US3550696A - Control of a well - Google Patents

Description

United States Patent 1111 3,550,696

[72] Inventor John W. Kenneday [56] References Cited n6us:6n,.Tex. UNITED STATES PATENTS [2H pp 349,238 2,082,329 6/1937 Foran et al 175/205 1 Filed July 25,1969 3,213,939 10/1965 Records 166/75 1 Patsmed 3,268,017 8/1966 Yarbrough 175/25 1 Asslsnee E859 Product Research m 3,338,319 8/1967 Griffin 175/25 Continuation of application Ser. No. 625,767 Man 24, 1967 abandoned Primary Exammer.lames A. Leppink Continuation-impart of application Ser. No. Schneider 517,036, Dec. 28, 1965, now abandoned.

ABSTRACT: Well control method and apparatus for auto- [54] g Y F' matically maintaining an essentially constant bottomhole presm sure in a well borehole during circulation of drilling fluid from [52] U5. Cl. 175/25, a surface inlet down a drill string located in the borehole and I 166/75 up the annulus between the drill string and the borehole wall [51] Int. Cl. ..F21b 33/03, to a surface annulus discharge by controlling the rate of F2lb 7/00 drilling fluid discharge from the annulus discharge in response [50] Field of Search 166/5, .6, to variations of drilling fluid pressure at the surface inlet from 75, 267, 244; l75/25, 57, 65, 205 a preselected fluid pressure.

Pmmmaczslsm I 3550.696

SHEEI 1 .UF 3

INVENTOR.

i I JOHN w. KENNEDAY,

@18- BYM/dfig.

ATTORNEY.

PATENIfinniczslsm 3,550,696

SHEU 3 [IF 3 GPOWER FLUID DRIVE Mu0 FROM PIT CONST'ANT RATE CONTROLLER if I SWIVEL -|7 DRILL PIPE\ PRESSURE 1 SENSING 1 coum RoLLzR 1 INVENTOR.

JOHN W. KENNE DAY,

ATTORNEY.

CONTROL A WELL This application is a continuation of Ser. No. 625,767, filed Mars. 24, 1967, by John W. Kenneday, entitled, Control ofa Well," which was a continuation-in-part application based on application Serial No. 517,036, filed Dec. 28, 1965, by John W. Kenneday, entitled Control of a Well," both now aban- ,technique of well control. This technique is more easily explained by considering a well bore containing a drill pipe as an open ended U-tube The bottomhole pressure is common to both columns and is reflected in the drill pipe and annulus pressures. Variations in either pressure are transmitted through the respective hydrostatic columns and these variations are of equal magnitude and in the same direction. Applying this principle to the dynamic process of circulating an extraneous fluid (gas, oil and/or salt water) from a well, drill pipe pressure can be controlled by regulating the annulus back pressure at the surface such that a constant bottomhole pressure is maintained.

The present invention may be briefly described as a method for automatically controlling a well during drilling operations inwhich the drilling fluid is circulated through a hollow drill string and up the annulus between the drill string and the wall of the well. In accordance with the present invention, when a sudden surge of pressure is encountered, the annulus between the drill string and the wall of the well is sealed. After the drill bit encounters the higher pressure formation, pressure is imposed on the circulating drill fluid in an amount sufficient to overcome the pressure in the drill string. Flow of drilling fluid from the annulus is controlled responsive to the pressure of drilling fluid into the drill string.

In accordance with the present invention,'the drilling fluid is circulated by pumping to maintain a substantially constant bottomhole pressure. Pressure of the circulating drilling fluid controls the amount of drilling fluid returned from the annulus.

In summary, the preferred steps of I the well control procedure may be:

( l Detect a well kick.

(2) Shut-in the well while rotating the drill pipe.

(3) Measure drill pipe pressure and annulus pressure at the surface.

(4) Start circulating the well with annuluspressure greater than (e.g., 100 p.s.i. greater than) shut-in annulus pressure.

