Well-drilling Method And Apparatus Involving Determination Of Pressure Of Drilling Fluid

Abstract

The pressure of drilling fluid in well-drilling apparatus is determined from the amount of deformation of a member which is attached to the drilling string and subjected to forces exerted by the pressurized drilling mud. The method involves determination of the pressure drop across the drilling bit by studying the extent of deformation of deformable members on the drilling string and changing the jet nozzles in the drilling bit when the pressure drop falls outside a prescribed range.

Description

Modern well-drilling practices commonly employ rotary jet bits of the type generally shown in U.S. Pat. Nos. 3,070,182; 3,084,751; 3,115,200; 3,129,777 and 3,137,354, all of which are incorporated herein by reference. These bits have nozzles providing a plurality of pressurized jets of drilling fluid which usually is drilling mud. The drilling mud strikes the bottom of the hole to quickly lift the cuttings and to create sufficient turbulence around the bit to keep the cutter teeth properly cleaned. It is known that the satisfactory operation of such drilling bits is dependent upon the pressure drop across the bit, i.e., the difference between the pressure of the drilling mud within the drilling string and its pressure in the annular portion of the borehole which lies outside the drilling string. The optimum pressure drop is 48 percent of the surface pressure existing at the discharge side of the circulating pump. Therefore, when the surface pressure is 3,000 p.s.i., the pressure drop across the drilling bit should be approximately 50 percent of this which is 1,500 p.s.i.

It has been customary to use computational methods for determining the pressure drop across the drilling bit. This involves consideration of a theoretical pressure loss in the drilling bit and a theoretical pressure loss in the annulus. When the theoretical computations indicate that the pressure drop is outside prescribed limits, the jet nozzles on the drilling bit are changed approximately every 8 hours. This is done when the drilling bits are changed.

The adoption of the apparatus and procedures described in this specification will eliminate the need to compute a theoretical pressure loss across the drilling bit and will result in a more accurate assessment of the true pressure drop, thereby giving more accurate guidance as to whether and how much the jet nozzle size should be changed.

Briefly, the invention involves one or more deformable members attached to the drilling string and exposed to forces generated by the pressure of the drilling fluid. Preferably but not essentially, the drilling fluid acts against a piston which urges a hardened spherical body against the deformable member. The piston may carry or comprise either the hardened spherical body or the deformable member. Knowledge of the physical characteristics of the deformable member, the area of the piston and well-known relationships of the Brinell hardness equation permit computation of the pressures in the vicinity of the drilling bit. These pressures may be sensed internally of the drilling string, in the annulus which is external of the drilling string or it may be sensed directly as a pressure differential.

A further understanding of the invention will be facilitated by a study of the following description and the accompanying drawings which illustrate typical embodiments of the invention. In the drawings;

FIG. 1 is a diagrammatic view of a typical drilling rig which incorporates the apparatus of the invention;

FIG. 2 is a sectional view through the drilling string in the vicinity of the drilling bit, showing separate devices for measuring the pressure of drilling fluid both internally and externally of the drilling string;

FIG. 3 shows another embodiment which utilizes a single impression plate, the deformation of which is used to ascertain the pressure existing inside and outside the drilling string; and

FIG. 4 is a sectional view of a portion of the drilling string embodying yet another form of the invention which enables direct computation of the pressure differential existing between the interior and exterior of the drilling string.

For the most part, the apparatus in FIG. 1 is conventional. A drilling bit 2 is located at the lower end of a drilling string 4 which is formed in the usual way of a plurality of tubular sections which depend through the borehole 6. The drilling bit is a roller jet rock bit which has three rotatable cone-type cutters. The bit also has a plurality of jet nozzles which project high-velocity streams or jets of drilling fluid directly against the bottom of the borehole to lift the cuttings and create sufficient turbulence to keep the cutter teeth properly cleaned.

The drilling string is rotated in a conventional manner by a Kelley-engaging rotary table 7. Drilling fluid, preferably drilling mud, is forced under very high pressures into the interior of the drilling string by a circulating pump 12 which discharges into a conduit 14 which, in turn, carries the drilling fluid to the swivel 16 and into the interior of the drilling string 4.

