US3760891A

US3760891A - Blowout and lost circulation detector

Info

Publication number: US3760891A
Application number: US00255160A
Authority: US
Inventors: J Gadbois
Original assignee: Offshore Co
Current assignee: SONAT OFFSHORE DRILLING Inc
Priority date: 1972-05-19
Filing date: 1972-05-19
Publication date: 1973-09-25
Anticipated expiration: 1990-09-25

Abstract

A method and apparatus for rapidly detecting blowouts and lost circulation in a drilling well, having particular application to a well being drilled at sea from a floating drilling platform. The return rate of flow of drilling fluid which will be affected by a blowout is monitored and transduced into an electrical signal proportional thereto. The magnitude of the electrical signal is monitored and accumulated over selected overlapping intervals of time to form a sequence of accumulated values. Each accumulated value in the sequence is compared with at least one of the previous accumulated value in the sequence and the difference therebetween determined. This difference is compared with preselected values denoting the allowable limits of the return flow rate of the fluid. If the difference between the accumulated values exceed the preselected values, an indication is provided of the commencement of a blowout or lost circulation. This abstract is not intended to define this invention which, of course, is measured by the claims, or to be limiting in any way as to the scope of this invention.

Description

O Unlted States Patent 1191 1111 3,760,891

Gadbois Sept. 25, 1973 BLOWOUT AND LOST CIRCULATION Primary Examiner-David H. Brown DETECTOR AttorneyJoe E. Edwards et a1.

[75] Inventor: John F. Gadbois, Houston, Tex I [73] Assignee: The Offshore Company, Houston, [57] ABSTRACT A method and apparatus for rapidly detecting blowouts [22] Filed: May 19 1972 and lost circulation in a drilling well, having particular application to a well being drilled at sea from a floating 1 1 pp 2551160 drilling platform. The return rate of flow of drilling fluid which will be affected by a blowout is monitored 52 us. 01. 175/48, 73/155 and transduced an electrical Signal PIOPOYtiOnal 511 1m. (:1 E2lb 47/10 therew- The magnitude ofthe electrical Signal is moni- 58 Field 61 Search 175/48 50 40 8- tored and mumulaed overselwed overl"ipPlngimervals of time to form a sequence of accumulated values. Each accumulated value in the sequence is compared 56] References Cited with at least one of the previous accumulated value in the sequence and the difference therebetween determined. This difference is compared with preselected values denoting the allowable limits of the return flow UNITED STATES PATENTS 2,290,179 7/1942 Hayward 175/48 X 2'966'059 12/ Dower /48 X rate of the fluid. If the difference between the accumu- 3,595,075 7/1971 Dower 73/155 lated values exceed the preselected values, an 1nd1ca- 3,602,322 8/1971 GOISuCh /48 .d d f h f bl 3,614,761 10/1971 Rehm et al. 175/48 x 6 F e commellcemeflto a 3,646,808 3 1972 Leonard 175 40 x 10st clrculatlon- T1115 abstract 15 not mended to define this invention which, of course, is measured by the FOREIGN PATENTS OR APPLICATIONS claims, or to be limiting in any way as to the scope of 156,125 4/1962 U.S.S.R 175/48 this invention 186,915 12/1964 U.S.S.R 175/48 12 Claims, 4 Drawing Figures PATENTEDSEPZS i975 SNEEI 1 BF 2 llllllllllk f PAIENTEU 3.760.891

sum

2 or 2 1 BLOWOUT AND LOST CIRCULATION DETECTOR BACKGROUND AND OBJECTS OF THE INVENTION This invention relates to a blowout and lost circulation detector and method.

In drilling a well, particularly an oil well, there exists the danger of drilling into an earth formation that contains high pressure fluids. When this occurs, the high pressure fluid from the formation flows into the well bore and displaces the drilling fluid (mud) up the well. If this occurence is not controlled rather quickly, the drilling fluid may be substantially displaced and the high pressure fluid may flow freely up the well bore. This is termed a blowout.

On the other hand, occasionally the well is drilled into an earth formation which is very porous. In such a situation, there may be a tendency for the drilling fluid to flow freelyinto the surrounding earth formation. This is termed lost circulation.