(5) Observe drill pipe pressure at circulation rate of step (6) Circulate the well while automatically maintaining the drill pipe pressure essentially constant at the observed drill pipe pressure. 4

In the present invention in which a hollow drill string is employed, and means for introducing drilling fluid into the hollow drill string including a pump means and means for discharging drilling fluid from the annulus between the wall of the well are provided, the apparatus comprises first valve means in the discharge means for controlling the discharge of the drilling fluid at a controlled pressure. Means are provided for supplying operating fluid under regulated pressure to the first valve means and a pressure sensing control means which may be a pressure-responsive pilot (second) valve means provided on the introduction means for controlling the pressure of drilling fluid into the hollow drili String by controlling discharge of drilling fluid friom the annulus with the first valve means. The pressure sensing control means is operatively connected to the first valve means which is responsive to the con-' trol means; pressure of the drilling fluid introduced into the drill string controls discharge of drilling fluid from the annu lus. Thus, in accordance with the present invention, the control means is operatively connected to the first valve means which is responsive to the pressure of drilling fluid circulated past the control means into the drill string.-

In addition, means may be provided for interrupting transmission of the control signal responsive to changes in drill pipe pressure a predetermined amount of time (the time required for the pressure wave or pulse resulting from opening and closing of the first valve means to travel down the annulus and up the drill pipe to the control means) to permit the control means to sense the pressure effects in the drill pipe caused by previous changes in the position of the first valve means before making an additional change in the position of the first valve means. This lag" time may vary, for example, up to 20 seconds for well depths up to 10,000 feet, respectively. Lag time is a function of column length and pressure transmissibility factors, principally fluid density.

The present invention is quite important and advantageous in that entry of formation fluids, such as gas, into the well is balanced and control of the discharge of extraneous fluids safely and automatically through the annulus choke (first valve means) line is provided while maintaining a selected bottomhole pressure. Thus, in accordance with the present invention, automatic control of annulus returns by circulating pump pressure allows unskilled personnel to handle a sudden increase in pressure due to gas entry without running into the danger of sticking the pipe and encountering lost returns. Also, by controlling bottomhole pressure, further entry of formation fluids is avoided and formationfracturing is prevented by avoiding excessive pressure on the well by permitting fast and efflcient well control and release or discharge of the unwanted fluids. Additionally, by having the pump operation at a preselected (constant) rate (or rates) and by controlling operations relative to the hydrostatic column in the well,

operations may be conducted automatically. The advantage of a constant rate is that it makes circulating friction loss constant.

The present invention will be further reference to the drawing in which:

FIG. 1 represents schematically one mode and embodiment of the present invention;

FIG. 2 illustrates schematically another embodiment of the invention;

FIG. 2A is an enlarged, partly sectional view of one of the components shown in FIG. 2;

FIG. 3 illustrates schematically still another modification of the invention;

FIG. 3A is a detailed view of the pressure sensing controller apparatus illustrated in FIG. 3; and

FIG. 3B is a detailed view of another pressure control component illustrated in FIG. 3.

Referring now to the drawing which represents a best mode described with l and embodiment and particularly in FIG. 1, numeral 11 designates the earths surface into which 192 well 196 has been drilled comprising a portion 13 in which a casing 14 has been cemented with cement 15 and an openhole portion 16 in which the drill string 17 including a bit 18 is arranged. The drill string 17 is suspended in the well (by means not shown) from the drilling rig l9 and the drill string is rotated by suitable rotary table 20 resting on a foundation 21. In the cellar 22 of the well, a blowout preventer 23 is provided for sealing off the casing 14. Blowout preventer 23 is preferably of the type which allows pipe rotation. The'casing 14 is provided with one or more flow or choke lines 24 in which a pressure-responsive valve 25 is provided for controlling the discharge from the annulus A in the well 12. The flow line 24 discharges into a mud pit 26 whence suction is taken through line 27 into'pump 28. Line 29 discharges drilling fluid from pump 28 to a swivel 30 and thence down through the drill string 17 and up the annulus A as has been described.

Arranged on the conduit 29 is a pressure sensor control 32 which may be a pressure-responsive diaphragm or piston or a pressure-responsive pilot valve. Power fluid, such as air or any other gas, at a selected pressure is introduced by line 34 against a diaphragm or piston in pressure-responsive valve 25 to allow the valve 25 to open and/or close in response to variations from a preselected drill pipe pressure. Pressure from sensor 32 is transmitted by line 35 against the diaphragm or piston or other pressure biasing means, such as a spring of pressure-responsive valve 25, such that pressure-responsive valve 25 may either open or close (throttled) to allow more or less drilling fluid to be discharged through line 24 into mud pit 26.