As previously mentioned, it is known that the efficiency of a jet roller bit is dependent upon the pressure differential across the bit, i.e., the difference in the pressure of the drilling fluid in the interior passageway of the drilling string and the pressure in the annulus defined by the sidewalls of the borehole 6 and the exterior of the drilling string 4, both pressures being measured in the vicinity of the drilling bit.

Rather than calculating a theoretical pressure drop across the drilling bit on the basis of pump discharge pressures and the dimensions of the passageways for the drilling fluid, this invention provides means attached to the drilling string and movable therewith into the borehole for sensing and recording pressure-responsive forces which exist in the vicinity of the drilling bit. Preferably, the pressure-produced forces are created by subjecting one face of a piston to the drilling fluid in order to urge an inelastic, permanently deformable impression block (which may be the piston itself) against a deforming tool or, conversely, to force the deforming tool against an impression block. It is also preferred that the deforming tool have a hard spherical surface of a known radius of curvature so that when the hardness of the impression block is known, the magnitude of deformation-producing forces may be initially computed using the well-known Brinell hardness equations and tables.

In the embodiment of FIG. 2, a portion of the piston serves as the deformable impression block. Separate devices are used for obtaining impressions which indicate the internal and external pressures in the vicinity of the drilling bit. The means for obtaining an indication of the internal pressures is an assembly 20 which is carried by a plug 22 threadedly connected to a sub 23 in the drilling string 4. The plug 22 is rotationally attached to and removed from the exterior surface of the sub by a special wrench which engages the recesses 24 in its outer surface. A shoulder portion 26 engages a counterbored surface on the sub to fix the final position of the plug 22 and to hold the plug 22 in place. A notch 28 on the plug houses an O-ring 30 which assists in preventing leakage of drilling fluid around the exterior of the plug 22.

The plug 22 has a concentric bore 32 which serves as a cylinder for the impression block piston 34. At the innermost end of the bore 32, a spherical recess 36 accommodates a hardened metallic ball 38 which suitably has a diameter of 10 millimeters. Loss of the piston 34 is prevented by a radially contractable lockring 40, and leakage around the periphery of the impression block piston 34 is deterred by the O-ring seal 42. A central portion of the piston 34 is provided with a recess 44 engageable by an extraction tool which facilitates removal of the piston from the bore 32.

Of course, when the apparatus of FIG. 2 is located within a borehole in a drilling rig and the drilling fluid in the interior passage of the drilling string is placed under pressure, this pressure will act on one face of the impression block piston 34 to urge it against the ball 38 which serves as a deforming tool. The piston 34 is formed of a material of suitable hardness preferably having a resistance to cold flow and creep. The ball 38 is of a harder material than the piston 34 so that the force between these elements will produce on the interior surface of the impression block piston 34 a dished indentation which has a circular periphery. Such indentations are also produced when performing a conventional Brinell hardness test, and criteria have been established to indicate the relationship between the force between a pair of bodies, the shapes of the bodies, and their hardness. Since, in this instance, the hardness of the impression block 34 is known, and the diameter of the ball 38 is known, the only variable involves the force which tends to produce the indentation in the impression block piston 34. This force may be computed and divided by the area of the piston 34 exposed to the pressurized drilling fluid in order to find the pressure of the drilling fluid. Easily computed tables may give the fluid pressures directly as a function of the diameter of the indentation.

It will be evident that the pressure in the annulus which surrounds the drilling string may be measured much in the same manner as the interior pressure by using a similar device having a piston exposed to the drilling fluid in the annulus. This is done by a plug 46 which is attached to the drilling string in a manner identical to the plug 22. The plug 46 has its central bore 48 exposed to the exterior of the drilling string. The impression block piston 50 is retained by a lockring 52 and peripherally sealed by an O-ring 54. The hardened spherical ball 56 confronts the surface 58 of the impression block piston 50 so that any forces exerted on the outer face of the piston by the pressurized drilling fluid will urge the members 56 and 50 together, thereby providing an indentation in the impression face of piston 50 which will permit determination of the pressure in the annulus.

