r“TABLE 1. Liquid Gradient.

APIGravi tyDegrees

8 07 57 06 56 05 55 04 8464 44 24 0383 63 43 23 02 82 62 42 22 01 8151 2

FluidGradient

Lb/Sq In/Ft0.2900.2970.3040.3120.3200.3290.3380.3420.3450.3490.3530.3580.3620.3660.3700.3750.3800.3840.3890.3940.3990.4050.4100.4190.427

SpecificGravi ty

by James N. McCoy, President, Echometer Co., Wichita Falls, Tex.

0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0WATER PERCENTAGE

0.6

DIRECTIONS:1. CONNECT OIL API GRAVITY AND WATER SPECIFIC GRAVITY

WITH A STRAIGHT LINE.2. READ LIQUID GRADIENT AS A FUNCTION OF WATER PERCENTAGE.

fresh water

saltwaterr a n g e

0.433

0.4770.5000.520

,

1 .ooo

1.1001.1541.200

Fig. 1. Liquid gradient. 1 psi is approximately 2 ft of water1 psi is approximately 3 ft of oil

nformation about producing and static bottom-hole pressures in gas lift wells is useful in designingand operating gas lift installations and measuringover all efficiency.

The static bottomhole pressure can be accuratelymeasured in the normal gas lift well using an acousticliquid level instrument. The normal gas lift well isassumed to be a continuous flow well in which a packeris placed immediately above the formation at the bot-tom of the tubing. The inside of the tubing is open fromthe bottom to the top of the well. Gas lift valves placedin or on the tubing are either casing pressure operatedor tubing fluid pressure operated.

Fig. 2 gives the pressure exerted by the gas column.If the packer is set a considerable distance above the

formation, the total volume of liquid above the forma-tion must be calculated. The percentages of oil andwater produced by the well times the volume of liquidabove the formation indicates the volume of water andthe volume of oil. Height of the water column is deter-mined by dividing the water volume by the casingvolume per ft. Then the heights of water column and oilcolumn are known. The liquid column gradients aregiven in Table 1.

Bottomhole PressureThe static bottomhole pressure is easy to measure Gas lift valves in a casing pressure actuated system

accurately in a gas lift well of this type. Simply close in operate as follows: The gas lift valve opens when it isthe tubing and gas injection line at the surface and exposed to a casing annulus pressure in excess of aallow the well to stabilize. Then determine the liquid pre-set value. The valves are arranged in the tubinglevel in the tubing using the acoustic liquid level in- string so that the highest pressure operating valve isstrument. The surface tubing pressure reading should at the top, and the operating pressure of each valvebe made at time of test. The static bottomhole pres- decreases with depth. Gas injection in the casing an-sure is the summation of the surface tubing pressure nulus lightens the load in the tubing above ‘the operat-plus the gas column pressure plus the pressure ex- ing valve. All valves below this point are open anderted by the column of liquid above the formation. The permit liquid or gas to pass into the tubing if the tubingliquid above the formation consists of water and oil in pressure is less than the casing annulus pressure. Thethe same ratio that is produced by the well. In a well liquid level in the casing annulus will be depressedwhich has tubing extending to the formation, the aver- until the pressure in the casing annulus at the bottomage gradient in the column can be determined by using valve is equal to the pressure in the tubing at theFig. 1. The hydrostatic pressure of the liquid column is bottom valve, or until the bottom gas lift valve isdetermined by multiplying the height of the liquid exposed to gas. Check valves prohibit back flow fromcolumn by the average gradient of the liquid column. the tubing into the annulus. As the liquid bleeds from

Reprinted from PETROLEUM ENGINEER, August 1976

Bottomhole Pressures

GAS COLUMN PRESSURE, PSI

OF STATIC BOTTOMHOLEPRESSURE FROM SURFACE DATAAVG. TEMP. GRAD. 1.6 DEG F = 100 FT + 74 DEG F

Fig. 2. Gas column pressure.

