US3633687A

US3633687A - Apparatus for separating and measuring gas in drilling fluid

Info

Publication number: US3633687A
Application number: US884502A
Authority: US
Inventors: Alfred Gordon West; Chester M Harden; Clyde E Perce
Original assignee: Individual
Current assignee: Individual
Priority date: 1969-12-12
Filing date: 1969-12-12
Publication date: 1972-01-11
Anticipated expiration: 1989-01-11

Abstract

Apparatus is provided for use in wells, allowing the well to be drilled with minimum hydrostatic head and yet insuring against the possibility of blowouts, and for measuring the gas flow rate from a separator which includes a plurality of parallel pipes of differing sizes. Each pipe has a flow rate detector, and suitable valves are included for passing the gas flow into whichever pipe is desired, depending on the flow rate. If the flow rate is great, a large pipe is used; if the flow rate is small, a small pipe is used.

Description

Ol-l1-72 OR 3%)339687 Ullltu uuwwu l .ll [H] [72] Inventors Alfred Gordon West [56] References Cited 3 81: d 501 sch M D UNITED STATES PATENTS 2 082 329 6/1937 Foran et al 175/48 Clyde E. Perce 600 Lidon, all of Midland Tex 7970] 2,700,897 2/1955 Arps 175/50 X [21) Appl. No. 884,502 Primary Examiner- Ernest R. Purser [22] Filed Dec. 12, 1969 AnorneysAmold, Roylance, Kruger & Durkee, Tom- [45] Patented Jan. 11, 1972 Arnold, Donald C. Roylance, Walter Kruger, Bill Durkee,

Continuation-impart of application Ser. No. Frank vaden, and Louis 670,559, Sept. 26, 1967, now Patent No.

498 393. This Ii ti D 12 1969, 3; 884 ca on cc ABSTRACT: Apparatus is provided for use in wells, allowing the well to be drilled with minimum hydrostatic head and yet insuring against the possibility of blowouts, and for measuring [54] APPARATUS FOR SEPARATING AND MEASURING the gas flow rate from a separator which includes a plurality of GAS IN DRILLING FLU") parallel pipes of differing sizes. Each pipe has a flow rate de- 4 Claims, 2 Drawing Figs. tector, and suitable valves are included for passing the gas flow into whichever pipe is desired, depending on the flow [52] U.S. Cl 11775724086 raw If the flow rate is great a large pipe is used; if the flow 51 Int. Cl ..E21b21 0o, me Small a used E21b 47/00 [50] Field of Search 175/29, 38,

' trol.

APPARATUS FOR SEPARATING AND MEASURING GAS DRILLING FLUID REFERENCE TO OTHER APPLICATIONS flhis application is a'continuation -in-part of our copending BACKGROUND OF THE INVENTION One of the most critical problems in drilling an oil or gas 7, well into the earth is the tendency to lose pressure equilibrium under certain well conditions, resulting in blowout" of the well. A blowout is the wasteful blowing of oil and gas out of the well and the complete loss of control of pressure. In view of the tremendous capital investment necessary to drill a well (drilling costs often run to many hundreds of dollars per day and the drilling has often been proceeding for many days when the greatest risk of blowout occurs) it is seen that a blowout is almost always an enormously expensive thing. In addition to the expense involved, a blowout is one of the most devastating and destructive things that can happen to a well; wells which have blown out are almost always damaged (damage which may be realized throughout the life of the well), and often must be completely abandoned. Even if the well can be brought under control, a great deal of time is often lost in drilling and special equipment and extra labor (which may not be readily available) are needed to bring the well pnder con- A still further hazard of the blowout is that thefriction of the equipment at the well head can be sufficient to cause a fire in the blowout gas, and of course such a fire is not only extremely expensive but very dangerous to the workmen at the rig, and ruinous to the rig equipment.

Blowouts most often occur in formations which contain high pressure gas pockets. In the drilling process a drilling fluid or drilling mud is customarily circulated down the borehole to clean the hole of cuttings and lubricate the drill bit. Blowouts can be prevented by establishing pressure control with this drilling mud. That is, if the pressure of the hydrostatic head of the drilling mud is at all times kept greater than the bottom hole pressure of the well, blowouts may be prevented. And the hydrostatic head of the drilling mud can be varied by varying the densityof the mud, as is well known in the drilling art. Further, the normal formation pressure of the strata being penetrated may be determined by standard methods. Consequently, it is the present practice to maintain the hydrostatic head at a level considerably greater by some safety factor" than the normal bottom hole pressure.

