US2320492A - Method of determining fluid pressure and production capacity of oil wells - Google Patents

Description

C. P. WALKER METHOD OF DETERMINING FLUID PRESSURE AND June 1, 1943.

PRODUCTION CAPACITY OF OIL WELLS Filed Aug. 11, 1958 1N VENTOR.

N/iv M11/re ATTORNEY.

Patented June 1, 1943 METHOD OF DETERMINING FLUID PRES- SURE AND PRODUCTION CAPACITY OF OIL WELLS Cranford I. Walker, San Marino, Calif.

Application August l1, 1938, Serial No. 224,416

27 Claims.

My invention relates to methods of testing produclng oil wells and has particular reference to a method for determining the pressure of the oil at the inlet of a mechanical pump employed to remove oil from the well and to a method for determining the maximum production capacity of the well.

In producing oil from wells employing mechanical pumping apparatus to lift the oil to the ground surface, it is extremely desirable to know whether or not the particular pumping apparatus used is operating in that wel] with such eiciency that it is removing the oil at substantially the same rate at which it flows into the well from the formation. Many conditions may exist under which the particular pump employed in the well will not operate to achieve the maximum production possible in that well. For example, the pump may have sufficient size, length of stroke and speed to properly and efficiently pump the well but due to the fact that it may be located at such point below the surface of the oil that adequate gas separation has not taken place, each stroke of the pump takes in a considerable quantity of gas as well as a quantity of liquid and the volumetric cihciency of the pump under those conditions is relatively low.

If the operating pressure at the pump is found to be high, by simply raising the pump to a higher level in the liquid at which adequate gas separation does take place, the same pump operating at the same speed and with the same stroke may produce a greater quantity of oil from the well. On the other hand, it may be that by installing a gas anchor or by substituting a different type of gas anchor for that which may be already in use, the same eiect of increased production may be accomplished. In certain types of wells the volumetric eiliciency of the pump may be improved by lowering it to a, point below that at which the liquid and gas enter the Well, resulting in an increased rate of production.

Another condition may be that the length of stroke and the speed of operation of the pump is not adequate to properly pump the iiuid at reasonable efficiency and by merely altering one or the other of these factors a considerably increased production will result.

Of course, the particular pump which has previously been selected for the'well may be of insuicient size and capacity to pump that particular well at its maximum efficient rate and an entire new pump installation may be required.

In initially selecting the pumping equipment to be used in a particular well, an estimate is made from the characteristics of the Well during its iiowing period or a comparison of the pumping equipment employed in adjacent wells in the same field is made and from. this information the particular size and character of the equipment is selected. Due either to the improper initial selection of the pumping equipment or due to changes in the characteristics of the Well after the pumps have been installed, or to improper positioning of the pump in the Well, many Wells are found which are not producing at their maximum eiicient rate.

In view of either production restrictions imposed by law or imposed by agreement between the oil producers, it is frequently necessary to produce a well at some predetermined percentage of its maximum capacity so that without adequate information as to the potential of the Well it is substantially impossible to determine whether or not any given well is producing either at its maximum rate or at an agreed percentage of such rate.

Moreover, the desirability of making changes in the pumping equipment Will of course depend upon the amount of increased production to be expected after such changes are made, so that in order to determine whether relatively inexpensive changes, such as the raising of the pump or alterations in its speed or stroke or the use of a more effective gas anchor, will be sufficient, will depend upon a knowledge of the maximum potential of the well, while if such maximum potential is known and indicates that the expense of a complete new pump installation would be justied, such more expensive change may be made.

As will be understood by those skilled in this art, a knowledge of the operating pressure at the inlet of the pump, compared with the production rate of the pump will indicate whether or not the pump is removing all of the available liquid and will permit the operator to determine Whether or not changes in the pumping equipment should be made. It is also desirable to know the operating pressure at the pump inlet at intervals over a period of time, in order to determine the decline in the productivity of the formation tapped by the well. Much other interesting and desirable data may be determined if a ready mea-ns is provided yfor determining the operating pressure at the inlet of the pump.

Heretofore in order to obtain this information it has been usual to remove the pump from the well and to insert pressure bombs or other pressure recording devices into the well at considerable expense and considerable loss of production from the well while such tests were made.