Pressure sensor 32 transmits pressure increases in drill pipe 29 above a preset selected pressure to the pressure-responsive means in valve 25, which is balanced by the power fluid. Decreases in drill pipe pressure cause the power fluid to close down automatically on valve 25. First valve means 25 (motor valve) may be of the type shown and described on page 1834 of the I962-63 Composite Catalog of Oil Field Equipment and Services type I25PP or I25PA (Fisher Governor Company).

Alternatively, a Bourdon type tube type regulator with reset (time delay) and a motor valve could be used. The Wizard type controller illustrated on page 1836 of the 1962-63 Composite Catalog of Oil Field Equipment and Services" modified by pressure sensing and control signaling piping redirected to sense and signal as described later herein with respect to the embodiment of FIG. 3 would be satisfactory.

A directory powered input pressure-responsive controller motor valve (combination unit) which would be suitable for use as the pressure sensing and valve elements is shown on page 2168 of the I96263 Composite Catalog of Oil Fuel Equipment and Services.

Thus, in accordance with the present invention, assuming blowout conditions to exist in the well (mud hydrostatic column is insufficient to overbalance formation pressure in, for example, a permeable gas-bearing zone), the blowout preventers are closed to seal the annulus (the pump may be shut down to halt circulation) and drill pipe pressure is read as by gauge 38 connected to line 29 to determine the required increase in mud hydrostatic column which is the minimum necessary to balance the bottom hole formation pressure. The well is then circulated, discharging from the annulus through the choke line 24 using pump pressure to overcome formation pressure. Pilot valve 32 and pressure-responsive throttle valve 25 control discharge of fluid from the annulus.

In practice, when a sudden increase in pressure is noted in the well, such as by influx of gas from a permeable formation, such as 37 encountered by the drill bit 18, the blowout preventer 23 is closed to seal the annulus A. The influx of extraneous fluid, such as gas, into a well while drilling may be detected by 1) increase in mud pit levels; (2) increase in rate of mud returns; (3) unexpected sustained drilling break; (4) swab check with drill pipe; or (5) fillup and flowback measurements during trips. Once fluid influx is detected. the pump is shut down and the well is shut in. The drill pipe preferably continues to rotate. Drill pipe pressure. readings are made from gauge 38. Pump 28 is operated at a selected essentially constant rate (or rates if pump rate is to be periodically changed) and controls the flow of drilling fluid through conduit 29 to drill string 17 by virtue of pressure on valve 25 which would be opened or closed, depending on the variations in fluid pressure from pilot valve 32. This series of operations allows the advantages ofthe present invention to be obtained.

In the modification of the invention illustrated in FIG. 2, a dampener unit 72 is located in line 35. This unit may be used to interrupt transmission of pressure pulses from conduit 29 to an input pressure controller 71 which controls the signals to the pressure-responsive means 36 in throttle valve 25. Dampener 72 may be any well-known, commercially available apparatus suitably of the dash pot type provided with an adjustable orifice to regulate the time required to return to pressure transmitting position. The time delay means, although shown as a dampener positioned in line 35 for purposes of illustration, may be part of the signal line 73, controller 71 or valve motor 36. Also shown in FIG. 2 are a manually operated valve 31 and a pressure gauge 33 located in choke line 24 upstream of throttle valve 25. AIso,-a valve may be provided in line:

35 as shown.

A suitable dampener is illustrated in FIG. 2A. A housing 75 contains chambers 76 and 77 separated by a barrier containing an opening 79 through which a rod 80 extends. Seals 81 are arranged in opening 79 to prevent fluid communication between chambers 76 and 77. A piston 82 provided with a small equalizing orifice 83 is arrangedon one end of rod 80 in chamber 76 between inlet line 35 and signal line 84. A compression spring 85 biases piston 82 to the right as shown in FIG. 2A. Another piston 86 is arranged in chamber 77 on the other end of rod 80. Piston 86 is provided with a flapper valve 87 containing an orifice 88 which opens with movement of piston 86 to the left and closes with movement of piston 86 to the right as shown in FIG. 2A. Conduit 84 contains a floating piston (or diaphragm) 90 which is used to avoid mixing of fluids.