There are two grave difficulties with the use of this safety factor technique. The first is that it does not compensate for the abnormal conditions which sometimes develop, and consequently is only partially effective in preventing blowouts. Secondly, in using the prior art methods a relatively large differential between hydrostatic head and normal bottom hole pressure must be maintained, which means that the hydrostatic head is at nearly all times much greater than is really necessary.

Maintenance of the hydrostatic head of the drilling fluid at a level above what is necessary is in itself a significant problem to the driller. For example, it has been shown that a lighter hydrostatic head results in a faster penetration rate. And of course, a faster penetration rate is desirable in drilling of wells because of the savings in time required to drill a well (and hence the cost of drilling), realized. Further, a higher hydrostatic head will result in injury to certain formations which are low in pressure. That is, if the pressure in the borehole is significantly greater than the pressure in an adjacent formation, the formation might be fractured by the drilling fluid in the borehole, thereby permanently injuring the formation and forever damaging its permeability to thereby result in permanent decrease of the productivity of the well. Such fracturing is also undesirable because a great deal of the drilling fluid can be lost into the fractured formation. It hasalso been shown that longer bit life is achieved with a lighter hydrostatic head. That is, the bit will not wear out so quickly if the mud weight is decreased. This is extremely important since when the bit does wear out, the drilling must, be stopped and a trip" must be made to replace the bit. This can often take many hours of valuable downtime on the rig. It also results in a buildup of ftrip gas" during the trip, and this gas further contributes to the blowout problem. For these and other reasons, it is alrnostale ways desired to drill with the smallest possible hydrostatic head. 1

As hazardous and as expensive of aproblem as it is, a blowout does not occur instantaneously. Rather, it is something which builds up over a period. of time, (a time period which may be short, however, if the formation permeability is great) and then, perhaps is rather instantaneous in its result. For this reason blowouts can be prevented and continuous pressure control established by the use of the presentinvention, while the driller is able to continuethe drilling using a relatively light hydrostatic head.

SUMMARY OF THE INVENTION The present invention ,provides,apparatus for maintaining pressure control whiledril ling a well such as an oil or gas .well, to thereby prevent the occurrence of a blowout. Such ,apparatus includes, in conjunction with means for circulating drilling fluid into the borehole means for separating free gas from the drilling fluid upon its return to the earth's surface, and means for measuring the gas flow rate of the free gas from the separator.

The apparatus for measuring the flow rate of the free gas includes a plurality of parallel pipes of differing sizes. Each pipe has a flow rate detector, and suitable valvesare included for passing the gas flow into whichever pipe is desired, depending on the flow rate of the gas. As will be more fully discussed below, the size of pipe used is directly-proportional to the flow rate, the larger pipes being employed for the larger flow rates.

BRIEF DESCRIPTION OFTHE DRAWINGS FIG. l is a schematic elevational view, partially in section, of a borehole during the drilling process, wherein the method of the present invention isemployed; and

FIG. 2 is a side view of apparatus according to one embodiment of this invention. I

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS A schematic illustration of a well-drilling operation employing the present invention may be seen in FIG. I.

Here it is seen that a borehole. 10 has been drilled into the earth extending from the surface 12 to a point 14 beneath the surface, through a plurality ofdifferent geological formations l6, 18, 20 and 22. It will be readily understood that as drilling proceeds the borehole 10 will get deeper and deeper.

The borehole 10 is being drilled in this instance by a rotary drill bit 24 suspended at the end of a string of drill pipe or a drill string 26. The bit 24 is rotated by suitable power means 34 located on the derrick floor 36 at the earths surface. Suitable drill collars 28 may be positioned onv the drill string just above the drill bit 24 in order to weight the bit and cause it to more effectively drill the formation.

The borehole wall may be protected by the installation of a casing 30 to any desired level in the borehole. In those regions protected by the casing 30, the problem of fracturing discussed above is overcome.