I have discovered that the minimum gas pressure which is required to maintain the liquid level depressed down to the pump inlet, while the pump is operating at its normal speed and stroke, is equal to the operating pressure of the liquid at the pump inlet for the normal or existing casing pressures, so that by determining the minimum pressure required to hold the liquid level so depressed, an accurate and reliable determination of the operating pressure at the pump may be made.

This is made possible by reason of the fact that when the well is producing, the downwardly directed forces of the gas pressure within the casing and the weight of the column of liquid above the pump inlet is just balanced or is in equilibrium with the upwardly directed forces resulting from the pressure with which the oil is flowing from the formation into the well (minus, of course, the weight of the column of liquid between the pump inlet and the point at which the oil passes from the formation into the well). Thus if this equilibrium is maintained and the liquid column above the pump inlet is depressed down to the pump inlet, it renews that the pressure of gas which isjust suflicient to hold the liquid level at the pump inlet is the same as the normal operating pressure at the pump inlet regardless ol.' how far above the pump inlet the liquid normally stands.

I have discovered that in some wells the arrival oi' the liquid level at the pump inlet by gradual increase of casing pressure is accompanied by a distinct change in the rate of production from the pump, the rate increasing sharply probably due to the fact that the gas which had previously separated and normally escaped through the casing is now separating and attempting to escape through the pump tubing, thereby lightening the column of liquid in the pump tubing and causing the well to "flow in addition to whatever lifting effect is produced by the mechanical pump.

In other wells, however, any attempt to fur- 45 crease of pressure has increased the back pres- 50 sure against the formation and retarded the rate of flow of oil from the formation into the well.

It is therefore an object of my invention to provide a ready method of accurately determining the operating pressure at the pump inlet, with- 55 out the necessity of removing or altering the pumping equipment and without interruption of the production from the well.

Another object of my invention is to provide a ready method for determining when the uid 60 level has been depressed to the inlet of the pump, by utilizing only the pumping equipment employed in the well and without the necessity of additional apparatus and instruments.

It is another object of my invention to provide a ready method for determining the productivity index of the well and to determinev the maximum production capacity of thewell, without the necessity of employing apparatus other than that normally employed for producing the particular well under consideration.

Other objects and advantages oi' my invention will be apparent from a study of the following specifications, read in connection with the BC- y 75 was found reduced, the pressure exerted upon the companying drawing, wherein Fig. 1 is a diagrammatic view of a chart which may be plotted from information obtained by my method and from which the relation between the casing pressure and production rate may be utilized to determine the operating pressure at the pump, to determine when the liquid level has been depressed to the pump inlet and to determine the maximum production capacity of the form of chart by which the static pressure at the pump inlet obtained by any means may be compared with the operating pressure at the pump inlet for any rate of production less than normal and by which the `maximum production capacity of the well may be determined.

In practicing my method for determining the operating pressure at the pump inlet, I first note the normal rate of production from the well with f the existing equipment operating at its normal speed and normal length of pump stroke and at the same time note such pressure, if any, as may exist at the casing head under such normal operating conditions. Since most wells make a sufcient amount of gas to permit the utilization thereof as a means for applying pressure at the liquid surface, I apply a gas pressure regulator to the casing head of the well and set the same to permit the gradual building up of the gas pressure in the casing until Athe liquid level is depressed to the pump inlet. I prefer to build up the casing pressure in relatively small increments in a series of steps. During all of the steps of building up of pressure inthe casing, the pump continues to operate so that when the pressure has been built up in one increment I permit the pump to continue its operation until the pressure at the casing head becomes constant and the production rate of the pump under the new pressure becomes stable (that is, the average rate at which the well is then producing is the same as it would be if this new casing pressure and all other conditions remained unchanged over a period of several days) This stabilizing of the pressure and production rate conditions may require holding the pressure atthe new value for a period of from 4 to 24 hours". When such conditions become stable, I note the casing head pressure and the production rate of the pump at the new pressure.