As shown in FIG. 2A, piston 82 is in balanced or static position in which position P =P When P becomes greater than P because of increased pressure in conduit 29, piston 82 is pressured rapidly to the left to cover the opening of signal conduit 84 into chamber 76 which causes spring 85 to compress and flapper valve 87 to open as piston 86 is moved to the left. Then spring 85 slowly moves piston 82 to the right against the resistance of fluid passing through orifice 88 to return piston 82 to its static position at which P equals P The time required for piston 82 to return to its static position as it is delayed by passage of fluid through orifice 88 in valve 86 is the lag time. A decreased pressure P, will result in a similar operation, except lag time is measured as the piston 82 initially moves to the right.

In operation, flow dampener 72 interrupts passage of pressure pulses transmitted through line 35 for approximately the amount of time required for a pressure pulse to travel from throttle valve 25 down annulus A and up drill string 17 to line 35. Such delay or lag time is desired in order to reduce the magnitude of the pressure variations which occur from actuation of throttle valve 25 in response to drill pipe pressure variations sensed by pilot valve 32. The "lag time permits the pressure signal from line 29 to actuate and control the position of throttle valve 25 at the time it has sensed the pressure in line 24 resulting from a previous change in the position of valve 25.

Referring now to FIG. 3 in which identical parts are identified by identical numerals as those in FIG. 1, in this mode and embodiment, means are provided for automatic setting of pump pressure control level as a function of pump rate to compensate for changes in circulating fluid friction. The embodiment of FIG. 3 provides for (I) automatically maintaining essentially constant drill pipe pressure by controlling annulus fluid discharge responsive to drill pipe pressure thereby keeping the bottomhole pressure essentially constant (embodiment of FIGS. land 2) plus (2) means for automatically adjusting the drill pipe pressure an amount equivalent to changes in circulating friction resulting from changes in pump rate, thereby keeping bottomhole pressure essentially constant for different circulating rates. Automatic adjustment of the preselected drill pipe pressure to compensate for changes in circulating friction can be achieved by various mechanical, electrical, hydraulic-mechanical, pneumaticmechanical or fluidic control devices well known in the art which sense input circulating rate (pump stroke counter, meter circulation rate, measure input power consumption) and issue a control signal. Control is, in turn, readily correlated to and calibrated for circulating friction by mechanical, electrical, hydraulic, pneumatic or fluidic devices.

One desirable manner of hydraulically-mechanically adjusting the preselected drill pipe control pressure level is illustrated in FIG. 3. A circulating pump 28 takes suction through line 27 from mud pit 26 as described with respect to FIG. 1. Pump 28 circulates drilling fluid through line 29 and through drill string 17 as described in FIG'. 1 and up annulus A and thence through line 24 which is controlled by a throttle valve 40. Throttle valve 40 is actuated through a diaphragm4l and a biasing or spring means 42. The top side of diaphragm 41 has power fluid working against the diaphragm. 41 which is introduced thereto through line 43 while power fluid is introduced on the underside of diaphragm 41 aiding the spring 42 by line 44. This will be described in more detail hereinafter. Line 45 branches from line 29 and-connects into an input pressure sensing controller generally indicated by the numeral 46 and shown in detail in FIG. 3A. Input pressure sensing controller may be of conventional type. A description of a suitable controller adaptable for use in the system of FIG. 3 may be found on page 1836 of the l962--63 "Composite Catalog of Oil Field and Pipeline Equipment." As seen in FIG. 3A, the input pressure sensing controller comprises a Bourdon tube 95 which is connected to line 45 and to a sliding valve 96 by means of linkage 97. Sliding valve 96 is provided with upper and lower valve elements 98 and 99, respe ctively. Valve element 98 is arranged for movement within an inner movable cylindrical valve member 100 which is connected to red 66 and movable thereby; Cylinder 100 is provided with ports 101 and 102 which, depending upon the position of valve element 98, control flow of fluid, between l conduits 47a and 44. Similarly, valve element 99 is positioned within a cylindrical valve member 103 provided with ports 104 and 105 which control, depending upon the positionof valve element 99., the flow of fluid between conduits 47b and 43. Cylinders 100 and 103 are connected together for unitary movement.