Blowout prevention is most effective when the commencement of an influx of high pressure fluid into the well can be quickly detected and controlled before an appreciable height of the drilling fluid column is displaced from the well. Loss of drilling fluid is kept to a minimum when the commencement of the loss can be quickly detected and the flow of the fluid controlled before an appreciable amount has passed from the well into the earth formation. It is old in the art to detect such an influx or loss of fluid be comparing the flow rate of the drilling fluid into the well and the flow rate of the return fluid out of the well. A substantial increase in the rate of the return fluid flow when there was no increase in the rate of the fluid flow into the well is indicative of the blowout. A substantial decrease in the rate of the return fluid flow when there was no decrease in the rate of the fluid flow into the well is indicative of lost circulation.

However, in drilling offshore wells from floating platforms, the measurement of the return fluid flow from the well is complicated by the motions of the vessel or drilling platform. These motions, particularly heave, produce cyclic variations in the return fluid flow so that it becomes difficult to determine whether an increase return flow is due to a blowout or due to the vessel motion.

The improved blowout and lost circulation detector and method according to this invention monitors, preferably at the surface, the flow rate of the returning drilling fluid and transduces this rate of flow into an electrical signal which has one of its parameters, such as current or voltage, proportional in magnitude thereto.

The magnitude of this chosen parameter of the electrical signal is measured, either continuously or at constant time intervals. Commencing at a first point in time, the measured values of this parameter are accumulated over a selected time interval and an accumulated value is obtained. Commencing at a second point in time, the measured values of this parameter are again accumulated for the same selected time interval and another accumulated value is obtained. This process is continued whereby a sequence of accumulated values is obtained. Each accumulation preferably overlaps in time with the preceding accumulation so that certain measured values of the chosen parameter of the electrical signal are included in both. Each accumulated value is compared with at least one of its predecessor in the sequence and the algebraic difference therebetween determined. The absolute magnitude of such difference is then compared with preselected limiting values chosen to denote the transient changes in the return rate of flow representative of normal conditions, such as the cyclic heave of the vessel. If the absolute magnitude of the difference between any of the selected accumulated values exceeds the limiting values, indicating means are actuated which, depending upon the algebraic sign of the difference, warn the operators of a p07 tential blowout or lost circulation.

It is, therefore, an object of this invention to provide an improved method and apparatus for detecting blowouts and lost circulation in wells wherein only the return flow rate of the drilling fluid, rather than the flow rate of the drilling fluid both into and out of the well, need to be monitored.

It is an additional object of this invention to provide an improved apparatus and method for detecting blowouts and lost circulation in a well being drilled at sea wherein a minimum of error is introduced by the motion of the drilling platform.

BRIEF DESCRIPTION OF THE DRAWINGS These and other objects and advantages of the present invention are hereinafter set forth and explained with reference to the drawings wherein like reference numbers indicate like parts;

FIG. 1 is an elevational view of a typical well drilling apparatus, showing the flow of drilling fluid into and out of the well and the preferred apparatus for measuring the rate of the return flow: 1

FIG. 2 is a graph of the height of a fluid column representative of the return flow rate of the drilling fluid versus time;

FIG. 3 is a conjunctive graph of the time intervals between the accumulated values of the transduced electrical signal;

FIG. 4 is a block diagram of the electrical components of a preferred blowout detector according to this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 illustrates schematically certain portions of an offshore well drilling apparatus. Secured to and located below the floor of a vessel or drilling platform is a conductor pipe 10. Positioned within this conductor pipe is the drill string 12. The conductor pipe 10 and drill string 12 extend from the drilling platform or vessel downwardly through the wellhead at the ocean bottom and into the well. Drilling fluid or mud 14 is forced downwardly through the interior of the drill string. This mud 14 exits through the lower end of the drill string and is forced upwardly in the annulus between the drill string and the conductor pipe. At the surface, the return flow of the mud is transmitted to the mud processing equipment through diverter conductor 16.

The method and apparatus of detecting blowouts and lost circulation according to this invention measures the rate of flow of the returning mud. Although the return flow rate may be measured by any of numerous commercially available devices, preferably a pressure tubing 18 is attached to conductor pipe 10 to provide a relatively small diameter fluid column. The height (h) of this fluid column will vary proportionally to the rate of flow of the returning mud. Transducing means 20,

such as a pressure cell, is in communication with the fluid column and functions to transform a selected parameter, such as pressure or height, of the fluid column into an electrical signal having one of its parameters, preferable voltage, proportional thereto. Thus, changes in the rate of flow of the returning mud, whether caused by subsurface blowouts, lost circulation, or the cyclic motion of the drilling vessel, will be reflected in the electrical signal generated by transducer 20.

