Title:
DRILLING MUD SYSTEM
United States Patent 3613806


Abstract:
Apparatus for monitoring and maintaining the hydrostatic head of a column of drilling mud in a well bore substantially constant as pipe is pulled from or run into the hole in well-drilling operations. A mud tank is provided with weighing means to measure the weight of fluid in the tank and connected by suitable conduits to the well bore so that all fluid flowing into or out of the well bore passes through the tank.



Inventors:
MALOTT RAYMOND A
Application Number:
05/023125
Publication Date:
10/19/1971
Filing Date:
03/27/1970
Assignee:
SHELL OIL CO.
Primary Class:
Other Classes:
166/75.11, 175/218
International Classes:
E21B21/08; (IPC1-7): E21B35/00; E21B41/00
Field of Search:
175/24,48,38,40,207,218 73
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Primary Examiner:
Leppink, James A.
Claims:
I claim as my invention

1. In an apparatus for drilling a well in which a drill pipe extends down a borehole and drilling mud is circulated down the drill pipe, returned up an annulus surrounding the drill pipe and discharged from the well through a mud discharge line, apparatus for maintaining the hydrostatic head of a column of drilling mud in the well bore substantially constant as drill pipe is removed from the borehole, the apparatus comprising:

2. The apparatus of claim 1 including pump means coupled to said conduit means and disposed to pump fluid from the mud tank into the well.

3. The apparatus of claim 1 including fluid depth indicator means to measure the depth of fluid in the mud tank.

4. The apparatus of claim 1 wherein the weighing means comprises at least one transducer means.

5. The apparatus of claim 4 including a base structure and pivot means whereby said mud tank is pivotably mounted with respect to a substantially horizontal axis adjacent one end of said mud tank of said base structure and is supported above the base structure near the opposite end by said transducer means.

6. The apparatus of claim 3 wherein said fluid depth indicator means comprises float means operatively connected to a signal means coupled to a volume recorder means.

Description:
BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of well drilling; and, more particularly, to a system for the control of the hydrostatic head of a column of drilling mud in a well.

2. Description of the Prior Art

In the drilling of wells with conventional rotary drilling equipment, it is common practice to circulate a drilling liquid or "drilling mud" down a hollow drill pipe, around a drilling bit, and up the annular space between the drill pipe and the walls of the well bore. The drilling mud cools the drilling bit, carries cuttings to the surface and provides a means for preventing fluids contained in earth formations penetrated by the well from "blowing out." A conventional drilling mud may be a suspension of clays, clayey materials, and weighing materials in crude oil or water. The density of the mud may be varied by properly adjusting the amounts of suspended matter in the mud.

To prevent blowouts, the hydrostatic head of the column of drilling mud in the well bore must be sufficient to create a pressure in the borehole opposite a fluid-containing formation greater than the pressure of the fluids within the formation. However, it is known that drilling rates may be increased and mud costs lowered by using a relatively light mud. Thus, a common practice is to employ a drilling mud having a density such that the column of mud in the well borehole produces pressures along the wall of the borehole only slightly greater than the blowout pressures expected to be encountered.

Where such a small margin of excess back pressure is maintained on the formations penetrated by a well, control of the proper relationship between the hydrostatic head of drilling mud and formation pressure becomes critical. This is especially true at times when the drilling operations tend to cause a change in the height of the mud column, such as when drill pipe is run into or pulled out of the borehole in the process of changing a wornout bit. For example, when pipe is pulled out of the borehole, there is generally a lowering of the fluid level in the well since the pipe removed is no longer displacing liquid. Prior art methods for maintaining the borehole pressure due to the hydrostatic head as the drill stem is pulled from the hole include manually activated periodic filling of the hole with mud and the employment of apparatus to automatically keep the hole full.

However, where the density of the mud added to borehole to fill the space formerly occupied by the removed drill pipe differs from the density of the liquid that had been displaced by the drill pipe, simply keeping the hole full of mud may not assure maintenance of the desired pressures in the well bore. If the drilling mud added to the borehole at the surface is less dense than that displaced by the lowermost portion of the drill stem, the hydrostatic head of the fluid column in the well bore prior to the raising of the drill stem will be greater than the hydrostatic head of the fluid column after some pipe has been removed and some mud added at the surface to fill the borehole. This difference may be understood by considering that in a given borehole the pressure exerted by a column of fluid of constant height is proportional to the total weight of the fluid column. The contribution of the drill pipe to the weight of the fluid column is equal to the buoyant force exerted on the drill pipe by the mud column which, according to Archimedes' principle, equals the weight of the fluid displaced by the pipe. Thus, if the drill pipe removed from the hole formerly displaced a liquid of greater density than the mud added to the hole to fill the volume formerly occupied by the drill pipe, the net effective weight of the column of mud, and thus the back pressure on formations near the bottom of the hole, will decrease.

SUMMARY OF THE INVENTION

It is an object of this invention to provide an apparatus that will give drillers of wells an improved and more accurate means for controlling pressures in a borehole due to a head of drilling mud, thus making it possible to work within narrower limits with respect to the differential pressure relationships between formation and mud column hydrostatic pressures. This improves the primary blowout control and reduces drilling costs by allowing less dense mud to be used safely, thus increasing penetration rate and decreasing mud cost.