Thus, a change'in the position of rod 66 causes the positions of cylinders 100 and 103 to change and change in the position of rod 97 moves valve elements 98 and 99 which in turn control flow of fluids through conduits 44 and 47a and 43 and 47b. The source of power fluid 48 may be suitably a gas bottle under pressure. The power fluid is supplied through valves actuated by the Bourdon tube of controller 46 to lines 43 and 44 through the slide valve which in turn is actuated by positioning rod 66 as will be described further.

Operatively connected by power means. 50 to pump 28 is a power fluid pumpSI whose pumping rate is proportional to the pumping rate of pump 28. Pump 51 receives power fluid by way of line 52 from a source not shown to obtain a pressure output which is essentially proportional to the change in power fluid volume output by discharging most of the power fluid through constant rate controller 54 and the remainder through choke control valve 73. This power fluid may suitably be a hydraulic fluid, but other suitable. fluids may be used. The discharge from-pump 51 is by way of line 53 to a constant rate controller 54 which suitably is a constant rate controller such as described on page 2l68 of the 1962-63 Composite Catalog of Oil Field and Pipeline Equipment," with power fluid being discharged therefrom to a source by line 55. A branch line 56 leads from lines 53 and provides power fluid under a variable pressure, which pressure is varied proportionally to changes in power fluid pump 51 volume output and hence proportionally to changes in circulating pump 28 volume output.

Thus, power tluid pump 5] pumps at a rate directly proportional to the pump rate of circulating pump 28. This proportional rate of power fluid output (proportional volume) is used to obtain a pressure put output which is proportional to change of volume output by throttling power fluid through choke control valve 73. Variable pressure upstream of the choke, which is thus made proportional to change of circulating pump 28 power fluid volume, is introduced on one side of actuating piston 63. Well control circulating pressure in line 29 is introduced onto the opposing piston area of actuating piston 63 through lines 59-.-60. Movement of piston 63 responding to changes in the difference of pressure across choke 73 and relative to changes in the output volumes of pumps 51 and 28 are thereby converted to linear motion of connecting linkage 66. Linear motion of linkage 66 readjusts the preselected drill pipe control pressure level of controller 46. The linear linkage drive will shift the null" or control I pressure level an amount equal to the change in circulating friction due to change in output of pump 28. Controller 46 will then control to the new control pressure level (or new null" position) without influence from linkage 66 until operating rate of pump 28 is again varied. g

Constant rate controller 54, differential pressure sensor 57, check valve 62 and discharge line 59 are devices to facilitate conversion of proportional power fluid volume to a proportional change in power fluid pressure and subsequently to a proportional linear movement, to exhaust excess power fluid, and to avoid backflow of well control fluid from line 29 into the power fluid lines, controls and storage should equipment malfunction. Differential pressure sensor 57 shown in detail in FIG. 38 includes a spring-biased piston -connected to a valve member 111 having a valve opening 112 by hollow rod 113 provided with a port 114 which fluidly communicates lines 59 and one side of piston 110 to balance pressure in lines 59 and 56. Q

Constant rate controller 54 receives fluid from pump 51 and line 53 and returns a major portion of' the circulating power fluid to storage by line 55. Controller 54 will normally be set to bypass full discharge of pump 51 at the minimum desired operating rate of pump 28 and. will thereby minimize discharge of power fluid through lines56,-6l, 59 and into the well circulating fluid input line 29. In addition, this optional :mode of operation of rate controller 54 increases power fluid through input line 56 from zero for minimum operating level of pump 28 directly relative to the differential rate of operation of pump 28. Null preadjustment of throttling valve position for some operating level for pump 28 adjusts back pressure imposed on the bottom area of piston 63 to opposed pressure from circulating pump pressure in lines 29, 59, 60 and imposed on the top area of piston 63 at the preselected input circulating control level. For each operating rate of pump 28, excess power fluid from pump 51 is exhausted at a constant rate and constant pressure by valve 73 into line 59 and into the circulating system line 29.

