and Grace instruments give a number of readings for different r.p.m. values.

(16) The largest diameter section of the tube could be dimensioned to give a reading equivalent to a 3 r.p.m. reading from, e.g., a Fann 75, the next section 3 a reading equivalent to a 6 r.p.m. reading from a Fann 75, the third section 3 a reading equivalent to a 100 r.p.m. reading from a Fann 75, etc. The other standard r.p.m.s are 200, 300 and 600 and further smaller diameters of the device may be provided to model the higher shear values created by these r.p.m.s.

(17) However, it is not necessary to dimension the different sections in this way. At a known flow, i.e. shear rate, it is possible to calculate a shear stress based on the delta pressure loss, i.e. comparable to a rotating rheometer where the rotational speed is known. From this, the shear stress can be calculated.

(18) The sensor apparatus is mounted within a tool forming part of the BHA of a drillstring and transported with the advancing drillstring down a well or partially drilled well, or during a drilling operation, to the depth of interest. This will be described in more detail below in connection with all the embodiments and with respect to FIG. 5.

(19) A second embodiment of sensor apparatus 21 is shown in FIG. 2. This embodiment is simpler than the first, comprising simply a tube 22 of constant diameter with an upstream and downstream pressure sensor P.sub.1 and P.sub.2. This embodiment, like the first, would be mounted in a BHA. However, this embodiment would require the flow rate of drilling fluid through the device to be varied in order to give readings for different shear stress equivalent to the r.p.m. settings of, e.g., a Fann 75 device. This could be arranged by having a separate pumping apparatus associated with the tool, by having an adjustable flow diverter device, or by varying the drilling fluid flow rate through the drillstring from the wellhead pump. In the latter case, a disadvantage would be that continuous sensing of fluid properties would not be possible, since normal operations would have to be suspended for a period in order to vary the drilling fluid flow rate.

(20) A third embodiment of sensor 31 is shown in FIGS. 3a and 3b. This embodiment comprises a cylindrical housing 32. Within the housing 32 are separate bores 33a-33f, each having a different radius R.sub.1, R.sub.2, etc. Within each bore 33 is a pair of pressure sensors P.sub.1, P.sub.2 arranged along the bore. In use, flow through the bores would of course be in parallel. If a flow diverter is used to regulate the flow through the device, it may need to include some arrangement to regulate separately the flow through each bore. In most respects, however, the operation of the third embodiment is similar to the first embodiment.

(21) A fourth embodiment of sensor 41 is shown in FIG. 4, installed in a section of drillpipe or BHA tool 47. The sensor apparatus is very similar in principle to the first embodiment, with the exception that it occupies substantially the entire diameter of the drillstring or, more probably, a tool of a bottom hole assembly. It comprises a housing 42 with a length in the region of 6 metres having a constant external diameter of, e.g., 6⅝ inch (166 mm), but having a stepped internal bore going down to a minimum of 2.5 inches. In this embodiment, all the drilling fluid flow passes through the sensor. For this reason even the smallest diameter section has a relatively large bore so as not to restrict unduly the flow of drilling fluid. Because of the different diameter sections providing different shear, there is no need artificially to alter the drilling fluid flow rate. As in the first embodiment, pressure sensors are located in each section to detect a delta P for each diameter.

(22) FIG. 5 shows a sensor tube 52 installed in a drillstring/drillpipe or a tool 58 forming part of a BHA. For simplicity, the second embodiment of sensor apparatus is shown installed within the drillpipe/tool 58 but it could be replaced by the first embodiment (but not the third embodiment).

(23) The sensor tube 52 is shown installed centrally within the bore of the drillpipe/tool 58 such that the majority of the interior cross section of the drillpipe/tool is free for drilling fluid to flow past the sensor (overall drilling fluid flow referenced Q.sub.FDP). A portion of the flow Q.sub.tube passes through the sensor.

(24) The inventors believe that better results will be obtained if the sensor is prevented from rotating with the remainder of the drillpipe/BHA 58. Tools are well-known which are designed to be part of a BHA but to be prevented from rotating with the drill string—for example non-rotating stabilizers. It is proposed that a similar arrangement is employed for preventing rotation of the sensor apparatus with respect to the tool or drillstring, or preventing rotation of the tool in which the sensor apparatus is installed. However, it may be possible to remove the rotational effects by the use of mathematics, so the prevention of rotation may not be necessary.

(25) It will be appreciated that the number of different diameters of bore provided in any of the embodiments is not fixed. In general the more different diameters, the more data will be collected and the more useful the result will be. Obviously, using fewer diameters will make for simpler and less expensive apparatus which also, being smaller, may interfere less with the normal running of the well.

(26) In all four embodiments, the pressure sensors will measure the pressure of the flow at a certain point and the difference in pressure between the start and end of the flow through a certain diameter of bore can be established. The pressure delta, together with the flow rate, diameter of the bore, the temperature and pressure (normally available from other instruments) and mud weight (ppg) can be used to establish a shear stress (Pa) and shear rate (sec.sup.−1) according to the formula
Shear Rate T.sub.wall=R/2*dp/dl, R=radius, dp/dl=delta pressure loss
Shear Stress γ.sub.wall=(3N+¼N)*8ν/D,

(27) The shear stress and shear rates may be used to give the data output needed for updating the simulation models in real time.

(28) In closing, it should be noted that the discussion of any reference is not an admission that it is prior art to the present invention, especially any reference that may have a publication date after the priority date of this application. At the same time, each and every claim below is hereby incorporated into this detailed description or specification as a additional embodiments of the present invention.

(29) Although the systems and processes described herein have been described in detail, it should be understood that various changes, substitutions, and alterations can be made without departing from the spirit and scope of the invention as defined by the following claims. Those skilled in the art may be able to study the preferred embodiments and identify other ways to practice the invention that are not exactly as described herein. It is the intent of the inventors that variations and equivalents of the invention are within the scope of the claims while the description, abstract and drawings are not to be used to limit the scope of the invention. The invention is specifically intended to be as broad as the claims below and their equivalents.

REFERENCES

(30) All of the references cited herein are expressly incorporated by reference. The discussion of any reference is not an admission that it is prior art to the present invention, especially any reference that may have a publication data after the priority date of this application. Incorporated references are listed again here for convenience: “Determination of drilling fluid rheology under downhole conditions by using real-time distributed pressure data”, Vajargah et al., Journal of natural Gas Science and Engineering.