By properly selecting'the amount of each increment of increased pressure such that several steps occur before there is any noticeable decrease in production rate, I am assured against a too rapid depressing of thevliquid surface to the pump inlet. In most wells it will be observed that at one of the steps the production rate is markedly reduced and Il note such reduced production rate and the casing pressure which was required to produce the reduced production rate. I then continue the steps of increasing the cas-l ing pressure', permitting the conditions to become stable, and noting the change in the production rate of the pump under the new pressure conditions. At some' point between the last step of increased pressure without reduced production rate and the next step at which production rate liquid column has been sumcient to depress the level of the liquid down to the pump inlet. To determine accurately at what pressure the level of the column reached the pump inlet, I plot the information obtained in the series of steps upon a chart or graph paper as shown in Fig. 1, in which the ordinates represent production rates from the pump while the abscissae represent the corresponding pressures at the casing head.

By plotting this information it will be noted that during the initial steps the stable production rate did not change even though casing pressure was increased and all of the points plotted, lsuch as those indicated at A, B and C, lie upon a straight line. It will also be noted that the ploting of the information obtained by the further steps give decreased production rates for increased casing head pressure and usually such plotted points, represented by D, E and F, will lie in a straight line. Thus by extending a straight line through the points D, E and F until they intersect a prolongation of a straight line through points A, B and C, the intersection J will represent the casing head pressure which wasv required to depress the liquid level to the pump inlet. From an inspection of the chart shown in Fig. 1, it will be noted that there is a sharp break in the line representing the production rate from the pump and this break designates the arrival of the liquid level at the pump inlet. From such chart the pressure required to depress the liquid level to the pump inlet may be read in pounds per square inch of casing pressure.

While, as stated above, in most instances, when the casing pressure is sufficient to cause a change in the production rate, it will be found that there is a fixed ratio between the change in casing head pressure and any change in production rate which may result therefrom, it may be found in some wells that this ratio is notixed and that a line extending through points D, E and F will not be a truly straight line, By making a sufilcien't number of tests upon such well as to produce a relatively great number of test points on the chart shown in Fig. 1, the trend of the curve passing through all of such points may be determined and it may be extrapolated to locate other points beyond the plotted test points or to intersect the straight line A-B--C.

In order to utilize the information obtained by the tests and the plotting of the test points as hereinbefore described to accurately determine the pressure at the pump inlet, I calculate the effect in pressure at the pump inlet for each production rate by adding to the casing pressure for that rate the effect in pounds pressure of the column of gas above the liquid surface. For example, the location of the pump inlet is known from the records of the well and by measuring the gravity of the gas a computation of the effect in pounds pressure of the column of gas above the pump inlet may be made. In the example shown in Fig. l, if it be assumed that the casing head pressure at the point J were 100 pounds per square inch and the effect of the column of gas is calculated to be l pounds per square inch, the total pressure at the pump inlet is 115 pounds per square inch for the noraml rate of production of the pump which was assumed to be 200 barreis, per day. By plotting the pressures at the pump inlet, determined by this method, against the production rates for .each such pump inlet pressure, a graph similar to Fig. 1 may be produced and the break" in the line may be noted in the same manner as described for the plotting of the casing head pressures. Thus the operator of the particular well under test may use this pressure information in any of the well known manners to determine whether or not changes in location, stroke or speed of the present pumping equipment should be made, whether or not more adequate gas anchors are required or what other alterations should be made to effectively increase the production from that well.

The information obtained by the plotting of points D, E and F as shown in Fig. 1 may also be used to determine the maximum production capacity of the well, to permit the operator to decide how much alteration or change should be made to permit the maximum production from the well or to permit him to reach such percentage of the maximum capacity allowed in vthat particular field or for that particular well. This may be readily accomplishedby extending the line F-E-D-J upwardly until it intersects the axis of the ordinates representing zero pressure, such intersection being indicated by the point H on the chart shown in Fig. 1 and represents a 'maximum production capacity of 300 barrels per day, that is, represents the amount of oil which could be produced by the well if the pumping v equipment were operated in such manner as to tional instruments or equipment. As shown in` Fig. 2, the pressure in pounds per square inch at the pump inlet is plotted against the several reduced production rates found for the -test points D, E, F of Fig. 1 by adding to the casing head pressure for each of these points the effect in pounds per square inch of the weight of the column of gas above the pump inlet.

