US2360742A - Apparatus for determining production potentials of oil wells - Google Patents

Description

Oct. 17, 1944. F 1 T01-H Er AL 2,360,742`

APPARATUS Pon DETEPMINING PRODUCTION PoTENTmLsl oP oIL WELLS V -Filed March 8, 1941 l 2 sheets-sheet 1 oct. 17, 1944.

APPARATUS FQR'DETERMINING PRODUCTION POTENTIALS 0F OIL WELLS F. J. 'ro'rH Erm. 2,360,142

Filed March a, 1941 2 shee'Ls-s'he'epa Przssurz ai Pump in Lbs/5mn.

0 IOO 200 500 400. 500 600. OO BOO Produdon in barrels pzr pag I Fig.5

Patented Oct. 17, 1944 APPARATUS FOR DETRMINING PRODUC- TION POTENTIALSI OF OIL WELLS Fred J. Toth and Donald S. Nutter, Bakersfield,

Calif., assignors to Shell Development Company, San Francisco,` Calif., a corporation f Delaware Application March 8, 1941, Serial No. 382,400

1 Claim.

This invention pertains to apparatus for the measurement of fluid levels, pressures and densities in oil Wells, and for the determination of the production potentials of i Wells, and relates more specifically to apparatus for effecting such measurements in flowing, gas lift or pumping type wells.

Determinations of oil well potentials or productive capacities, involving fluid level, pressure and density measurements, are necessary for purposes of efficient and economical production of mineral oil, and for purposes of compliance with proration laws regulating so-called production allowables from oil wells.

Various methods have been applied for effecting these measurements in wells, involving, for example,the use of bailers lowered on sand lines for fluid level measurements and the use of pressure recorders connected into the pump string near the pump for the measurement of pressure at any depth within the Well.

Since these methods, however, generally require the pumping operations to be stopped; there are also in use so-called sonic methods whereby all necessary measurements may be effected, and the production potential of a pumping well may be determined without stopping pumping operations.

These methods, for example, as described in the U. S. Reissue Patent No. 21,383 to Walker, comprise broadly the following steps: Closing the annular space between the Vcasing and the pump tubing while Voperating the pump at a predetermined constant rate; allowing the-pressure of the formation gas accumulating in said annular space to depress the level of the fluid standing in the well; effecting level measurements by the sonic method and noting theannular space gas pressures corresponding to each of the measured levels; determining from these level and pressure measurements the density of the Well fluid and the pressure at the pump; changing the rate of pumping and repeating said measurements and determinations, and establishing, by combining the data thus obtained, the well potential or the maximum-production capacity of the Well, which may be expressed in terms of barrels per day.

The above methods, however, are inelfective liquid and a relativelysmall amount of gas, since the gas pressure building up in the well when thecasing head is. closed is insufficient-to depress the iiuid level as required for the use of saidv method. The application of extraneous gas pressure, as, for example, from a compressor, isV prohibitively expensive' and wasteful. Neither the natural, nor the artificial gas pressure method can be effectively used in wells havingleaky casmgs. .Y

Furthermore, in Wells producing a low gravity and/or high viscosity crude..oil which has a tendency to retard the liberation of minute quantities ofentrained formation gas, there is usually formed in the annular space between the casing and the tubing, a layer of more or less stable foam. This layer, whose height is indefinite and subject to change, may sometimes detract from the accuracy ofthe sonic method, since the latter is based primarilyon the principle of the reiiection of sound from a true liquid surfacea The above-outlined method has furthermore the disadvantage of consuming a relatively large amount of time due to the necessity of taking level and pressure measurements at a `plurality of points, and of giving the pressureat the pump only by an indirect extrapolation process instead of by direct reading. Likewise, pressure Yand level determinations at some other points of special interest in the well, such for example, as the mid-point of a perforated interval of the casing or liner string, often have to be effected by the same indirect process of extrapolation, because it is not always possible to depress the iiuid in the well to a level sufliciently low to take direct readings at said points.

It is, therefore, the object of the present invention to provide apparatus for the measurement of-oil well conditions such as pressures and fluid levels and densities, and for the determination of well potentialsfwhich apparatus is applicable in wells producing relatively small amounts of gas.

It is also an object of this invention to provide apparatus for effecting the above determinations in wells producing oil which results in the formation of a foam layer in the annular space between the casing and the tubing.