Under Null" condition, the override control system to adjust circulating pressure control pressure level is balanced out and exerts no control. At this preset drill pipe pressure level or null position, the variable input rate automatic well control system functions exactly as if it were a constant input rate system. Thus, it senses input pressure through line 45 with Bourdon tube type controller 46 supplying control signals to annulus discharge vale valve 42 to control annulus fluid discharge flow and pressure and thereby adjust pressure in line 29 to the desired control pressure level.-

At some other circulating rate (above preestablished minimum) for pump 28, excess power fluid from pump 51 and line'53, which is not bypassed by controller 54, is forced through line 56, pressure sensor 57 and check valve 62. This excess volume in throttling through choke valve 73 builds back pressure in line 61 in proportion to the the throttling flow rate. This increased pressu reupstream of choke valve 73 is exerted 0n the bottom piston area of piston 63 and the pressure sensing pilot 57. Excess power fluid from pump 51, not bypassed through controller 54, is throttled through choke valve 73 at a changed rate and pressure and discharged into circulating system line 29. Check valve 62 prohibits back flow of well circulating fluid into the power fluid system. Pilot sensor 57 senses pressure upstream of check valve 62 in line 61 and immediately downstream of throttling point in choke valve 73 to control throttle valve- 73. Differential pressure control adjustment corrects the range of operating back pressure on one end of piston 63 to oppose the range of circulating pressure-from line 29 imposed on the other end of piston 63 for correct positioning of piston 63 to exactly the desired adjustment in control pressure level (null position).

Thus, the variable pressure of power fluid also acts on sensor valve 57 and is opposed by circulating pump 28 pressure.

These pressures act to adjust valve 73 and to power fluid to piston 63. Branch line 59 leads to sensor valve 57 and connects by line 60 to actuator 58. Line 61 connects actuator 58 with valve 57 and also connects with line 59, which is provided with a one-way check valve 62. The actuator 58 is provided with a piston 63 which is biased downwardly by spring 64 and biased upwardly by spring 65. Rod 66 connects to the piston 63 and transmits vertical motion (as shown in the drawing) to the input pressure sensing controller.

In the mode and embodiment of FIG. 3, the pump 28 pumping drilling fluid to the drill string 17 causes circulation from the annulus A through line 24 back to the mud pit 26.When formation fluids from formation 37 begin to flow to the open hole 16 and to the annulus A, the blowout preventers 23 are closed and the valve 40 is throttled. As the pressure in line 29 is transmitted to actuating means 58 and to the input pressure sensing controller 46, the input pressure sensing controller 46 and the actuator 58 cause throttling of the valve 40, the actuating rod 66 allowing power fluid pressure to be asserted above or below the diaphragm 41 to close or open the valve 40, as the case may be. This control means 46 thus is operatively connected to the control throttle valve 40 which also is a control means and compensation is made to maintain a selected overburden pressure and to compensate for drilling fluid rate. By sensing the pressure changes due to changes in the fluid in the open hole 16 and annulus A, it is possible to maintain the pressure at a selected point to prevent the sudden surges. Thus, in accordance with the mode and embodiment of FIG. 3, compensation is made for changes in drilling fluid rate due to the changes in the friction of the drilling fluid as it flows through the pipe.

The hydraulic-mechanical system described herein and shown in the drawings is not the only manner and means contemplated for achieving automatic well control in accordance with this invention. Various types of mechanical, electrical, pneumatic or fluidic systems and combinations thereof may be substituted for the hydraulic-mechanical system illustrated as will be understood by those skilled in this art. For example, an electric-pneumatic or electric-hydraulic system which comprises an electric control panel, pressure transducers (to convert pressure measurements to electrical signals) and sole noids (to convert electrical signals to pneumatic or hydraulic signals) may be used to control automatically operation of a pneumatically or hydraulically operated adjustable choke in the annulus discharge line in response to fluid pressure changes in the well inlet line. In such a system, a lag" time correction adjustment can be made directly on the control panel.