Such pressure and production rate points may be plotted as shown in Fig. 2 as points L-M-N corresponding to the test points D-E-F, respectively, while the point K is plotted for the point J of Fig. 1. Again assuming, as is the usual case. that the ratio between pump inlet pressure and production rate is fixed, the points L-M-N lie in a straight line so that by extending a line drawn through these points until` it intersects the axis of the ordinates representing zero lpressure at the pump inlet, the maximum capacity of the well, namely, 300 barrels per day, is determined, while extension of the line in the opposite direction until it intersects the axis of the abscissae representing zero production, the static pressure at the pump inlet is determined, such point being indicated at Q in Fig. 2. Using the same values of casing press-ure, gravity of gas, height of column of gas and production rate as hereinbefore stated, the static pressure at the pump inlet on the particular well under test is found to be 350 pounds per square` inch.

Now the ratio between the difference in pressure at the pump inlet and the corresponding difference in production rate under such pressure conditions represents the productivity index of the well and is represented by the slope of the line O-Q. Such productivity index may also be computed without the necessity of plotting the chart shown in Fig. 2 by mathematically comparing the diiference between the pressure at the pump inlet for any two rates of production with the difference in the corresponding rates of production. Thus if as indicated at the point L the production rate is 185.5 barrels per day and the pressure required to produce this production rate is 137 pounds per square inch. andthe production rate at the point N is found to be 90 barrels per day and the pressure 249 pounds per square inch, the difference in the two pressures is 112 pounds per square inch while the difference in the two production rates is 95.5 barrels per day. The productivity index would be 95.5/1l2=.8526 barrels per day per pound of difference in pressure at the pump inlet.

Knowing the productivity index, the maximum production capacity of the well may be computed by making use of either the test point L or the test point N. If point L is selected, the maximum production capacity with the pump located where it was for the test would be (137 lb. .8526)+185.5 BID=302 B/D.

The productivity index may also be used as a practical means for computing the approximate static pressure at the pump inlet; for example, again using the test point L, the static pressure is represented as allowing the casing pressure to build up above normal sumciently to permit the reaching of a l stabilized production rate lower than the normal rate', such single test may be plotted as hereinbefore described as pounds pressure at the pump inletcompared withl the reduced production rate upon a graph or chart as shown in Fig. 3, this point being indicated by the point S.

0n the same chart the static pressure at the pump inlet may be plotted on the aero production abscissae, this point being indicated at R. Assuming. as is the usual case, that the ratio between pressure at the pump inlet and the production rate is ilxed. it follows that a straight line drawn between the points Rand S and continued until it intersects the aero pressure ordinate, such intersection indicated at U will represent the maximum production rate of the well. Also an approximation of the pressure at the pump inlet under normal production can be determined by noting the point where line R--S-U intersects a line representing the nox"- mal rate of production, such point being indicated at T;

` While greater accuracy may be achieved by building up the casing pressure in steps and permitting the conditions to become stable at each step before taking the casing pressure and production rate readings, reasonably satisfactory resultsmay be obtained in some wells by allow ing the casing pressure to built up continuously over a relatively long time, but so slowly that, during the initial portions of the test the production rate of the pump remains at, or substantially at. normal and continuing to increase the casing pressure at this same slow rateY throughout the test. Such slow increase of casing pressure will produce production rates-which may be considered as stable" rates for the purposes of the plotting and computations stated herein. From time to time during the building up of the casing pressure, readings or recordings of the casing pressures may be made and readings or recordings of the 4production rates at these selected casing pressures may be made. Such readings may then be plotted as indicated in Fig. 1. vFor some time the production will remain normal even though pressure is increased and then the production rate will be found to drop steadily as the pressure increases so as to produce the test points corresponding to A, B, C, D, E and F, and the pump inlet pressure, well potential and productivity index may be calculated as hereinbefore described. v

In making the pressure and production rate measurements, it may be found that it is necessary to either slow down the pump or to reduce the length of stroke after the rate of production drops below normal, since in many wells it will be found that the pumps become gas-locked and cause the rate of production and the fluid level to iluctuate widely, thus giving erroneous results. By reducing either the pump speed or the length of stroke or both and watching the fluid level as by means of fluid level ilnding apparatus, as is described in my copending applications Serial Nos. 162,699 and 164,534, and maintaining a proper adjustment between the speed and stroke of the pump and the casing pressure, the fluid level may be held at the pump inlet and the test measurements as outlined herein will be accurate.