The apparatus of this invention is useful in connectionwith means for circulating a drilling fluid which may bewater or oil, but is commonly "mud" (typicallya mixture of clay and water with suitable additives), during drilling, from the surface down the center 32 of the hollow-drill string 26, past the drill bit 24 at the bottom 14 of the hole,.and thenceup the annular .area 33 outside the drill string between the drill string and the casing 30 or borehole wall. Means for circulating drilling fluid this manner are well known in the priorart.

As the bit 24 is rotated with the drill string by the power means 34, it cuts away at the adjacent formation 22, forming loose cuttings at the bottom of the hole. As the drilling mud sweeps the bottom of the hole, it carries these cuttings upwardly through the borehole to the earths surface. The drilling mud also cools the bit, and serves to maintain the hole gauge by its continual sweeping action.

But the drilling mud provides another vital function and that is to serve as a balance against the bottom hole pressure to thereby prevent blowout. The hydrostatic head is directly related to the depth of the borehole and the density of the drilling mud, by the following equation:

p=0.052dh where p is the hydrostatic head in p.s.i.,

d is the density of the mud in pounds per gallon, and

h is the depth of the column in feet.

Mud density may be regulated by conventional and wellknown methods (literature available from National Lead Company, Baroid Division, for example) by the addition of weighting additives to the mud before it is introduced into the borehole, and may be readily determined by suitable measurements well known to those of skill in the art. Therefore, it is a relatively simple matter to calculate the hydrostatic head and to adjust the hydrostatic head to any desired value at all times.

The pressure of the fluid in the pores of a rock formation determines the formation pressure" of that particular strata. The various formations l6, 18, 20 and 22 through which the borehole penetrates exist at different formation pressures. Since the normal pressure gradient is known for the particular area of drilling, the normal formation pressure may be calculated simply by multiplying the normal gradient times the depth of the borehole. To balance this formation pressure, the driller adjusts the hydrostatic head by increasing or decreasing the density of the drilling mud as mentioned above. And then to allow for pressure surges and higher than expected pressures, a safety factor is added; that is, the hydrostatic head is maintained at a value considerably greater (for example, 750 p.s.i. greater) than the normal formation pressure. This safety factor is necessary because, as mentioned above, the formation pressure varies even within one formation; further, the drill may suddenly hit a relatively high-pressure gas pocket.

There are at least two principal difficulties with the use of these safety factors in present practice, as discussed above. The first and most critical is that they are sometimes not large enough to compensate for the extremely high-pressure gas pockets which are sometimes encountered, especially in certain areas of Texas and Louisiana. If such a gas pocket is encountered and the gas pocket formation is of sufficient permeability, a blowout may occur. The second problem with the safety factor practice is that the safety factors used are (for the great majority of situations) overly large, to compensate for problems such as that just described. Use of these large safety factors means that the hydrostatic head is maintained at a level significantly above and beyond what is generally needed, resulting in lower bit penetration rate, shorter bit life, consequent increase in cost of drilling the well, and the risk of fracturing a low-pressure formation.

ln addition to these difficulties, when it is determined that a high-pressure pocket has been encountered (and a blowout has not occurred), drilling through such pocket under present practice is slowed to a very slow pace, resulting again in increased cost of drilling.

Some formations are known as fhigh-pressure, low-volume" formations. Because of their low permeability, there is no real blowout danger in some of these formations even though the gas exists in the pores of the strata at high-pressure. This is simply because not enough of the high-pressure gas can find its way into the borehole. These formations are often of no real concern to the driller, as far as the blowout problem is concerned, so in some such formations only, the bottom hole pressure may safely be greater than the hydrostatic head.

As the drilling mud sweeps the bottom of the borehole, as seen in FIG. 1, gas from the formation being cut by the drill bit will enter the borehole and will be returned to the surface up the annulus 33 by the mud. Some relatively small amount of the gas will be dissolved in the mud, while other gas, which may be termed free gas, will not be dissolved but will be returned to the surface by the drilling fluid. In accordance with this invention, when this gas-containing mud reaches the surface it is transported by suitable means such as the pipe 38 to a separator 40 suitable for separating the free gas from the mud. Suitable choke means may be. included in the line'38 to reduce the pressure of the mud-gas mixture to a level which may be properly handled by the separator. In this connection, it has been found particularly advantageous to use the twostage system illustrated in FIG. 1, comprising the adjustable choke 84 and the hydraulic choke 86. The two-stage pressure drop efiected in this manner has been found to be effective in preventing equipment damage due to high gas pressures.