The nature and objects of the present invention having been fully described and illustrated and the best mode and embodiment contemplated set forth, what l wish to claim as new and useful and secure by Letters Patent is:

lclaim: 1. A method for controlling a well during drilling operations in which the drilling fluid is circulated from a pump to a hollow drill string, through the drill string and up the annulus between the drill string and the wall of the well to establish a column ofdrilling fluid which comprises:

sealing the annulus when pressure in the well increases to a point sufficiently great to overcome the hydrostatic pressure ofthe column ofdrilling fluid in the well;

automatically adjusting the pressure of the circulating drilling fluid to compensate for increased well pressure; and

circulating drilling fluid at said adjusted pressure from the pump into the drill string at a controlled, substantially constant rate while directing all of the drilling fluid flowing from the pump into the drill string and while simultaneously controlling the discharge of drilling fluid from the annulus in direct response to variations in the pressure of the drilling fluid in the drill string, thereby controlling the flow on the annulus whereby the pressure on the drilling fluid introduced into the well is varied and said flow is controlled.

2. A method in accordance with claim 1 in which the 4. In a method for controlling a well in which drilling fluid is pumped from a pump to a surface inlet of a drill string, down the drill string and up the annulus between the drill string and the borehole wall to a surface annulus discharge by maintaining constant bottomhole pressure the improvement compris ing; automatically adjusting the annulus circulating fluid discharge rate higher and lower in response to increases and decreases, respectively, in the inlet circulating pressure while maintaining input flow rate of drilling fluid into the drill string substantially constant, and while directing all of the drilling fluid flowing from the pump into the surface inlet.

5. A method as recited in claim 4 including the step of reducing the magnitude of the pressure variations resulting from variations in the rate of fluid discharge from said annulus in response to variations in inlet circulating pressure.

6. A method for controlling awell to compensate for changes in bottomhole pressure when drilling wells in accordance with rotary drilling techniques in which drilling fluid is circulated from a pump to a surface inlet of a drill string, down the drill string and up the annulus between the drill string and the borehole wall comprising the steps of; automatically and simultaneously maintaining essentially constant drill string pressure by directly controlling annulus fluid discharge responsive to drill string pressure while maintaining input flow rate of drilling fluid essentially constant while'directing all of the drilling fluid flowing from the pump into the surface inlet, thereby keeping bottomhole pressure essentially constant.

7. In a method for controlling a well in which drill fluid is pumped from a surface inlet down the drill string and up the annulus between the drill string and the borehole wall to a surface annulus discharge by maintaining constant pressure the improvement comprising:

automatically adjusting the annulus circulating fluid discharge rate higher and lower in response to increases and decreases, respectively, in the inlet circulating pressure for a selected pump rate; automatically adjusting drill string pressure an amount equivalent to changes in circulating friction resulting from changes in said selected pump rate; and

automatically adjusting the annulus circulating fluid discharge rate higher and lower in response to increases and decreases, respectively, in the inlet circulating pressure for said changed pump rate, thereby keeping bottomhole pressure essentially constant for different pump rates.

8. A method as recited in claim 7 including the step of reducing the magnitude of the pressure variations resulting from variations in the rate of fluid discharge from said annulus in response to variations in inlet circulating pressure.

9. A method for controlling a well to compensate for changes in bottomhole well pressure when drilling wells in accordance with rotary drilling techniques in which drilling fluid is circulated down the drill string and up the annulus between the drill string and the borehole wall comprising the steps of:

automatically and simultaneously maintaining substantially constant drill string pressure for a selected fluid circulating rate by directly controlling annulus fluid discharge responsive to drill string pressure; automatically adjusting drill string pressure an amount equivalent to changes in circulating friction resulting from changes in pump rate; and

automatically and simultaneously maintaining substantially constant drill string pressure for said changed rate by directly controlling annulus fluid discharge responsive to drill string pressure, thereby keeping bottomhole essentially constant for different circulating rates.

l including the step of 10. A method as recited in claim 9 including the step of reducing the magnitude of the pressure variations resulting from variations in the rate of fluid discharge from said annulus in response to variations in inlet circulating pressure.