Assuming a constant, known input flow rate, it is possible to calculate the expected output flow rate and, assuming all potential variables to be constant, it is possible to calculate the magnitude of the voltage of the electrical signal which will be generated by transducer as the result of this expected return flow rate. However, as illustrated in FIG. 2, the voltage component of the typical electrical signal generated by transducer 20 does not assume a constant magnitude. The normal variations of the electrical signal from the expected magnitude (shown as line 22) of voltage are the result of the irregular cyclic motions of the drilling vessel. Irregular variations in the transduced electrical signal, are evidence of potentially dangerous conditions.

The method and apparatus for detecting blowouts from the return rate of flow of the drilling fluid according to this invention utilizes a plurality of accumulations of the values of the voltage of the transduced electrical signal to obviate the effects of the normal, transient changes in the return flow rate. As illustrated in FIG. 3, commencing at point in time 24, all values of the voltage of the transduced electrical signal are accumulated for a time interval t, to produce a first accumulated value. After a preselected interval of time 1 another accumulation is performed over the same time interval 1, to obtain a second accumulated value. FIG. 3 illustrates ten accumulators being utilized to produce a sequence of accumulations of the values of the voltage components of the transduced electrical signal over time intervals t The commencement of the operation of each accumulator is delayed in time by interval t from the commencement of the preceding accumulation. The ratio of 1 /1 is equal to the total number of accumulators, so that monitoring of the transduced electrical signal is continuous.

The time interval t between the commencement of the operation of the various acumulation operations should depend upon the number of accumulators utilized and the time interval I that each accumulator operates. The time interval should depend not only upon the number of accumulators utilized but also upon the desired accuracy of the system. The predominant motion cycle of the drilling vessel should be determined and the accumulation time interval t be regulated to be relatively long in comparison to such predominant motion period so that a large number of motion cycles of the drilling vessel are included in each accumulation.

Each accumulated value of the voltages is compared with at least one of the preceding accumulated values of the voltages and the algebraic difference therebetween determined. More than one comparison may be made simultaneously. For example, ifit is desired to detect only changes of great magnitude (kicks") in the return rate of flow, then each accumulated value of the voltages preferably is compared with the immediately preceding accumulated value of the voltages. But if it is desired to detect somewhat slower changes in the return rate of flow, each accumulated value of the voltages might be compared with the fifth preceding accumulated value of the voltages. Preferably both of these comparisons may be made with each accumulated value of the voltages.

By accumulating the values of the voltages over a time period relatively long with respect to the cyclic period of the drilling vessel or other regularly occurring condition in the drilling operation and then comparing each accumulated value with at least one of the preceding values to determine an algebraic difference, the cyclic motions of the drilling vessel are greatly dampened. However, to eliminate completely the cyclic effects of the vessel on the return rate of fluid flow, each algebraic difference is then compared with preselected limiting values denoting allowable changes in the return rate of flow of the drilling fluid. If the absolute magnitude of the algebraic difference is greater than the preselected limiting values, then a condition exists of which the operators of the well should be aware. The algebraic sign will determine whether the change in the return rate of flow is indicative ofa blowout or lost circulation.

FIG. 4 illustrates diagrammatically the preferred electrical components of a blowout and lost circulation detector according to this invention. The output of pressure/electrical transducer 20 is coupled to the input of an input switching circuit 32. Also coupled to the input of switching circuit 32 is a timing signal from clock 34. The output of input switching circuit 32 is coupled to the inputs of each of 10 accumulators 36. Input switching circuit 32 functions, responsive of its timing signal, to phase the transduced electrical signal to the individual accumulators in a preselected order, at preselected time intervals and for preselected time intervals. Each accumulator 36 functions to accumulate the voltage component of the electrical signal applied to its input.