The separator 40, which may operateat any convenient pressure as for example I25 p.s.i., may be elevated so that the mud may return by gravity through a mud return line 44 to a mud pit 46, for later recirculation into the well.

The gas exits the separator 40 through a gas line 42, and is thence measured to determine the volume of gas released from the separator. Such measurement may be made in any suitable manner, one example being illustrated in FIG. 2.

A multiple flow line system which has been found to be particularly advantageous for the measurement of gas flow under all conditions is shown in FIG. 2. The primary advantage of this particular system is that it allows accurate and precise flow measurements for gas flow rates ranging from very high to very low. In accordance with this system, several lines of different sizes are connected in parallel, with instruments for measuring flow rate positioned in each line. Since it is difficult to get accurate and precise measurements when there is very little flow through a' large line, or when there is large flow through a small line, the flow is directed by the operator through the appropriate line by suitable flow control means, depending on the volume of flow.

in this embodiment, the line 42 from the separator may be 6 inches in diameter. Parallel line 50 may be four inches in diameter and may be secured by an elbow connector 51 to the line 42 at a point spaced from the separator 40. Another parallel line 54, which may be three inches in diameter, may be secured in a similar manner by appropriate elbow connection 55, to the line 50.

The parallel lines are in fluid communication at a point spaced from their connection with each other, with a transverse line 62 which communicates with a flare 64 whereby the waste gas is desirably burned a safe distance from the rig.

Suitable flow control means such as valves are located at appropriate positions in the lines, as seen in FIG. 2. Valve 52 is located in line 42 downstream from the connection of line 42 with line 50. Valve 58 is located in line 50, downstream from its connection with line 54. In transverse line 62,

suitable valves

68, 70 and 72 are located between line 42 and flare 64, line 42 and line 50, and line 50 and line 54, respectively. A small line 60 for use with very small flow rates may be located transverse to the

parallel lines

42, 50, 54 and secured in fluid communication between the

lines

54 and 42. This line may be for example 3/4 inch in diameter, and may contain an orifice 66 of any suitable size therein.

A rupture plate 56 which may be adapted for rupture when the pressure reaches a certain level, for example I00 p.s.i., is located in line 54 near its connection with line 50.

Suitable flow measurement means, such as pitot tubes, are

located in the various lines. Tube 74 is positioned in line 42 between the separator 40 and the connection with line 50; tube 76 is positioned in line 50 between its connections with the

lines

42 and 54, tube 78 in line 54 between its connections with the lines 50 and 60, and tube 80 in the line 60 between its connections with the

lines

54 and 42, preferably at the orifice 66. Each of these tubes or other flow indicators is suitable for measuring the flow rate of the gas passing through the line wherein it is located, and suitable means are connected with each such tube for continuously recording the data received on a recorder 82. ln this manner, a continual record of the flow rate of gas returning from the well may be kept.

The manipulation of the various valves in order to effect flow through'ariy of the desired lines is readily apparent by reference to FIG. 2. Thus, when the flow rate is relatively large and flow'is desired through the line 42, the valve 52 is opened and the valve 58 is closed. But if 'flow through line 50 is desired, then the valve52 is closed and the valve 58 is opened.

Since the'flow rate of the gas released from the separator is, assuming constant permeability, directly proportionalto the bottom hole pressure in the well, these flow measurements enable the driller to keep a constant, up-to-the-minute account of what is happening, pressurewise, downhole. And by comparing the flow rate measurements with similar measurements taken earlier, the rate of change in the flow rate may also be determined. These measurements thus allow the driller to determine whether a gas zone is depleting or whether additional gas is being encountered, and thus predict what is happening downhole at any given moment. The operator knows when the drilling has proceeded into a high-pressure gas pocket. The hydrostatic head is then adjusted when necessary by decreasing or increasing the density of the drilling mud by mixing with the mud the appropriate additives.