I claim:

l. 'I'he method of determining the pressure required in the casing of an oil well to depress the liquid level therein to the level of the inlet ofa'pump immersed in said liquid which cornprises changing the casing pressure, in steps,

allowing the pump to operate at each step until the production characteristics of the well become stable, and noting the stable casing pressure at which the rate of production by the pump undergoes a distinct change, which pressure will be the pressure required to depress the liquid level to the level of the inletof the pump.

2. The method of determining when the liquid level in a well reaches the inlet of a mechanical pump located therein, which comprises altering the casing pressure in a series of steps, to depress the liquid level toward the pump inlet, operating the pump during each of said steps, noting the rate of production .by the pump when the production rate and pressure become stable at each of said steps, and noting the stable casing pressure at which the rate of production by the pump undergoes the first distinct change, which change occurs when the level of the liquid is at the pump inlet.

3. The method of determining the casing pressure necessary to depress the liquid level in a well to the. entrance of a pump employed in the well, which comprises incrementally changing the casing pressure, allowing the rate of production to become stable for each value of casing pressure, and plotting the casing pressure against the corresponding production rate as coordinates. and prolonging a straight line drawn through the plotted points representing less than normal rate of production and through an ordinate corresponding to the normal rate of production, the intersection of the line with the ordinate representing normal production rate dening the that obtainable with a lesser pressure in the casminimum casingpressure necessary to maintain the liquid level at the pump entrance.

4. The method o f determining the casing pressure necessary to depress the liquid level in a well to the entrance of a pump employed in the well, which comprises changing the casing pressure in steps, operating the pump at each oi said steps until the production rate and casing pressure become stable, plotting the casing pressure so used and the resultant rate of production as co-ordinates, extending a line through all oi the test points at which the rates of production are substantially normal and another line throughr the test points at which production rates are substantially abnormal, the intersection of said lines denoting the casing pressure necessary to maintain the liquid level at the pump inlet at the normal rate of production.

5. The method of determining the pressure of the liquid at the inlet of a pump operating in a well, which comprises gradually changing the casing pressure over a sulcient range to effect a reduction in the rate of production by the pump, allowing the pump to operate to stabilize the production rate for any selected value of casing pressure, adding to the stable casing pressures for each rate the eiect in pressure of the column of gas located above the pump inlet, and plotting the sum of the casing pressure and said eiect in pressure against the rates of production as co-ordinates, noting the point at which a line drawn through the test points makes a definite change in its course, said point then representing vthe normal pressure of the liquid at they inlet of the pump under normal operating conditions of casing pressure and production rate.

6. 'I'he method of determining the operating pressure of liquid at the inlet of a pump employed in a well, which comprises gradually changing the casing pressure, allowing the pump to operate until the productionlrate and casing pressure are stable for each pressure value, adding to the stable casing pressures for each rate the eiect in pressure of the column of gas located above the pump inlet, and plotting the sum oi the casing pressure and said effect in pressure against the rate of production for each casing pressure as co-ordinates, extending a. line through the test points at which substantially no change in production rate occurs, extending another line through the test points showing abnormal production rates, the intersection of said lines representing the normal pressure of the liquid at the inlet of the pump under normal operating conditions of casing pressure and production rate. 7. The method of determining the operating pressure at the inlet of a pump employed in an oil Well for any rate of production from said pump, which comprises maintaining a constant pressure in the casing sufficient to cause a substantial reduction in the rate of production below that obtainable with a lesser pressure in the casing, measuring said casing pressure at the casing head, and adding to the casing pressure so employed the effect in pressure of the column of gas above the pump inlet, the resultant sum representing the fluid pressure at the pump for the test rate of production.