0.6 0.7 0.8 0.9 1.0 1.0SPECIFIC GRAVITY - (AIR = 1)

EXAMPLE:

THE GAS COLUMN PRESSURE OFA0.727

the annulus into the tubing and exposes a gas lift valveto gas, the gas will pass into the tubing if the tubingpressure is less than the casing annulus pressure. Agas lift valve above this point will close if the pressurein the casing annulus is reduced below its pre-setoperating value by the loss of gas to the lower valve.The bottom valve is always open unless it is the injec-tion valve and sufficient gas is not available to maintaina pressure in excess of its operating pressure, then itwill throttle or intermit. If the bottom valve is ex-posed, gas injection through it has occurred, and thebottom valve may still be the gas injection valve.

A tubing operated gas lift valve system operates asfollows: The gas lift valve will open if the pressure inthe tubing exceeds a pre-set value. The lowest operat-ing pressure valve is set near the top of the tubing, andthe operating pressure settings increase with depth.When gas injection into the casing annulus starts, gasis injected into the valve nearest the top which hassufficient fluid pressure in the tubing to open thevalve. The bottom valves will be open and permitliquid to pass from the casing annulus into the tubing.

Gas valves will become exposed as the liquid leveldrops, and gas injection will begin if the casing annuluspressure exceeds the tubing pressure and sufficienttubing pressure exists to open the gas lift valve. Uppergas lift valves usually will close since injection at thelower depth decreases the tubing pressures and closesthe upper valve. In a normal installation, if liquidexists above the bottom valve, the bottom valve will beopen and the pressure in the tubing will be equal to thepressure in the casing annulus. If the liquid is belowthe bottom gas lift valve, gas injection has and maystill exist at the bottom valve. If the valve is throttling,the pressure drop across the valve is not known. Whena valve is throttling, the pressure in the tubing alwaysis less than in the casing annulus.

Note the following facts about gas lift systems whichhave liquid above the lowest valve.l When the system is first put in operation and the

well is permitted to stabilize, the pressure in thecasing annulus opposite the bottom valve is thesame as the pressure in the tubing because thebottom valve is open.

Bottomhole Pressures

Casing

2 TUBING

GAS LIFTVALVE

LIQUID

PACKER

SCREENEDORIFICE\

E F G

Fig. 3. Types of gas injection systems.

l The casing annulus pressure opposite the lowestvalve in a well which has been in operation for along period of time indicates the lowest pressurewhich has existed in the tubing since last equaliza-tion of tubing and casing annulus pressures.

l If the tubing pressure opposite the lowest gas liftvalve is suspected to have been lower at some timesince the last tubing and casing annulus equaliza-tion, liquid could be added to the casing annulusuntil the liquid level ceases to rise. Then the cur-rent pressure in the tubing opposite the bottomgas lift valve would be the same as the casingannulus pressure.

l The liquid level in the casing annulus could be andprobably is several gas lift valves lower than thepoint of gas injection because of the difference inthe gradient of the gas free liquid in the casingannulus above the bottom valve and the gradient ofthe gaseous fluid column which exists in the tubingbetween the gas injection valve and the bottomvalve. If the pressures in the tubing and the casingannulus at the bottom valve are equal, then thepressure drop in the tubing from the bottom valveto the gas injection valve plus the pressure dropacross the gas injection valve must be equal to theliquid column pressure in the casing annulus abovethe bottom valve plus the gas column pressure

between the casing annulus liquid level and the gasinjection valve. Since the casing annulus liquid isgas bubble free, the height of the casing annulusliquid will be small compared to the distance to thegas injection valve in a gaseous well. The followingequation offers approximate numbers.

D x GF + PGLV = HLL x GLL + ∆PGCJD = distance between bottom gas lift

valve and gas injection valve,n.

IL

G F = gradient of the fluid in thetubing between the bottom valveand the gas injection valve,psi/ft

PG L V = pressure drop across gasinjection valve, psi

HL L = height of the liquid level abovethe bottom gas lift valve. I

G L L = gradient of the liquid in thecasing annulus above the bottomvalve, psi ft.

∆P = the difference in the gas columnG C -pressure at the liquid level andthe gas injection valve, psi(can be found from Fig. 2).