The outputs of the individual accumulators 36 are coupled to output switching circuit 38 which functions to control the sequence of accumulator output comparison so that the proper accumulated values are coupled to the input of first comparator 40. Output switching circuit 38 sequentially couples the output signals of the selected accumulators 36 to the input of first comparator 40, which functions to compare the two signals, determine the difference therebetween and attach the proper algebraic polarity thereto to denote a gain or loss. The output of first comparator $0 is coupled to the input of a second comparator 42 which functions to determine whether the absolute magnitude of the signal received at its input falls within preselected limits denoting allowable gain or loss. The preselected limits are dependent upon the accuracy of the system and should be utilized to filter out increases or decreases in the accumulated values caused by the cyclic motion of the drilling vessel and other regular variations of the drilling operation. Whenever the signal received at the input of second comparator 42 falls outside the preselected limits, second comparator 42 generates an output signal which, depending upon the algebraic sign of the signal generated by comparator 40, actuates indicating means 44 to alert the operator of the danger of a blowout.

A secondary output from first comparator 40 may be available for application to a readout, recorder or memory device 46 to facilitate further comparisons on a longer time basis.

The components illustrated in the preferred embodiment of the blowout detector may be any of numerous commercially available electronic switching and logic circuits well known to those skilled in the art.

Thus, this invention provides an improved method and apparatus for rapidly detecting blowouts and lost circulation in a drilling well by monitoring only the return flow rate of drilling fluid, wherein normal cyclic changes in such return flow rate are filtered. As such, the method and apparatus has particular application to offshore drilling.

Many variations in the form of the preferred embodi- 'ment will now be apparent to those skilled in the art.

For instance, numerous other means, such as piezoelectric pressure transducers, may be utilized to determine the rate of flow of the returning drilling fluid. Additionally, different electrical circuitry may be utilized to accumulate the transduced signal over the selected time intervals. Therefore, the invention should not be limited to the illustrated embodiment, but rather should extend to the full scope and spirit of the invention defined in the appended claims.

What is claimed is:

l. A method of detecting the commencement of blowouts and lost circulation in a well in which a drilling fluid is being utilized, including the steps of:

transducing the rate of flow of the drilling fluid out of the well into an electrical signal proportional thereto;

commencing at a first point in time, accumulating said electrical signal over a first selected interval of time;

commencing at a second point in time occurring a second selected interval of time after said first point in time, accumulating again said electrical signal over said first interval of time;

repetitively performing said step of accumulating said electrical signal over said first selected interval of time to obtain a continuous sequence of accumulated values, each step of accumulation commencing at said second selected interval of time after the commencement of the preceding step of accumulation; and

comparing each of said accumulated values with at least one selected accumulated value preceding it in said sequence to determine the difference in value therebetween, a preselected difference between said accumulated values being indicative of the commencement of one of a blowout and lost circulation.

2. The method according to claim 1, including the step of:

comparing each of said differences in accumulated values with selected limiting values denoting selected limits in transient changes of the return flow rate of the drilling fluid,

said preselected limiting values being such that normal changes in the return flow rate produce differences between said acccumulated values that fall within said limiting values and blowouts and lost circulation produce differences between said accumulated values that do not fall within said limiting values. 3. A method according to claim 1, including the step of: 1

actuating means for indicating the commencement of one of a blowout and lost circulation responsive to any of said differences between said accumulated values falling outside said preselected limiting values. 4. A method according to claim 1, including the steps of:

determining whether each of said accumulated values is greater than or less than the ones of said preceding accumulated values with which it is compared, and assigning an algebraic sign to each of said differences therebetween indicative thereof;

comparing each of the absolute magnitudes of said differences in accumulated values with preselected limiting values denoting selected limits in the transient changes of the return flow rate of the drilling fluid; and

actuating means for indicating the commencement of one of a blowout and lost circulation if any of the absolute magnitudes of said differences exceeds said preselected limits;

said actuating means indicating the commencement of a blowout if said algebraic sign indicates said return rate of flow is increasing and indicating the commencement of lost circulation if said algebraic sign indicates said return rate of flow is decreasing. 5. A method according to claim 1 wherein the step of transducing produces an electrical signal having one of its parameters of voltage and current proportional in magnitude to said return flow rate.

6. A method of obtaining a signal representative of the return flow rate of drilling fluid in a well and free of ambiguities caused by cyclic and regularly occurring variations, comprising:

transducing said return flow rate into an electrical signal having one of its parameters of voltage and current proportional in magnitude thereto;

repetitively accumulating the magnitudes of said parameter of said signal over a first selected interval of time to obtain a sequence of accumulated values, each step of accumulation commencing at a second selected interval of time after the commencement of the preceding step of accumulation;

comparing each of said accumulated values with at least one selected preceding accumulated value in the sequence to determine the difference in value therebetween; determining limiting values representative of changes between said accumulated values produced by normal cyclic and regularly occurring variations in the return flow rate;

comparing said difference between accumulated values with said selected limiting values; and

generating an electrical signal responsive to the difference between said accumulated values exceeding said selected limiting values.