It is still necessary in formatioii's which are not relatively low volume formations, to maintain the hydrostatic head at a level somewhat greater than the bottom hole pressure. It will be recognized that the differential employed will vary somewhat depending on the formations being drilled and other factors, but in many contexts of use, it is found that a pressure differential of about 200-250 p.s.i. is sufficient. This is considerably less than the differential necessary for safety with conventional drilling processes. And it is the difference between the hydrostatic head levels which may be utilized with the present invention, as compared to those which must be used with prior art methods, which is important-rather than the absolute value of these hydrostatic head levels. These absolute values, of course, are to a great extent governed by formation properties. As an example, if it is determined that a 250 p.s.i. differential is appropriate for use in a given situation, the mud density is continually adjusted so that the hydrostatic head is maintained at a level exceeding the bottom hole pressure by 250 p.s.i. Complete pressure control is thus maintained without danger of blowout.

When a high-pressure gas pocket is struck, the driller will have time using the method of the preset invention to increase the mud density sufficiently to avoid blowout. This is because there is a delay or lag time between the bits striking the gas pocket, and any possible blowout, because formations are not sufficiently permeable to cause instantaneous blowout. Of course, this lag time varies greatly depending on the permeability of the formation and the pressure of the gas pocket, and therefore it is greatly desired that fairly immediate corrective action be taken whenever necessary. This of course is one of the great advantages of the present invention.

It is here noted that there are presently methods for determining bottom-hole pressure, such as that known as the drill stem test. But such methods are expensive, dangerous, and cannot be continuously performed during drilling of the well. If it is desired to take such a test during drilling of the well while the process of this invention is being used, then such a spot check will allow the driller to better correlate the data received. For example, if the flow rate is increasing, the driller can tell just exactly how much of the increase is due to an increase in bottom-hole pressure and how much is due to increased porosity of the formation. Such procedures are not generally necessary however. Since the formation porosity is generally known at least to some degree, the measured flow rate of the gas returning from the well (and the rate of change therein) has been found to give a direct and accurate indication of the bottom-hole pressure picture. In other words, although the porosity of the formation does have an effect on the flow rate measurements, such effect can usually be discounted to great extent because of the knowledge the driller will have of the formation being dr illed, and the safety factor allowed.

It is seen that the invention has, provided an apparatus for well control which providesfor complete pressurecontroh'at all times with minimum risk of costly and dangerous blowout... Further, the invention provides an apparatus whichallows for faster penetration rates and longer bit life', resulting in reduced drilling costs. The apparatus of. the present'invention. also is seen to allow for faster drilling through strata wherein the formation pressure is -high,.and to provide protectionagainst the fracture of low-pressure formations.

It is further .seen that the present invention. provides apparatus suitable for'accurately and preciselymeasuring the flow rate of free gas emanating from the-well, regardless of whether the flow is great or smalli- The apparatus is desirably portable so that it may be readily transported form rig to rig.

What is claimed is: i

1. Apparatus for use in connection with the drilling ofwells, allowing the well drilling to proceed withminim um-hydrostatic head on the column of drilling fluid while at'the same time minimizing the possibility of blowout, said apparatus being utilized in connection with means for circulating drillingfluid through the borehole so that the drillingffluid'. carries cuttings and free gas from theborehole to the earth's surface, including:

separator means for separating said free gas from said drilling fluid;

means for receiving said free gas after separationthereof,

said means including a conduit: system comprisinga plurality of generally parallel conduits of differingsizes, arranged in a manner such that the gas flow maybe directed into any one of said conduits depending upon the approx imate quantity of said flow; means in said conduit system for measuring when desired the gas flow rate therethrough; and,

means for flaring said gas. after it has passed through. said conduit system.