8. The method of determining the operating pressure at the inlet of a pump employed in an oil well for any rate of production from said pump, which comprises maintaining a constant pressure in the casing suicient to cause a substantial reduction in the rate of production below ing, reducing the pump speed or length of stroke or both to minimize fluctuations in the rate oi' production and in the fluid level, adjusting the pump speed, pump stroke or casing pressure, or all oi them, to maintain the iluid level at the pump inlet, measuring said casing pressure at the casing head, and adding to the casing pressurev so adjusted the effect in pressure of the column of gas above the pump inlet, the resulting sum representing the iiuid pressure existing at the pump inlet for the test rate of production. 9. The method of determining th'e productivity index of a well employing a pump,which comprises maintaining a constant pressure in the casing sumcient to cause a substantial reduction in the rate of production below normal production, maintaining another constant pressure in the casing suilicie'nt to cause afdiiierent substantial reduction in the rate of production below normal, adding to each of the casing pressures so employed the effect in pressure of the column of gas above the pump inlet, to determine the pressure at the pump inlet for the corresponding rates of production, the ratio of the difference between the two rates oi production to the diierence between the two corresponding pump inlet pressures representing the productivity index.

10. 'I'he method oi determining the productivity index of a well employing a pump, which comprises maintaining a constant pressure in the casing suillcient to cause a substantial reduction in the rate of production .below normal production, maintaining another constant pressure in the casing suilicient to cause a diierent substantial reduction in the rate of production below normal, adding to ,each of the casing pressures so employed the effect in pressure of the column of gas above the pump inlet, to determine the pressure at the pump inlet for the corresponding rates of production, plotting the two rates of production and the two corresponding pump inlet pressures as co-ordinates, the'slope of aline passing through said plotted points representing the A productivity index.

11. 'Ihe method of determining the productivity index of a well employing a pump, which comprises maintaining a constant pressure in the casing suilicient to cause a substantial reduction in the rate of production below normal production, adding to said casing pressure the effect in pressure of the column of gas above the pump inlet to determine the pressure at the pump inlet for the reduced rate of production, plotting this pump inlet pressure against the corresponding production rate and the static pressure at the pump inlet obtained by any method against zero production rate as like co-ordinates, and determining the slope of a straight line extending through said plotted points, said slope representing the productivity index of the well under test.

12. The method of determining the production capacity of a well having a pump therein, which comprises changing the casing pressure in a series of steps, operating the pump at each of the steps until the production rate and casing pressure become stable, plotting the production rate and casing pressure at each of said steps as co-ordinates, extending a line through each of the plotted test points representing reduced production rates, and extending said line to intersect an ordinate representing zero casing pressure, said intersection then representing the maximum capacity of the well for the test location of the pump.

the steps until the production rate and casing pressure become stable, adding tc each casing` pressurethe effect in pressure of the column oi" gas above the pump inlet to determine the pressure at the pump inlet for each corresponding rate oi' production, plotting the pump inlet pressures and the'corresponding rates of production at each oi' ,said steps as coordinates, extending a line through each oi.' the plotted test points representing reduced production rates, and extending said line to intersect an ordinate representing zero pump inlet pressure, said intersection then representing the maximum capacity ot the well for the test location of the pump.

14. The method o! determining the production capacity of a well having a pump therein. which comprises changing the casing ypressure in a series of steps, operating .the pump at each oi the steps. until the production rate and casing pressure become stable, reducing the pump speed or length of stroke or both when the casing pressure used is such. that the rate of production is less than normal, adjusting the pump speed or stroke or casing pressure or all of them to maintain the iluid level at the pump, adding to each casing pressure the eect in pressure of the column of gas above the pump inlet to determine the pressure at the pump inlet for each corresponding rate oi production, plotting the pump inlet pressures and the corresponding rates of production at each of said steps as co-ordinates, extending a line through each ot the plotted test points representing reduced production rates, and extending said line to intersect an ordinate representing zero pump inlet pressure, said intersection then representing the maximum capacity of the well lor the test location of the pump.

15. The method of determining the maximum production capacity of a well employing a pump,

which comprises maintaining a constant pressure in the casing sulcient to cause a substantial reduction in the rate of production below normal production, maintaining another constant pressure in the casing suiilcient to cause a different substantial reduction in the rate of production below normal, adding to each of the casing presy sures so employed the eiTect in pressure of the column of gas above the pump inlet, to determine the pressure at the pump inlet for the corresponding rates of production, plotting the pump inlet pressures and corresponding rates of production as co-ordinates, drawing a line through the two plotted test points, and extendof production as co-ordinates, drawing a line through the two plotted test points, and extending said line to intersect the abscissae representing zero production rate, said intersection representing the static pressure at the pump inlet.