7. In a well in which drilling fluid is being utilized, a blowout and lost circulation detector comprising:

means for sensing the rate said drilling fluid flows out of said well;

means connected to the output of said sensing means for transducing said return rate of flow into an electrical signal proportional thereto;

means connected to the output of said transducing means for obtaining a sequence of accumulated values of said transduced electrical signal, each of said accumulated values being obtained over a first selected interval of time and the commencement of obtaining each accumulated value occurring at a second selected interval of time after the commencement of obtaining the preceding accumulated value in the sequence; and

a first comparator connected to the outputs of said accumulating means for comparing each of said accumulated values with at least one selected preceding accumulated value in the sequence and for determining the difference in value therebetween, a

preselected difference between said accumulated values being indicative of the commencement of one of a blowout and lost circulation.

8. A detector according to claim 7, wherein: said means for determining said return flow rate includes a fluid conductor in communication with said drilling fluid and vertically positioned to provide a columnar receptacle for said drilling fluid, and

said means for transducing said return rate of flow into an electrical signal includes a pressuresensitive transducer in communication with said columnar receptacle which transforms the pressure of the drilling fluid therein into an electrical signal having one of its parameters of voltage and current proportional thereto.

9. A detector according to claim 7 wherein said means for obtaining said sequence of accumulated values comprises:

a clock generating a timing signal;

a first switching means connected to the output of said clock and to the output of said transducing means; and

a plurality of accumulators, each of which is connected to the output of said first switching means,

said first switching means coupling the output of said transducing means responsive to said timing signal to selected ones of said accumulators, at selected points in time and for selected intervals of time.

10. A detector according to claim 7, including: said first comparator also determines whether each of said accumulated values is greater than or less than the preceding accumulated values with which it is compared and assigns to said difference an algebraic sign indicative thereof;

a second comparator coupled to the output of said first comparator for comparing the absolute magnitude of each of said differences in accumulated values with preselected limiting values denoting selected limits in transient changes of the return flow rate of said drilling fluid; and

means connected to the output of said second comparator for indicating the commencement of one of a blowout and lost circulation whenever any of the absolute magnitudes of the differences exceeds said preselected limiting values, said means indicating one of a blowout and lost circulation depending upon said algebraic sign assigned to said difference.

1 l. A detector according to claim 7, including: a second comparator connected to the output of said first comparator for comparing each of said differences in accumulated values with preselected limiting values denoting selected limits in transient changes of the return flow rate of the drilling fluid,

said preselected limiting values being such that normal changes in the return flow rate produce differences between said accumulated values that fall within said limiting values and blowouts produce differences between said accumulated values that do not fall within said limiting values.

12. A detector according to claim 11, including: means connected to the output of said second comparator for indicating the commencement of one of a blowout and lost circulation when any of said differences between said accumulated values fall outside said preselected limiting values.

Claims

1. A method of detecting the commencement of blowouts and lost circulation in a well in which a drilling fluid is being utilized, including the steps of: transducing the rate of flow of the drilling fluid out of the well into an electrical signal proportional thereto; commencing at a first point in time, accumulating said electrical signal over a first selected interval of time; commencing at a second point in time occurring a second selected interval of time after said first point in time, accumulating again said electrical signal over said first interval of time; repetitively performing said step of accumulating said electrical signal over said first selected interval of time to obtain a continuous sequence of accumulated values, each step of accumulation commencing at said second selected interval of time after the commencement of the preceding step of accumulation; and comparing each of said accumulated values with at least one selected accumulated value preceding it in said sequence to determine the difference in value therebetween, a preselected difference between said accumulated values being indicative of the commencement of one of a blowout and lost circulation.

2. The method according to claim 1, including the step of: comparing each of said differences in accumulated values with selected limiting values denoting selected limits in transient changes of the return flow rate of the drilling fluid, said preselected limiting values being such that normal changes in the return flow rate produce differences between said acccumulated values that fall within said limiting values and blowouts and lost circulation produce differences between said accumulated values that do not fall within said limiting values.

3. A method according to claim 1, including the step of: actuating means for indicating the commencement of one of a blowout and lost circulation responsive to any of said differences between said accumulated values falling outside said preselected limiting values.