It-is also an object of this invention to provide apparatus for effecting the above determinations without withdrawing the pump string from the well and without stopping the pumping operations, by means comprising devices lowered into the annular space between the casing or the walls of the well and said pump string.

It is also an object of this invention to provide apparatus for effecting the above determinations, which may, if desired, be used without stopping the pumping operations and without closing the annular space between the vcasing and the tubing, or without applying an artificial gas pressure to said space in order to depress the fluid level.

It is also an object of this invention to provide apparatus for effecting the above determinations, whereby the pressure at the pump or at any other desired point, such for example, as the mid-point of a perforated interval of the casing or liner string, may be measured by direct readings rather than by a process of indirect extrapolation.

These and other objects of this invention will be clearly apparent from the following description taken with reference to the appended drawings, wherein:

Fig. 1 shows the general arrangement of the apparatus used in applying the present method;

Fig. 1a is a cross-section view of a detail of the apparatus of Fig. l, taken along line Iar of Fig. 1;

Fig. 2 shows a valve, chamber and sinker assembly which may be lowered into a 'well according to the present method; and

Figs. 3, 4 and 5 are graphs showing the manner in which the data obtained by the use of the present method are combined to obtain the desired Well potential values.

Briefly stated, the method of the present invention comprises the steps of effecting simultaneous pressure and level measurements in the annular space between the walls of the well or the casing and the pump string by means of apparatus such as a pressure bomb lowered into said annular space on a measured wire line, or a string of flexible, small diameter tubing to which gas pressure is applied from the surface. More specifically and in greater detail, a preferred embodiment of the present invention comprises apparatus for lowering into the annular space between the casing and the pump tubing a string of flexible small-diameter tubing, submerging the lower end of said flexible string in the bore-hole uid to any desired level, measuring said level, applying to the upper end of said flexible string a gas pressure capable of balancing the liquid head under which the lower end of said string is submerged, measuring said equilibrium pressure, and calculating from said level and gas pressure measurements the total pressure at the lower end of the flexible string. By repeating these determinations for different levels or different constant pumping rates and, if desired, for different values of gas pressure in the annular space between this casing and the tubes, the well potential and the productivity index may be determined `in a manner similar to that used in the sonic methods referred to above.

Figure 1 shows a well I, having a casing or liner string '2, closed by means of a flanged casing head 9, which supports a tubing string 6 provided with a pump 3 and means, such as a sucker rod string shown at IIJ, for supplying mechanical power to the pump, said pump being adapted to raise the well fluid to a suitable reservoir at the surface through a pipe II. The annular space between the casing 2 and the tubing 6 may be vented through a pipe 1 or closed by means of a valve 8 provided in said pipe,a gauge 'Ia being used to indicate the gas pressure in said annular space.

The casing head flange has a suitable perforation 9a in communication with a lubricator I2 and an oil saver I3, through which a string of copper tubing 5, carrying at its lower end a sinker bar or' a special assembly 4, to be described later, may be lowered into the annular space between the casing and the pump tubing of the well. This lubricator may also be used, if desired, to lower a pressure bomb on a wire line into the annular space, if it is desired to determine the pressure therein in this manner instead of the conventional method of connecting the bomb into the pump string. I

The stringi 5 consists of flexible copper tubing of relatively small diameter, for example, 11e

4 inch inside and 1/8 outside diameter. This tubing may be manufactured in any suitable lengths, such as 1000 feet, and may then be joined, by flux, acid or silver soldering, into strings having desirable lengths such as 2000 feet, 4000 feet or more, said solder joints having a tensile strength of 300 pounds or more.

Upon coming out of the well throughthe lubricator I2 and cil saver I3, the flexible copper tubing passes over a sheave IB, cooperating with a metering device I'I adapted to indicate the depth at which the lower end of the tubing 5 is located at any given moment. It is, however, obvious that said depth may be measured by other means, for example, by placing suitable markers on the tubing 5 and counting said markers, in which case the metering device I'I may be dispensed with.

After passing over the sheave I6,'the copper tubing is wound on a reel or drum I8 rotatably supported on a base or frame I9. The reel I8 may be operated mechanically or manually, as, for example, by means of a handle 23.

