Gas Lift

Gas Lift

• Methods

• Limitations

• Gas lift is the incorporation of externally supplied gas, injected into the flow stream at specific depths, to decrease the produced fluid flowing density and create a higher drawdown on the formation, resulting in higher production.

8/25/2015 1

George E. King Engineering GEKEngineering.com

Continuous Flow Gas Lift: Applications

• Producing wells that cannot flow naturally.

• Increasing production in flowing wells.

• Lift wells that flow solids.

• To initiate flow in wells with too much liquid column or saturation to

start flowing.

• Unloading gas wells and deliquification.

• To assist backflow of injectors and disposal wells.

• Produce water source wells.

8/25/2015 2 George E. King Engineering

GEKEngineering.com

Gas Lift

• Advantages – No restrictions in tubing

– Can use subsurface safety valves with only minor complications.

– Will lift sand laden fluids

– Flexible and changeable conditions, can be optimized

– Can be used in deviated wells (looses efficiency with increasing deviations)

– Works with reservoir gas, any GOR

– Low Capex and Opex.

– Requires minimum surface space

– Does not need electrical power.

• Disadvantages – Efficiency is lowest of common lift

methods. – Low pressure wells will produce

better with other lift methods. – Loses efficiency as deviation goes

beyond about 15 to 20o – requires more gas.

– Low gas volume may be a drawback, requiring more gas volume.

– Must have a high pressure gas source (high enough for lift)

– Will not work well with low API oils (gas expansion cools the fluid)


GEKEngineering.com

Gas Lift Basics

• Continuous gas lift simply supplements the natural flow process by adding additional gas into the produced fluid to reduce the hydrostatic head component and hence, the back pressure on the formation.


GEKEngineering.com

Gas Lift Basics

• Injection gas is supplied in a closed loop system, in which it is collected from the separators, then compressed, dried (if necessary), and supplied to the well.

• The lift gas is normally injected down the annulus and into the tubing through a gas lift valve (GLV), which contains a non-return check valve.

• Advantageous to locate the lift point as low as possible in the well to lighten as much head as possible.

• Generally only flows gas from one valve at a time.


GEKEngineering.com

Gas Lift Limits

• The gas supply pressure.

• The flowing pressure in the tubing at the valve – ensure adequate pressure and stable supply.

• The depth of the deepest gas lift mandrel.

• The differential across the valves required to keep the upper valves closed

• Well deviation – fluid segregation is a severe limit


GEKEngineering.com

Gas Lift Design

• The available kick-off pressure gradient is first plotted on the graph. Kick-off pressure is usually higher than normal GL operating system pressure

• Kill fluid gradient is plotted starting at the flowing tubing head pressure of the production system.

• The intersection of the kill gradient with the kick-off pressure gradient determines the location of the first GLV.


GEKEngineering.com

Artificial Lift Gas Lift Valve Spacing Example

Pressure (psi)

Depth

(ft

)

-8000

-7000

-6000

-5000

-4000

-3000

-2000

-1000

0

0 500 1000 1500 2000

Valve 1

Valve 2

Valve 3

Valve 4

Valve 5

Flowing Gradient

Unloading Gradient


GEKEngineering.com

Gas Lift Operation

• Gas passes through the first GLV and lightens the fluid column in the tubing.

• As tubing fluid gradient changes, supply gas in the casing moves down to second GLV, unloading fluid from casing & lightening the flowing fluid above the second valve.

• The process continues until the fluids in the casing annulus have been displaced, and the gas is passing through the bottom valve.


GEKEngineering.com

A simple gas lift system – the gas flows

from the annulus through a metering valve

in a mandrel and into the tubing, reducing

the density of the flowing fluid.

The pressurized gas increases in volume as

it rises, increasing the velocity along the

flow path and “dragging” the liquids upward

by the hindered settling effect and by

viscous coupling and turbulence (increased

mixing).

Gas Lift Elements


GEKEngineering.com

Gas Lift Valve Schematic

The gas lift valve transfers gas from the annulus gas lift supply to the

tubing through a orifice that is controlled by a ball and seat.


GEKEngineering.com

GL Well Unloading Sequence


GEKEngineering.com

G. L. Well Unloading Sequence Part 2


GEKEngineering.com

Gas Lift – final comments

• Production rate from a gas lifted well is a function of: – reservoir pressure,

– PI,

– water cut

– gas injection rate

– valve placement – this is frequently a limit because an insufficient number of G.L. mandrels were used when tubing was run.


GEKEngineering.com

Lift – when needed?

• When pressures decline.

• When tubing large/gas rate drops (wells loading).

• When water hits.

• To enhance production.


GEKEngineering.com

There is

no single

“best” lift

system.