17. The method o! determining thegproduction capacity of a well having a pump therein. which comprises operating the 'pump successively under at least two diiierent conditions of gas pressure existing at a predetermined point in the well casing, both gas pressures being sumcient to hold the liquid surface at the level ot the pump inlet with a stable production rate for each oi said gas pressures, plotting the rates o! production against the gas pressures required to achieve them, extending a line through points so plotted and to its intersection with an ordinate representing zero gas pressure on said predetermined point in the casing, said intersection then representing the maximum production capacity oi the well for the test location oi the pump.

18. The method o! determining tor any rate o! production the pressure at the inlet oi a pump located at or near the bottom of the production tubing in a well, which comprises operating the pump at a given rate oi speed with little or no pressure in the casing and measuring the rate of production; continuing the operation oi' the pump at the same speed. restricting and regulating the ilow of gas from the well casing to maintain a constant pressure therein sumcient to cause a substantial reduction in lthe stable production rate by the pump below the rate obtained with little or no pressure in the casing, measuring said casing pressure, gravity oi' the gas in the casing, gauging the rate of production, and computing the pressure on the iluid surface, the pressure on the iluid surface under said condition of reduced production rate being the pressure at the inlet o! vthe pump forsaid reduced production rate.

19. The method of determining for any rate of production the minimum gas pressure necessary in the annular space between the casing and tubing to depress the stable liquid level in a well to the entrance of a pump operating at or near the bottom of the production tubing therein,

ing said line to intersect the ordinate representing zero pump inlet pressure, said intersection representing the maximum production capacity of the well for the test location of the pump.

16. The method of determining the static pressure at the inlet of a pump employed in an oil which comprises changing the gas pressure in the casing in steps, continuing the operation of the pump at a constant speed for each of said steps until the production rate and pressure in the casing become stable, varying the pressure in the casing over a suilicient range to cause a change in the stable rate of production from the pump while it is operating at a constant speed, employing two or more different values of pressure in the casing which cause a change in the stable production rate and comparing the relation between the changes in gas pressure in the casing which affect the production rate and the corresponding changes in the production rate by the pump, to determine the pressure in the casing necessary to depress the liquid level to the entrance of the pump for any rate ot production.

20. The method of determining for any rate of production the pressure at the inlet o! a pump located at the bottom of the production tubing in a well, which comprises operating the pump at a given rate of speed with little or no pressure in the casing and measuring the rate oi production; continuing the operation of the pump at the same speed, restricting and regulating the flow of gas from the well casing to maintain a constant pressure therein suillcient to cause a substantial reduction'in the stable production rate by the pump below the rate obtained with little or no pressure in the casing, measuring said casing pressure, gravity of the gas in the casing, gauging the rate of production, and computing the pressure on the iiuid surface; the pressure on the iiuid surface under said condition' of reduced stable production rate being the pressure at the inlet of the pump for said reduced production rate; restricting and regulating the flow of gas from the well casing to maintain a different pressure therein sufficient to cause another and different rate of stable production which is less than the production rate obtained with little or no pressure in the casing, measuring said casing pressure, gravity of gas in the casing, gauging the rate of production, and computing thev pressure on the fluid surface; the pressure on the Iiuid surface under said second condition of reduced production rate being the pressure at the inlet of the pump for said second reduced rate; from the relation between the change in the pressure on the iuid surface and the corresponding difference in the two reduced rates of production, the pressure at the pump inlet for any rate of production may be determined.

21. The method of determining for any rate of production the pressure at the inlet of a pump located at the bottom oi' the production tubing in an oil well, which comprises restricting and regulating the flow of gas from the well casing to maintain a constant pressure in the casing sucient to cause a substantial reduction in the stable rate of production by the pump -below the constant stable rate obtained'with one or more lower values of stable pressure in the casing; repeating the test with a diierent value of casing pressure which is sufficient to produce another rate of production which is less than the constant rate of production obtained at lower values of casing pressure; the pressures on the uid surface for the reduced rates of production being the pressures at the pump inlet for these rates and from the relation between the change in -pressure at the pump inlet and the corresponding change in the production rate, the pressure at the pump inlet for any rate of production may be determined.