The tubing 5 is connected to a source of gas pressure 21, such as a tank or a compressor, by any suitable means, the following-being given by way of example and illustrated in Fig. la. The upper end of the tubing 5, wound as the innermost; layer of tubing on the drum I8 is led toward and into the shaft 2| of the drum, as shown at 5a. 'Ihe shaft 2| is made hollow and is provided at one end with a ball-bearing thrust packing box 25 mounted rotatably with regard to said shaft 2| and xedly with regard to the frame I9. A conduit 5b, provided with a valve 26, lconnects the bearing box 25 with the reservoir of pressure gas 2'I. By opening the valve 26, gas may thus be admitted to the tubing 5, the pressure of said gas being indicated by a gauge 24 mounted at any suitable place, for example, on the box 25 as shown in Figs. 1 and 1a.

'Ihe reservoir 2I may be filled with compressed air or gas of any type, the use of hydrogen being, however, preferred, as will be shown below.

lThe lower end of tubing 5 may simply open to the fluid within the well, being provided with suitable weighting means, such for example, as 10foot round stock steel sinker bar of about 1% inch outside diameter, as shown in Fig. l, or may have attached thereto a special assembly of the type shown in Fig. 2.

The arrangement of Fig. 2 comprises a metallic tubular member 40 having preferably an outside diameter of less than one inch and pronvices wahre-p and bottom closure pieces so and i9. "The lower end 'of tubing '5 Ais Yled through an 'p'en'ihg lin 'the top 'piece 30 `and is 'aHXe'cl with- 'in'the 'tube 40 by clamping means generally shown 'at 3l.

The clamping ymeans are provided with an axial 'passage 33, in register with the tube 5, and support, for example, by means of a ring 35, a flexible rubber sleeve 3b adjacent the inside wall `of tubing 40. The lower en'd of the sleeve '36 is affixed, by means of 'a ring 3'9, to a ball valve chamber held within the tubing 40 and generally shown at '44.

The tubing 40 is provided with lperforations y33 whereby pressure variations occurring in the well are 'applied from the outside to 'the chamber "formed by the flexible sleeve 36 and serve 'to vary 'the volume of said chamber.

The chamber within the sleeve '36 is in direct communication with the space within the Well by Ameans of passages '4l and-45 in the valve chamber 44, and perforations 256 in the tubing 40, the

-valve chamber being also provided with a non- Vreturn valve 42, which may be of the ball type and is actuated by a spring 43 to close the passage 4l against the entry of the well fluid. The lower portion of the tubular member 4i! serves as a housing for weighting Aor sinker means, such for example, as a body of lead shown at 41.

A thermometer, for example, of Ythe maximum registering type may also be housed within the tubular member til for purposes to be described below, as shown at 50.

The arrangement shown in Fig. 2 has therefore a threefold use. First, the surge chamber Witlr'n the flexible sleeve 36 serves Ito smooth the effect of the pressure Variations occurring within the well, whereby average, rather than instantanecus values, are indicated by the gauge 24 in applying the present apparatus; second, the

non-return valve 42 closes the small-diametertubing to the entry of the Weii fluid, thereby 'preventing the possibility of its becoming clogged by impurities carried Yby the fluid; and third, the whole assembly, and especially the lead body 41, serves as a sinker bar for the tubing 5.

In applying 'the present apparatus, the fluid in the well is allowed to assume a stable level such as shown at A in Fig. 1, while either stop-ping the pump 3, if it is desired to obtain the sos-called static well gradient, or operating said pump at a predetermined constant rate, if it is desired to ofbtain the so-called pumping or producing well gradient. The tubing 5, having at its lower end either the sinker bar shown in Fig. 1 or the arrangement shown in Fig. 2 is then lowered into the annular space between the casing and the tubing bythe use of the lubricator I2.

When the lower end cf the tubing 5 reaches any desired level, such for example, as shown at B in Fi'g. 1, as determined bythe metering device l1, the valve 26 is'momentarily opened, admitting a certain quantity of the pressure gas from reservoir 2 into the tulbing 5.

This causes within the tubing 5 a pressure surge which is gradually dissipated as the gas eX- pands and escapes by bubbling through the well ilud from the lower end of tubing 5 or from the openings 45 of the apparatus of Fig. 2, until said pressure reaches a constant value in equilibrium with the iiuid head under which the tubing is submerged.

By 'referring to curve B in Fig. 3, which illustrates a case wherein the lower end of the tubing 5 was held at a level B equal to 1500 feet, it

will be rseen that an` initial pressure surge of 350 lb`s`./sq. inch, as indicated by the gauge 24, decreased in 6 minutes'to a substantially constant value of 260 lbs. /s'q. inch, which is the equilibri-uni pressure at said leve-l'.