Gas lift is

widely

applied,

especially

offshore,

and is

very

versatile.

Lift system comparisons


GEKEngineering.com

Artificial Lift

• Need for Lift? The height of the liquid column holds a back pressure on the well. To improve production, remove or lighten the head.

q

P

GOR = 400 scf/bbl

GOR = 400 + 600 (gas lift)

rod pump - limited rate,

and problems with gas

ESP - minor backpressure,

problems with gas, reliability?

jet pump - still has backpressure,

requires power fluid path.

gas lift 0.15 to 0.25 psi/ft, lowers

backpressure, but doesn’t remove it.

natural flow = 0.2 to 0.3 psi/ft


GEKEngineering.com

Gas Lift

• Advantages

– Mandrels 5 to 7 ft long - can go around corners

– Lift large amounts of solids

– Lift very large amounts of liquids

Pennwell AL Charts, 1986 8/25/2015 18 George E. King Engineering

GEKEngineering.com

Gas Lift

• Disadvantages – limited by lowest hydrostatic column you can

generate with gas mix, usually 0.15 to 0.25 psi/ft, cannot draw it down all the way.

– Stratification of gas to high side - solids dropped, gas slugs in highly deviated wells and in any horizontal flow. System less efficient as deviation goes beyond 15 to 20o deviation from vertical.

– Takes a lot of gas near depletion to lift gas depleted reservoir fluid.


GEKEngineering.com

GL Solutions

• keep GL valves near vertical

• use tapered string

• recalculate gas lift requirements on a regular basis

• A gas lift engineer is needed on a full time basis to optimize big fields.


GEKEngineering.com

Gas Lift Valve Testing

• Have to know how valve performs to be able to model the lift system.

• Determine if the valve is performing as expected

• how much actual flow comes through the valve

• How to use the test data


GEKEngineering.com

Types

• Static

– Load rate

– travel

– opening and closing

• Dynamic

– flow coefficients

– live valve flow test


GEKEngineering.com

Factors affecting GL valve performance

• 1. Max effective stem travel (less than 1/16” to 0.18”) - must balance port size with travel to get performance. Some valves might be throttling while others may be wide open.

• 2. The load rate of a valve is a measure of the valves’ resistance to opening. The higher the load rate, the more pressure pressure required to move the stem off the seat.

– Load rate of a 1” IPO valve is about 1000 psi/inch


GEKEngineering.com


• 3. Flow coefficients are a measure of the valve’s resistance to flow. Valves with high flow coefficients curves have high flow rates. Flow coefficients are geometry dependent and cannot be changed.

– The shape of the flow path through the valve determines the flow coefficient.


GEKEngineering.com


• 4. Silicon fluid is used in the bellow (dome) to dampen vibrations. Silicon fluid expands with temperature and causes the dome pressure to increase more than the normal temperature correlation.

– Problem is than if one valve in the string has a different ratio of fluid to dome size - it will behave differently - may not close.

– Can be bad problem on refurbished valves


GEKEngineering.com

How can you use valve performance data?

• Use VPC data to:

– calculate gas flow rates through the valves - get a performance curve for the pressure conditions in force.

– perform troubleshooting

– use in computer prog to dynamically simulate a gas lift well.


GEKEngineering.com

Field (BP Forties) Optimization in the North Sea

• Gas, natural lift, artificial lift, commitment

• Well-by-well review – use of analysis software

– model individual wells

– monitor performance

– scrutinize individual wells downhole/ surface

– consider whole production system

– troubleshoot problematic wells

– evaluate recommended actions

8/25/2015 27

George E. King Engineering GEKEngineering.com

Continued…..

• Full Field Optimization

– model network

– model reservoir

– set up and model management system

– train operator engineers

– train personnel

– manage field optimization

– measure performance


GEKEngineering.com

Unlocking Value – Optimization of the lift system!

• Optimization leads to peaks in production - problem is making constant changes that keeps the production high.

• Peaks show potential value from optimization – rate will fall off fast if gas lift system is left alone.


GEKEngineering.com

Considerations

• Sand production

• drawdown limitations

• produced water handling

• water coning/preferential production

• scale

• gas capacity and availability

• plant considerations

• well start up philosophies


GEKEngineering.com

Optimization Procedure

• Focused and regular scrutiny of wells

• reconcile differences and take action

• input data into full field model

• compare actual with predicted

• run full field optimized model

• construct working document

• longer term issues go to management mtg

• report weekly figures

• monitor against measures

• support artificial lift studies


GEKEngineering.com

When do you need lift? • Do an Inflow Performance Relationship (IPR)

analysis

• Do a nodal analysis on the effect of back pressure.