11. A system for controlling a well by maintaining constant bottomhole pressure in the well through which drilling fluid is circulated froma pump to a drill string inlet and down the drill string and up the annulus between the drill string and the borehole wall to a surface annulus discharge comprising:

a closed flow path from said pump to said drill string inlet for directing all of the drilling fluid flowing from the pump into the drill string inlet;

fluid pressure responsive means arranged downstream of said pump; and

throttle valve means arranged in said annulus discharge cooperating with said fluid pressure responsive means to vary automatically the rate of fluid discharge from said annulus discharge in response to variations in inlet fluid pressure while maintaining input flow rate of drilling fluid constant. a i

12. A system as recited in claim llv including means arranged in said system for reducing the magnitude of the pressure variations resulting from variations in the rate of fluid discharge from said annulus in response to variations in inlet fluid pressure.

13;Apparatus for automatically maintaining constant bottomhole pressure in a well borehole during circulation of drilling fluid from a surface inlet down a drill string located in a borehole and up the annulus between the drill string and the borehole wall to a surface annulus discharge which comprises:

a surface pump for pumping drilling fluid through said surface inlet, down said drill string and up said annulus between the drill string and the borehole wall to said surface annulus discharge;

a closed flow path from said pump to said surface inlet directing all of the drilling fluid flowing from the pump into the surface inlet;

a pressure sensing means arranged downstream of said pump capable of indicating changes in pressure of said drilling fluid;

an adjustable choke arranged in said annulus discharge line;

and

control means responsive to said pressure sensing means for controlling a said adjustable choke means so as to maintain an essentially constant bottomhole pressure in said borehole while maintaining input flow rate of drilling fluid substantially constant.

14. Apparatus as recited in claim 13 including means for interrupting transmission of indications of said changes in drilling fluid pressure approximately the amount of time required for a pressure pulse to travel from said adjustable choke down said annulus and up said drill string to said pressure sensing means.

15. Apparatus as recited in claim 14 in which said interruption means is positioned between said pressure sensing means and said control means.

16. Apparatus as recited in claim'l4 in which said interruption means is positioned in said control means.

17 Apparatus as recited in claim 14 in which said interruption means is positioned between said control means and said adjustable choke.

18. A system for controlling a well by maintaining constant bottomhole pressure in the well through which drilling fluid is circulated from a surface inlet down the drill string and up the annulus between the drill string and the borehole wall to a surface annulus discharge comprising:

a pump for circulating said drilling fluid; fluid pressure responsive means arranged on said inlet;

throttle valve means arranged in said annulus discharge cooperating with said fluid pressure responsive means to vary automatically the rate of fluid discharge from said annulus discharge in response to variations in inlet fluid pressure to maintain bottomhole pressure essentially constant for a selected circulating pump rate; and means responsive to said pump output for automatically adjusting said throttle valve means and thereby said drill string pressure an amount equivalent to changes in circulating friction resulting from changes in said selected circulating pump rate, thereby keeping bottomhole pressure essentially constant for different fluid circulating rates.

19. A system as recited in claim 18 including means arranged in said system for reducing the magnitude of the pressure variations resulting from variations in the rate of fluid discharge from said annulus in response to variations in inlet fluid pressure.

20. Apparatus for automatically maintaining constant bottomhole pressure in a well borehole during circulation of drilling fluid from a surface inlet down a drill string located in a borehole and up the annulus between the drill string and the borehole wall to a surface annulus discharge which comprises:

a pressure sensing means capable of indicating changes in pressure of drilling fluid;

an adjustable choke arranged in said annulus discharge line;

control means responsive to said pressure sensing means for controlling said adjustable choke means so as to maintain an essentially constant bottomhole pressure in said well bore for a selected circulating pump rate; and

means responsive to said pump output and connected to said pressure sensing means for automatically adjusting said adjustable choke and thereby said drill string pres sure an amount equivalent to changes in circulating friction resulting from changes in said selected circulating pump rate, thereby keeping bottomhole pressure essentially constant for different fluid circulating rates.

21. A system as recited in claim 20 including means arranged in said system for reducing the magnitude of the pressure variations resulting from variations in the rate of fluid discharge from said annulus in response to variations in inlet fluid pressure.

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