The total subsurface pressure at level B or at any other level isV obviously equal to said equilibrium pressure plus the computed pressure head exerted by the gas column within the copper tub'- ing'. This last pressure may be computed by the well-known Dice formula:

wherein:

P=pressure due to static head of gas columnl in lbs/sq. in.

PG=abso1ute gauge pressure in lbs/sq. in.

S=specic gravity of gas referred to air d=depth in feet T=mean absolute temperature of gas in degrees Fahrenheit The mean absolute temperature of the gas' in tubing 5, which is substantially .the same as vthe mean temperature in the well, can be determined in a manner well understood by petroleum engineers', fr example, from-readings obtainedby means of the thermometer 50;

It will be seen that the specific gravity of the gas' within the tubing 5 is a factor entering in the above formula. ItV is', therefore, of great advantage to use for the purpose described a pressure gas .having a very small specific gravity whereby errors due Ito small inaccuracies in temperature or level measurements are minimized.

After measuring the equilibrium pressure and computing thetotal pressure at level B, the lower end of tubing 5 may be raised or lowered to any other desired level, such as C, which may, for example, be the mid-point of a perforated interval of the casing-or liner, and where direct pressure measurements may for this reasonl be considered c f importance.

levels B and C against corresponding depths, asv

shown in Fig. 4, and drawing a line I through the points B and C, the pressure gradient of the well may be obtained, from which the well pressure at the level of the pump may be determined. Thus, if the pump is located at a level D equal to 1800 feet, the Well pressure at the pump will be found to be 385 feet.

If permitted by other conditions of the Well, such as depth of the pump, length of the available copper tubing equipment, etc., the pressure at the pump 'may be determined by the present apparatus directly, by lowering the tubing 5 to the level D and effecting a measurement at said level. This procedure gives quicker results, as only one measurement is necessary in such case, and may likewise be used to check the result obtained by the extrapolation methodV described above. A reading obtained in this manner is shown at D1 in Fig. 4, checking closely the result obtained by extrapolation.

Knowing the depth of the levels B and C and wherein DF=density of uid in lbs/sq. in. per foot of fluid Pc=total pressure at level C in lbs/sq. in. PB -total pressure at level B in lbs/sq. in. Lc=depth from surface to level C` in feet La=depth from surface to level B in feet If the measurements described aboveare made while the pump 3 is stopped, the values obtained and p-lotted as shown by line I in Fig. 4 give the So-called static gradient of the well.

If the pump is then started and operated at a predetermined constantl rate for a length of time suicient for the fluid to recede to a new constant level, and the above measurements are repeated for the levels B and C, or for any different levels, so-called pumping gradients for the well may be determined.

Thus, vfor example, if the pump is operated at a constant rate to give a production of 150 barrels per day, the static fluid level A will recede to a lower constant level, and the pressure determined for the pump level will correspondingly decrease, for example, from a value of 385 lbs/sq. in. to a value of 315 lbs/sq. in., as shown at point Dn on the pumping gradient line II in Fig. 4.

By plotting the values of pressure at the level of the pump against the corresponding rates of production, it is possible to determine the Well potential or the maximum production capacity of the well. t

Thus, by plotting the above pump pressure values of 387, 315 and 2.75 lbs/sq. in. against corresponding production rates of 0, 150 and 225 barrels per day, respectively, as shown at points I, II and III in Fig. 5, it will be seen that the maximum potential production of the given Well,

the pressure at the level of the pump to zero, is

, equal to '765 barrels per day. From this value,

it is further possible to determine the productivity index of the well which is dened as the Well production in barrels per day for each pound per square inch of pressure at the pump.

If the well in which it is desired to carry out the above determinations produces gas, the antential may be effected in a manner and with which is the production rate required to reduce I results similar to the above, the pressure at any level within the Well being in such case equal to the sum of the fluid head obtaining at any point and of gas pressure applied to the surface of said fluid within the annular s'pace between the casing and the tubing, said gas pressure being indicated by a gauge la in communication with said annular space.

We claim as our invention:

In apparatus for measuring conditions determining the production of a well having a casing and a pump suspended therein on a tubing string, a string of flexible small diameter tubing insertable into the annular space between the casing and the pump strings of said well, the lower end of said exible string opening to a flexiblewall surge chamber externally exposed to Well pressures, a non-return valve in communication

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