• Look for slugging, surging effects in the facilities or on the instrumentation of the well.

• Will adding lift make an economic impact on production?

• What type of lift will create the best value? What lift system will the will the well and field conditions support?


GEKEngineering.com

Effect o f G as R ate on B H P

0

500

1000

1500

2000

2500

3000

0 500 1000 1500

Nitro g en Rate , sc f/m in

BH

P a

t 6

90

0 f

t, p

si

Friction Dominated

Hydrostatic

Dominated

Just adding more gas will not always increase lift.


GEKEngineering.com

Lift Systems

• gas lift - 95% of offshore lift systems

• rod pumps - shallower (d<11,000’ TVD), lower volume (rod breakage and problems increase with depth).

• ESP’s - high volume, high mantainence


GEKEngineering.com

Gas Lift

• operating depth limited by supply pressure

• would like to inject as deep as possible

• lift limited by the flowing gradient

– depth of gas lift injection

– amount of gas injected (back pressures)

– mixing of gas with liquid

– density of fluid to be lifted


GEKEngineering.com

CT Completions

• Gas Lift Completions

– GOM - Amoco - (internal report.)

– GOM - Conoco, W.O. C.T., March 1996)

– N. Sea - BP - Forties (W.O. C.T., March 1995)

– Alaska - BP - N.Slope (SPE 26538)

– Venez. - Lake Maracaibo (SPE 23749, 26968)

– Austin Chalk - Texaco (SPE 38404)

– Oman - PDO (SPE 36261)

– W. Tex-Shell (WO, Jan 97), Altura (SWPSC, 98)


GEKEngineering.com

Flow Velocity and Lift

• Flow velocity affects flow efficiency through liquid slippage, turbulence and friction.

• At low rates, gas tends to slip through liquids, allowing the liquids to remain in the tubing.

• At medium rates, gas drags liquid upward due to turbulence. This action reduces pressure gradient.

• At very high flow rates, friction of flowing fluid with the wall of the tubing causes excessive pressure drops, increasing the flowing gradient.


GEKEngineering.com

BHP and Lift Example – Tube Friction vs Critical Diameter for Lift

• 50% water cut, 35o API oil, 60 psi wellhead pressure, using Hagedorn and Brown gradients:

Tubing Flow GLR BHP

(in) (b/d) (scf/bbl) (psi)

2-3/8” 50 1,000 970

2-3/8” 400 1,000 1,070

2-7/8” 50 1,000 1,320

2-7/8” 400 1,000 940

• At 400 bpd, the 2-3/8” tubing shows excessive flow resistance. At 50 bpd, the 2-7/8” is allowing gas to slip through the liquid. Either case reduces production.


GEKEngineering.com

Gas Liquid Ratio (GLR) Effect

• Up to the point where friction becomes excessive, a higher GLR results in a lighter

fluid gradient. For 50% water cut, 35o API oil, 60 psi wellhead pressure:

Tubing Flow GLR BHP

(in) (b/d) (scf/bbl) (psi)

2-3/8” 100 500 1,260

2-7/8” 100 500 1,520

2-3/8” 100 2,000 680

2-7/8” 100 2,000 605

• The larger tubing has a larger gradient decrease because friction is not as severe.


GEKEngineering.com

Wellhead Pressures

• A small increase in wellhead pressure (WHP) can have a very severe change on BHP. Data for 2-3/8” tbg.

Flow GLR Flowing WHP Flowing BHP

(b/d) (scf/bbl) (psi) (psi)

200 200 50 990

200 200 80 1,155

• An increase in 30 psi at surface results in a 165 psi increase in flowing BHP.

• In most cases, the choke setting (the backpressure on the system) should be as low as possible. If gas volume is increasing too much (creating backpressure), then the system is not efficient. A small choke change may help reduce gas volume slightly and actually improve lift.

• How do you optimize a choke with a minimum of equipment?


GEKEngineering.com

Gas Lift mandrel collapse - The

root cause of failure was

thermal loading/ annulus

pressure buildup. What was not

realized was the low potential

collapse rating of the GLM. The

type used is MMG 2-4, the

mandrel has a slightly reduced

OD and a fully open ID and

therefore a reduced test

pressure/ collapsed rating.

Some Problems in Gas Lift – Mandrel Collapse

Bp Egypt 8/25/2015 41 George E. King Engineering

GEKEngineering.com

Collapsed Mandrel

BP Egypt

Collpases in the tubulars such as shown in the picture require the pulling of a string – increasing risk and decreasing economic of operations.

Case is needed when pressuring the annulus. Changes in gas pressure coupled with higher pressures from liquid fill (higher density fluid) can radically change


GEKEngineering.com

Gas Lift

Documents

Transcript of Gas Lift