The alternative embodiment shown in FIG. 3 is similar to FIG. 2 in the sense that a piston has one face exposed to the pressurized drilling fluid in order to create the indentation-producing forces. In FIG. 3, the impression plate 56 is held on the drilling string by a plate 58 which is secured by bolts 60. The plate has a planar inner surface mating a flat surface on the drilling string, and a curved outer surface which conforms to the external shape of the drilling string. The impression plate 56 is held in a machined recess and is provided with a peripheral O-ring 62 which assists in preventing leakage around the impression plate.

On the interior side of the impression plate, there is a bore 64 which communicates through a port 66 with the interior of the drilling string. Within the bore 64, there is a piston formed of an elastomeric sealing portion 68 which closely fits the bore 64 and has a surface exposed to the drilling fluid. The remainder of the piston is a hardened body 69 having a spherical outer surface which abuts the interior face of the impression plate 56. As in the embodiment of FIG. 2, pressure within the drilling string is exerted on the piston to create a force which urges together a spherical hardened body and a deformable impression plate member.

The pressures existing in the borehole annulus in the embodiment of FIG. 3 are sensed by an exterior piston which has a sealing portion 70 and a hardened portion 72. A spherical surface on the hardened portion 72 is urged against the impression plate 56 by the pressurized fluid within the annulus. The piston rides within the bore 74 and is subjected to the pressurized fluid due to the existance of the port 76.

It is evident that the interior and exterior pressures of the drilling mud may be ascertained when the drilling string is removed from the borehole by studying the two impressions made on the impression plate 56. Once the pressures are determined and the pressure differential is found, a decision may be made as to whether to change the sizes of the jet nozzles in the replacement drilling bit. Then, the replacement drilling bit and the drilling string are placed in the borehole, together with the pressure-sensing apparatus which then includes a new undeformed impression plate 56.

In the apparatus of FIG. 4, the extent of deformation of an impression plate provides a reading of the pressure differential since it is responsive both to the internal and external pressures existent in the borehole. In this embodiment, the drilling string includes a sub with a bore 76 partially closed at one end by a threaded plug 78. Within the bore 76, there is a movable piston assembly 80 which carries a peripheral O-ring 82 riding against the bore and an impression plate 84 attached to one face of the piston and in confronting relationship with a hardened steel ball 86.

As viewed in FIG. 4, the left side of the piston is exposed to the pressures of the drilling fluid in the annulus by means of a port 87. The internal side of the piston 80 is exposed to the interior pressures through a port 88. Therefore, the force exerted on the piston assembly 80 with the impression plate 84 will be directly dependent upon a pressure differential rather than the absolute pressures which exist inside and outside the drilling string. This differential-produced force will, of course, have an effect on the amount of indentation of the impression plate 84 by the hardened ball 86. Knowledge of the area of the piston 80, the Brinell hardness of the impression plate 84 and the diameter of the hardened ball 86 will enable persons running the drilling rig to determine the pressure differential in order to determine whether the sizes of the jet nozzles in a rotary jet bit require changing.

The apparatus described hereinabove is attached to the drilling string during normal drilling operations while the drilling string is rotated by a rotary table. The pressurized drilling fluid is forced downwardly through the drilling string by the circulating pump and through the jet nozzles against the bottom of the borehole. The return stream of drilling fluid passes upwardly through the annulus defined by the borehole walls and the exterior surface of the drilling string. The forces created by the pressure of the drilling fluid produce permanent deformation in one of the members 34, 50, 56 or 84. At the end of the bit run, the drilling string is withdrawn from the borehole and the deformable member is inspected. The extent of deformation permits the rig operators to ascertain pressures and/or pressure differentials which existed at the bottom of the borehole. If the pressure differential is too great, larger jet nozzles are used for the replacement drilling bit; and, conversely, if the pressure differential is not great enough, smaller jet nozzles are used. The drilling string with the replacement bit is returned to the borehole and drilling is resumed.