4. A method according to claim 1, including the steps of: determining whether each of said accumulated values is greater than or less than the ones of said preceding accumulated values with which it is compared, and assigning an algebraic sign to each of said differences therebetween indicative thereof; comparing each of the absolute magnitudes of said differences in accumulated values with preselected limiting values denoting selected limits in the transient changes of the return flow rate of the drilling fluid; and actuating means for indicating the commencement of one of a blowout and lost circulation if any of the absolute magnitudes of said differences exceeds said preselected limits; said actuating means indicating the commencement of a blowout if said algebraic sign indicates said return rate of flow is increasing and indicating the commencement of lost circulation if said algebraic sign indicates said return rate of flow is decreasing.

5. A method according to claim 1 wherein the step of transducing produces an electrical signal having one of its parameters of voltage and current proportional in magnitude to said return flow rate.

6. A method of obtaining a signal representative of the return flow rate of drilling fluid in a well and free of ambiguities caused by cyclic anD regularly occurring variations, comprising: transducing said return flow rate into an electrical signal having one of its parameters of voltage and current proportional in magnitude thereto; repetitively accumulating the magnitudes of said parameter of said signal over a first selected interval of time to obtain a sequence of accumulated values, each step of accumulation commencing at a second selected interval of time after the commencement of the preceding step of accumulation; comparing each of said accumulated values with at least one selected preceding accumulated value in the sequence to determine the difference in value therebetween; determining limiting values representative of changes between said accumulated values produced by normal cyclic and regularly occurring variations in the return flow rate; comparing said difference between accumulated values with said selected limiting values; and generating an electrical signal responsive to the difference between said accumulated values exceeding said selected limiting values.

7. In a well in which drilling fluid is being utilized, a blowout and lost circulation detector comprising: means for sensing the rate said drilling fluid flows out of said well; means connected to the output of said sensing means for transducing said return rate of flow into an electrical signal proportional thereto; means connected to the output of said transducing means for obtaining a sequence of accumulated values of said transduced electrical signal, each of said accumulated values being obtained over a first selected interval of time and the commencement of obtaining each accumulated value occurring at a second selected interval of time after the commencement of obtaining the preceding accumulated value in the sequence; and a first comparator connected to the outputs of said accumulating means for comparing each of said accumulated values with at least one selected preceding accumulated value in the sequence and for determining the difference in value therebetween, a preselected difference between said accumulated values being indicative of the commencement of one of a blowout and lost circulation.

8. A detector according to claim 7, wherein: said means for determining said return flow rate includes a fluid conductor in communication with said drilling fluid and vertically positioned to provide a columnar receptacle for said drilling fluid, and said means for transducing said return rate of flow into an electrical signal includes a pressure-sensitive transducer in communication with said columnar receptacle which transforms the pressure of the drilling fluid therein into an electrical signal having one of its parameters of voltage and current proportional thereto.

9. A detector according to claim 7 wherein said means for obtaining said sequence of accumulated values comprises: a clock generating a timing signal; a first switching means connected to the output of said clock and to the output of said transducing means; and a plurality of accumulators, each of which is connected to the output of said first switching means, said first switching means coupling the output of said transducing means responsive to said timing signal to selected ones of said accumulators, at selected points in time and for selected intervals of time.

10. A detector according to claim 7, including: said first comparator also determines whether each of said accumulated values is greater than or less than the preceding accumulated values with which it is compared and assigns to said difference an algebraic sign indicative thereof; a second comparator coupled to the output of said first comparator for comparing the absolute magnitude of each of said differences in accumulated values with preselected limiting values denoting selected limits in transient changes of the return flow rate of said drilling fluid; and means connected to the output of said second comparator for indIcating the commencement of one of a blowout and lost circulation whenever any of the absolute magnitudes of the differences exceeds said preselected limiting values, said means indicating one of a blowout and lost circulation depending upon said algebraic sign assigned to said difference.

11. A detector according to claim 7, including: a second comparator connected to the output of said first comparator for comparing each of said differences in accumulated values with preselected limiting values denoting selected limits in transient changes of the return flow rate of the drilling fluid, said preselected limiting values being such that normal changes in the return flow rate produce differences between said accumulated values that fall within said limiting values and blowouts produce differences between said accumulated values that do not fall within said limiting values.