2. Apparatus suitable for measuring the flow rate of gas emitted from a mud-gas separator, comprising: i

a first line having a selected diameter, in fluid communication with said separator;

a second line having a diameter smaller than the diameter of said first line, in fluid communication with said first line at a point spaced from said separator;

a third line having a diameter smaller than the diameter of said second line, in fluid communication with said second line at a point spaced from the connection of said second line with said first line;

suitable flow control means in said lines selectively directing flow through either said first line, said second line, or said third line;

a transverse line in fluid communication with each said first line, said second line, and said third line, and with a flare for burning waste gas;

measuring means in each said first line, said second line, and said third line, suitable for measuring the gas flow through each of said lines;

whereby gas from said separator can be selectively directed through either said first line, said second line or said third line depending upon the flow rate of said gas, to achieve accurate and precise flow measurements regardless of the volume of flow. 3. Apparatus suitable for measuring the flow rate of gas emitted from amud-gas separator, comprising:

a first line having a selected diameter, in'fluid communication with said separator;

a second line having a diameter smaller thanthe diameter of 7 whereby gas from said separE r can be selectively directed through either said first line or said second line depending upon the flow rate of said gas, to achieve accurate and precise flow measurements regardless of the volume of flow.

4. Apparatus suitable for measuring the flow rate of gas emitted from a mud-gas separator, comprising:

suitable flow control means in said lines for selectively directing flow through either said first line, said second line, or said third line;

whereby gas from said separator can be selectively directed through either said first line, said second line or said third line depending upon the flow rate of said gas, to achieve accurate and precise flow measurements regardless of the volume of flow.

I l 0 III III

Claims

1. Apparatus for use in connection with the drilling of wells, allowing the well drilling to proceed with minimum hydrostatic head on the column of drilling fluid while at the same time minimizing the possibility of blowout, said apparatus being utilized in connection with means for circulating drilling fluid through the borehole so that the drilling fluid carries cuttings and free gas from thE borehole to the earth''s surface, including: separator means for separating said free gas from said drilling fluid; means for receiving said free gas after separation thereof, said means including a conduit system comprising a plurality of generally parallel conduits of differing sizes, arranged in a manner such that the gas flow may be directed into any one of said conduits depending upon the approximate quantity of said flow; means in said conduit system for measuring when desired the gas flow rate therethrough; and, means for flaring said gas after it has passed through said conduit system.

2. Apparatus suitable for measuring the flow rate of gas emitted from a mud-gas separator, comprising: a first line having a selected diameter, in fluid communication with said separator; a second line having a diameter smaller than the diameter of said first line, in fluid communication with said first line at a point spaced from said separator; a third line having a diameter smaller than the diameter of said second line, in fluid communication with said second line at a point spaced from the connection of said second line with said first line; suitable flow control means in said lines selectively directing flow through either said first line, said second line, or said third line; a transverse line in fluid communication with each said first line, said second line, and said third line, and with a flare for burning waste gas; measuring means in each said first line, said second line, and said third line, suitable for measuring the gas flow through each of said lines; whereby gas from said separator can be selectively directed through either said first line, said second line or said third line depending upon the flow rate of said gas, to achieve accurate and precise flow measurements regardless of the volume of flow.

3. Apparatus suitable for measuring the flow rate of gas emitted from a mud-gas separator, comprising: a first line having a selected diameter, in fluid communication with said separator; a second line having a diameter smaller than the diameter of said first line, in fluid communication with said first line at a point spaced from said separator; suitable flow control means in said lines for selectively directing flow through either said first line or said second line; measuring means in each said first line and said second line suitable for measuring the gas flow through said lines; whereby gas from said separator can be selectively directed through either said first line or said second line depending upon the flow rate of said gas, to achieve accurate and precise flow measurements regardless of the volume of flow.

4. Apparatus suitable for measuring the flow rate of gas emitted from a mud-gas separator, comprising: a first line having a selected diameter, in fluid communication with said separator; a second line having a diameter smaller than the diameter of said first line, in fluid communication with said first line at a point spaced from said separator; a third line having a diameter smaller than the diameter of said second line, in fluid communication with said second line at a point spaced from the connection of said second line with said first line; suitable flow control means in said lines for selectively directing flow through either said first line, said second line, or said third line; measuring means in each said first line, said second line, and said third line, suitable for measuring the gas flow through each of said lines; whereby gas from said separator can be selectively directed through either said first line, said second line or said third line depending upon the flow rate of said gas, to achieve accurate and precise flow measurements regardless of the volume of flow.