Those skilled in the art will appreciate that the invention may assume numerous forms other than the disclosed and preferred embodiments. For example, the ball 86 in the FIG. 4 apparatus may be replaced by a piston of the type shown in FIG. 3. The deformable member may be of a diaphragmlike plate which is directly exposed to the drilling fluid by means of a port so that the amount to which the diaphragm is dished by the drilling fluid itself will be indicative of a pressure or of a pressure differential. The indenting members or tools, when used, are preferably of a spherical configuration, but the use of other shapes which, for example, may be conical or pyramidal is considered within the spirit of this invention. The pressure-sensing sub may also carry a known type of temperature sensitive paper which changes colors to give an indication of the temperatures of the drilling fluid in the vicinity of the bit. Since these and other modifications will naturally take place as the art develops, the invention is not limited only to the illustrated or discussed embodiments, but is to be considered in light of the terms and the spirit of the claims which follow.

Claims

I claim:

1. Well-drilling apparatus comprising;

a drilling string having a central passageway for drilling fluid and an external surface which is adapted to be spaced from the sidewalls of a borehole to provide an annular passageway for drilling fluid lying externally of the drilling string,

a roller bit at the lower extremety of the drilling string, said roller bit having rotatable cutter teeth and a plurality of jets for directing streams of drilling fluid against the bottom of the borehole,

a deformable body on the drilling string, and

means for subjecting said deformable body to deformation-producing forces generated by the pressure of the drilling fluid in at least one of the passageways.

2. Apparatus according to claim 1 wherein the means for subjecting said deformable body to forces generated by the drilling fluid is a piston having one face exposed to one of said passageways.

3. Apparatus according to claim 2 wherein a portion of said piston spaced from said one face thereof is the deformable body.

4. Apparatus according to claim 3 having a deforming tool with a hardened spherical surface fixed with respect to the drilling string and in contact with said portion of said piston, said deformable body being inelastic so as to be permanently deformed by the deforming tool.

5. Apparatus according to claim 1 wherein the deformable body lies against a deforming tool which has a hardened spherical surface and said means for subjecting the deformable body to deformation-producing forces is constructed to force together the hardened spherical surface and the deformable body.

6. Apparatus for ascertaining pressures of drilling fluid in an earth drilling string, comprising,

a. a support body connectable to the drilling string,

b. a deforming tool member mounted on the support body,

c. a deformable member mounted on the support body and confronting the deforming tool member, and

d. one of said members being movable toward the other member under the influence of the pressure of drilling fluid to deform the deformable member, whereby the degree of deformation of the deformable member will reflect the pressure of the drilling fluid.

7. Apparatus according to claim 6 wherein the deformable member is a reciprocable piston having one face confronting the deforming tool member and its opposite face exposed to the drilling fluid so as to permit the application of pressure thereagainst by drilling fluid, said deformable member being sufficiently inelastic to permit permanent deformation thereof.

8. Apparatus according to claim 7 wherein the deforming tool member has a spherical surface confronting the deformable member.

9. Apparatus according to claim 6 wherein the support body is tubular and includes two sets of members of the type described in claim 6, with the sets of members being constructed and located where one set of members is responsive to pressures within the tubular support body and the other set of members is responsive to pressures outside the tubular support body.

10. Apparatus according to claim 9 wherein each set of members is supported on a plug which is removably connected to the tubular body.

11. Apparatus according to claim 10 wherein the connections between the tubular body and both of the plugs permit removal of the plugs in a radially outward direction.

12. A method of drilling the earth with a rotary drilling bit having jets therein for directing a stream of drilling mud against the bottom of a borehole, comprising the steps of;

rotating a drilling string and a rotary drilling bit while directing a stream of drilling mud downwardly through jet nozzles in the drilling bit, and simultaneously subjecting one or more inelastic deformable members to forces developed by the drilling mud both in the interior of the drilling string and in the annulus surrounding the drilling string,

withdrawing the drilling string and the drilling bit from the borehole,

inspecting the deformable member or members to ascertain therefrom the pressure differential between the interior of the drilling string and the annulus to ascertain whether it is necessary to change the size of the jet nozzles, and,

placing a drilling bit and the drilling string back into the borehole to resume drilling operations.

13. The apparatus of claim 1 having means on the drilling string and adjacent to the deformable body for indicating the temperature of the drilling fluid.

14. The apparatus of claim 6 having means carried by the support body for indicating the temperature of drilling fluid at the support body.