22. The method of determining the production capacity of a well having production tubing and a pump therein, which comprises restricting and regulating the flow of gas from the casing in steps to obtain several different values of constant gas pressure in the casing, employing sufcient casing gas pressure for two or more of the steps to cause a substantial reduction in the production rate by the pump, operating the pump at the same speed for each step until the casing gas pressure and production rate become stable, measuring the casing gas pressure and rate of production for each step, from the relation between the changes in casing gas pressures which are suicient to cause a change in the production rate and the corresponding changes in the production rate by the pump, determining the production rate for any value of casing pressure, including zero pressure, which will be the maximum production rate obtainable from the well for the test location of the pump.

23. The method of varying and controlling the rate of production from a well equipped with a pump located at or near the bottom of the production tubing while maintaining the pump speed constant, which comprises varying the gas pressure on the surface of the liquid in the annular space between the casing and the tubing in stabilized steps until a stable pressure is reached at which the stable rate of production by the pump begins to change, while the pump operation is continued at a constant speed, continuing to change the gas pressure in the fluid surface in the same direction and in stabilized steps until the desired stable production rate is obtained.

24. The method of determining the casing pressure necessary to depress the liquid level in a well to the entrance of a pump operating therein which comprises changing the casing pressure over a range suiiicient to effect a' production at less than normal rate, and in so doing changing the casing pressure at a sufliciently slow rate such that the equilibrium between the upward pressure exerted by the inflow of liquid from the oil producing formation and the downward pressure exerted by the casing pressure and weight of the column of liquid above the formation is not disturbed, noting the casing pressures and corresponding production rates at a plurality Aof intervals during the test. plotting the selected casing pressure and the corresponding production rates as coordinates, and extending the line through the points so plotted which represent less than normal rate of production and through an ordinate corresponding to the normal rate of production, the intersection of said line and the normal production rate ordinate dening the minimum casing pressure necessary to maintain the liquid level at the pump entrance.

25. The method of determining the casing pressure necessary to depress the liquid level in a well to the entrance of a pump operating therein which comprises changing the casing pressure over a range suiicient to eiect a production at less than normal rate, and in yso doing changing the casing pressure at a sufficiently slow rate such that the equilibrium between the upward pressure exerted by the inflow of liquid from the oil producing formation and the down- Ward pressure exerted by the casing pressure and weight of the column of liquid. above the formation is not disturbed, noting the casing pressures and corresponding production rates over a range which includes at least two production rates less than normal to determine the relation between changes of casing pressure `and the correspondf ing changes in production rate, and comparing the observed relation to the normal production rate to determine the maximum value of casing pressure at which the production rate would -be normal which is the minimum casing pressure required to depress the liquid level at the pump entrance.

26. The method of determining the maximum production capacity of a well having a pump therein which comprises changing the casing pressure over a range suicient to eiect a production at less than normal rate, and in so doing changing the casing pressure at a suiiciently slow rate such that the equilibrium between the upward pressure exerted by the inow of liquid from the oil producing formation and the downward -pressure exerted by the casing pressure and weight'oi the column of liquid above the formation is not disturbed, noting the casing pressures and corresponding production rates over a value will be substantially the maximum production capacity of the well.

27. The method oi determining the production capacity oi' a well having production tubing and a pump therein, which comprises restricting and regulating the flow of gas from the wall casing in steps to obtain several diierent values of stable gas pressures in the casing, employing pressures for at least two of the steps each of which will cause a change in the rate of production from the well without changing the pump speed or stroke, allowing the well to produce'until the gas pressure and production rate become stablevfor each step, measuring the gas pressure and rate of production for each step and from the relation between the changes in gas pressure which cause a change in the production rate and the corresponding changes in the production rate determining the production rate for any value of casing pressure, including zero pressure, which will be the maximum production rate obtainable from the well for the test location of the pump.

CRANFORD l. WALKER.

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