The apparatus comprises a mud tank positioned to receive drilling mud discharged from a well through a mud discharge line, conduit means opening into the well below the mud discharge line to provide a flow path for fluid communication between the mud tank and the well, pump means coupled to said conduit means to pump fluid from the mud tank into the well, weighing means operatively connected to the mud tank to measure the weight of drilling mud in the tank, and display means to display the measured weight of the contents of the mud tank.

The mud tank may be provided with a fluid depth indicator means which may comprise a float means operatively connected to an electrical signal means coupled to a volume recorder means.

In a preferred embodiment, the weighing means may be a pressure-sensitive cell which supports one end of the mud tank above a base structure while near the opposite end of the mud tank is pivotably mounted (with respect to a horizontal axis) on the base structure as with a hinge means.

BRIEF DESCRIPTION OF THE DRAWING

The FIG. is a diagrammatic view, partially in cross section, of a well and well-drilling equipment to which the apparatus of this invention may be operatively connected.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the FIGURE, we see a rig floor 10 which is mounted a rotary turntable 11. Positioned below the turntable 11 is a mud conductor pipe 12 attached to a blowout preventer 13 that is coupled to a casing nipple 14 connected to a surface casing 15 by means of a casing spool 16. A string of hollow drill pipe 17 provides a means for rotating a drilling bit 18 to further extend a borehole 19 into an earth formation 20.

The mud conductor pipe 12 opens into an upper mud discharge line 21 and a lower mud discharge line 22 which carries a valve 23. A mud tank 24, which in this embodiment may have a volume of about 500 cubic feet, is positioned below the lower mud discharge line 22 to receive fluid flowing out of this line. The tank 24 is operatively connected to a means suitable for weighing the contents thereof. For example, in the embodiment of the FIGURE, the tank 24 is pivotably attached to a base structure 25 with a hinge means 26 near one end of the tank 24 while near the other end, it is supported above the base structure 25 by a weighing means which may be a transducer means such as pressure-sensitive cell 27 which is of a type which may be adapted to deliver a signal, preferably electrical, proportional to the weight on the pressure cell 27, via a circuit means 28, to a display means such as weight indicator gauge 29. The gauge 29 may be of a type that can be adjusted to read zero when the mud tank 24 is empty and to indicate the weight of the contents of the mud tank 24 in pounds or other suitable units when it is not empty.

Preferably, the tank is also provided with volume indicator means such as a depth-gauging stick or a floating ball 30 operatively connected by a connector means such as a rod 31 to a signal means 32 which provides a signal, preferably electrical, proportional to the depth of the fluid in the mud tank 24 to a volume recorder means 33.

A conduit means such as mud flow pipe 34 which opens into mud tank 24 and casing nipple 14 provides a flow path for fluid communication between the mud tank 24 and the borehole 29. The flow pipe 34 may be provided with a check valve 35 or other suitable means to prevent backflow through the flow pipe 34 into the mud tank 24 and with means such as a valve 36 to shut off fluid flow from the mud tank 24 to the borehole 19. A pump means 37 may be coupled to flow pipe 34 to pump fluid from the mud tank 24 into the borehole 19.

At those times in the course of drilling a well when the weight of the column of drilling mud 38 in the borehole 19 is of particular interest, as when pulling the drill pipe 17 out of the borehole 19, the mud tank 24 may be substantially filled with drilling mud 38 from a drilling mud source (not shown), and the initial readings of the volume recorder 33 and the weight indicator gauge may be recorded. The valves 23 and 36 are then opened and the pump means 37 made operative.

If the fluid level in the borehole 19 falls, as when a joint of drill pipe 17 is removed, drilling mud 38 pumped from the mud tank 24 through the flow pipe 34 and into the casing nipple 14 by the pump means 37 fills the borehole 19. If the borehole 19 is substantially full, drilling mud pumped from the mud tank 24 into the casing nipple 14 circulates up through the blowout preventer 13, into the mud conductor pipe 12, and out the lower mud discharge line 22 back into the mud tank 24. Thus, the borehole 19 is maintained substantially full of fluid, and all drilling mud flowing into or out of the top of the surface casing 15 passes into or out of the mud tank 24.

As the total weight of the drilling mud 38 in the mud tank 24 changes, the force on pressure cell 27 changes and the new total fluid weight is recorded on weight indicator gauge 29. Similarly, as the level of fluid in the mud tank 24 varies, the floating ball 30 is moved up or down causing volume recorder means 33 to record the new volume of fluid in the mud tank 24. Therefore, by noting changes in volume recorder means 33 and weight indicator 29, one may determine the total volume and the total weight of drilling mud 38 added to or removed from the borehole 19. If the weight of drilling mud 38 added to the borehole 19 is not as great as the contribution to the total hydrostatic head of the column of drilling mud 38 in the borehole 19 of the drilling pipe 17 that has been removed from the borehole 19, then the density of the drilling mud 38 in the mud tank 24 may be increased by adding weighing materials so that as this drilling mud 38 is pumped into the borehole 19, the hydrostatic head of the mud column will return to the desired magnitude.

While the operation of the apparatus of this invention has been described primarily with respect to use as drill pipe is removed from the borehole 19, it is understood that the apparatus may also be profitably used at other times in the course of drilling a well, as when pipe is run into the hole or to measure the volume and density of liquids flowing from the wellbore during a drill stem test. It is also recognized that the apparatus may be operated effectively without a pump means 37 if the mud tank 24 is positioned so that drilling mud 38 will flow into the casing borehole 19 through the flow pipe 34 by gravity drainage.