Further, note the following facts about gas lift systemsin which the bottom valve is exposed to gas:

0.8

FLOW RATE, BPD

200 400 6002000

1800

1600

800

600

10 20 30 40 50 60 70 80 90 100PRODUCING RATE, AS A

PERCENTAGE OF MAXIMUM

Fig. 4. Flow rate and producing rate efficiencyas a function of SBHP and PBHP.

• The bottom valve has been and may still be the gasinjection valve. To verify that the bottom valve isstill the gas injection valve, add liquid to the casingannulus. If the bottom valve again becomes ex-posed, the casing annulus pressure exceeds thetubing pressure and gas injection will occur.

• The pressure in the tubing at the bottom valve isthe pressure in the casing annulus at the bottomvalve less the pressure drop across the valve.

Fig. 3 shows 7 different types of gas lift systems.Reference to the above notes will help determine if theproducing bottomhole pressure calculated is the cur-rent producing bottomhole pressure or, possibly, theminimum producing bottomhole pressure since lasttubing and casing annulus equalization.

In case A (Fig. 3), the bottomhole pressure is thesum of the casing pressure plus the gas column pres-sure, less the pressure drop across the valve plus thepressure due to the flowing fluid column in the tubingbetween the bottom valve and the formation. In caseB, the bottomhole pressure is the sum of the casingpressure, plus the gas column pressure, plus the liquidcolumn pressure above the bottom valve plus the pres-sure due to the flowing fluid column in the tubingbetween the bottom valve and the formation.

In case C, the bottomhole pressure is the pressure inthe casing annulus at the bottom valve less the pres-sure drop across the valve. The pressure drop acrossthe valve can be estimated by the gas lift valve man-ufacturer. The pressure in the casing annulus at thebottom valve is the surface casing pressure plus thegas column pressure. In case D, the producing bot-tomhole pressure is the casing pressure plus the gascolumn pressure plus the liquid column pressure abovethe bottom valve.

In case E, the producing bottomhole pressure is thecasing pressure plus the gas column pressure minusthe pressure drop across the bottom valve plus thefluid column pressure between the bottom valve andthe formation. In case F, the producing bottomholepressure is the casing pressure plus the gas columnpressure plus the liquid column pressure above thebottom valve plus the fluid column pressure betweenthe bottom valve and the formation.

The pressure thus obtained is the minimum produc-ing bottomhole pressure that has occurred since thelast time liquid was permitted to enter the casing an-nulus. If the producing bottomhole pressure is greaterthan it was at some time in the past, an additionaltest should be made. Liquid should be injected to fillthe casing annulus until the height of l iquid hasstabilized. Then, the pressure calculated will repre-sent the current producing bottomhole pressure.

Completion RecommendationAccurate producing bottomhole pressures can be

obtained by completing a gas injection well as shown inFig. 3 by types D or G. The producing bottomholepressure is the casing pressure plus the gas columnpressure plus the liquid column pressure above thebottom valve or screened orifice.

Application of PressuresThe static and producing bottomhole pressures offer

valuable information for selecting a gas lift valve in-stallation. The static bottomhole pressure is beneficialfor selecting top valve setting. The flow characteris-tics given by the static and producing bottomholepressures are useful in selecting the depths for addi-tional valves, determining gas injection rates, and op-timizing the gas injection system.

The gas lift efficiency can be determined readily byutilizing the following technique in gas injection sys-tems shown by types C, D, and G in Fig. 3. Add liquidto the casing annulus until the liquid height stabilizes(which indicates the static bottomhole pressure). Startgas injection at a relatively low rate and measure thegas injection rate, oil production rate, water produc-tion rate, and the producing bottomhole pressure. In-crease the gas injection rate and continue measure-ments until additional gas injection no longer producesa reduction in the flowing bottomhole pressure and,consequently, no additional liquid.

The flow rate at any producing bottomhole pressurecan be determined by using Fig. 4. The data needed toconstruct the curve are the static bottomhole pres-sure, the producing bottomhole pressure, and the flowrate at that producing bottomhole pressure. The re-sulting chart shows maximum well,flow rate and theflow rates at different producing bottomhole pres-sures. The chart assumes the reservoir pressure isbelow bubble point. n