REVEAL Dual String Transient Gas Lift Wellbore Simulation · Introduction: Dual String Gas Lift Completions: Dual string gas lift installations comprise of two producing zones, each

Introduction:

Dual String Gas Lift Completions:

Dual string gas lift installations comprise of two producing zones, each with its own tubing string in a single

production casing, where the gas-lift-gas is injected at surface into the common annulus. Despite the known

operational issues associated with dual string gas lift completions, the benefits of the dual completion are

used from an economics point of view because of their reduced drilling costs when developing fields with

multi-stacked reservoirs. The dual string completion enables two independent horizons to be exploited from

a single wellbore, so long as the given reservoirs are not too great a distance apart i.e., <1000-ft. In some

offshore platforms, there may also be insufficient surface space to accommodate the required well count to

adequately develop the multiple reservoirs with single completions, hence the dual string completion is

favoured to address the limited well count foot print on the platform itself.

Compared to single completions, dual completions have a higher initial completion cost, and have more

operating problems than single well gas lift installations, and are also more difficult and expensive to work-

over. Dual string completions often produce less efficiently, usually because one string consumes the

available gas-lift-gas from the other string due to valve dome pressure and valve port size interaction under

prevailing and changing operating conditions.

Dual String Gas Lift Valve Type and Spacing Design:

From a dual gas lift design perspective, there are many rules of thumb approaches that are listed in the

literature, attempting to address the well unloading and operational instability issues typically associated

with dual string gas lift completions.

Complexity at the nodal analysis design level arises from only being able to address single-string completion

design and unloading, where the valve pressure and rate relationships are not fully captured due to the

dynamics of the second string being absent during the gas lift design phase. Usually it is the long string’s

gas lift design that is performed first, setting a maximum depth of injection for unloading and operation at

the dual string packer depth. The valve spacing on the short string is usually arbitrarily spaced one or more

joints above the valves designed for the long string. From only being able to design the valves based on

single string interaction adds further complexity due to the dynamic valve responses not taking into

consideration the impact of the other string during unloading and operation. Once the well has been

unloaded and in operation, there may be a requirement to shut-in the well again and change out the gas lift

valves using different dome pressures and port sizes by means of wireline workover techniques until a more

stable production status can be achieved, which is a costly iterative process.

IPM Modelling:

It is well known the value PROSPER offers the end-user community with the multitude of well design and

diagnoses capabilities that are available, including the single-completion transient gas lift simulator,

enabling dynamic well unloading and valve operation to be understood and resolved as required to rectify

multi-pointing gas injection, etc. Gas lift equilibrium curves can also be constructed that describe the actual

production rate when the well is gas-lifting from any particular depth.

Page 1

CAS E STUDY

REVEAL Dual String Transient Gas Lift Wellbore Simulation

May 2017

REVEAL has taken fully-coupled complex well design and performance to new levels, from being able to

describe multi-string completions that include ICD/ICV/AICD flow control devices, bringing understanding of

fluid contact movement to given production strategies for example.

Dual string gas lift optimization workflows are already available in RESOLVE and Digital Oil Field (DOF)

technology. The RESOLVE and DOF workflows enable optimization of existing dual string completions,

where valve deign and valve placement may not be optimum.

Petroleum Experts’ invests millions of self-funded dollars each year into R&D addressing the needs of our

clients and the industry. As a result of the R&D investment, among many other new IPM developments,

REVEAL has taken another invaluable step forward into the domain of transient wellbore simulation,

delivering unequalled well modelling capability. Simultaneously being able to capture both the transient

reservoir, and transient well responses, provides unlimited value to the production, completion and

operations engineers when investigating dual string gas lift completions’ performance, and developing

stable well operational strategies.

The transient wellbore modelling capabilities REVEAL offers production pressure operated (PPO) and

injection pressure operated (IPO) valves, and combinations thereof to be investigated, bringing

understanding and the performance of valve interactions during well unloading and operation.

Having the capability to perform transient wellbore analyses at the dual string gas lift design level delivers

invaluable knowledge of what gas lift valve dome pressures and port sizes are required for unloading, and

valve spacing requirements to deliver stable production prior to completing the well. Existing completions

with sub-optimal performance can also be investigated, delivering knowledge of actual valve interaction

response to given dynamic operating boundary conditions, then valve attributes re-designed to deliver

optimal well production performance.

Case study:

The following case study illustrates the extended well modelling capabilities REVEAL offers in context of

transient dual gas lift unloading and optimization, starting with an existing sub-optimal gas lift design that

currently operates on the long-string only. The long string is completed with IPO operated valves, and the

short string is completed with PPO gas lift valves. The well valve spacing is fixed, as it is an existing well,

however, using REVEAL’s ability to capture the transient valve throttling responses and transient well inflow

response, enables the dynamic valve behaviours to be understood. Valve attributes can be re-designed as

required, until the desired unloading and stable well production requirements have been satisfied.

REVEAL Complex Well Design:

The dual string gas lift completion can be created directly in REVEAL using the well Equipment Builder, or

created separately using RESOLVE’s WellBuilder DataObject, then simply imported directly into REVEAL.

Page 2

CAS E STUDY

REVEAL Dual String Transient Gas Lift Wellbore Simulation

May 2017

The following figures illustrate the dual string well construction each approach offers, and depict the existing

dual string gas lift completion design being investigated:

WellBulider Existing Dual String Gas Lift Completion View:

Detailed well equipment can be added and placed at the respective location in the completion as required

from the Equipment Browser.

Page 3

CAS E STUDY

REVEAL Dual String Transient Gas Lift Wellbore Simulation

May 2017

REVEAL Existing Dual String Gas Lift Completion View:

Well Unloading and Well Flow Analysis:

As gas is injected into the annulus, the kill fluid is expelled through the unloading valves of each string. The

following transient wellbore results show the gas injection impact on fluid density in the annulus, tubing and

second tubing, as well as indicating where the gas-lift-gas is being injected.

Page 4

CAS E STUDY

REVEAL Dual String Transient Gas Lift Wellbore Simulation

May 2017

Time = 1-Day Prior to Starting Gas Injection:

Gas Injection Started Showing Fluid Density Changes:

Page 5

CAS E STUDY

REVEAL Dual String Transient Gas Lift Wellbore Simulation

May 2017

Annulus, tubing and second tubing show kill fluid

density prior to gas injection

Gas density now in

annulus up to first unloading

valve

Kill fluid to packer depth

Long-string fluid density

reduction and positive flow

due to gas injection

Second tubing reverse

flowing and not unloading

Note in the above figure that due to the valve attributes of the second string, it does not unload and instead

reverse flows fluids into the reservoir.

Well Flowing Status after Unloading:

The following figure shows the annulus fluid is now gas up to the deepest point of injection being reached

for the long-string. The long-string shows as producing, with the second string not flowing:

Gas Lift Valve Injection Depth Analysis:

We will see from reviewing the valve results that the unloading sequence was unsuccessful, as the second

string (short) partially unloaded, then it was only the long-string that produced, essentially consuming all the

available gas-lift-gas. The transient valve response can be fully investigated, including throttling behaviours

to diagnose what happened, as well as provide a means to change valve attributes.

The gas lift design investigation may also consider changing valve types if desired until the well has been

correctly unloaded and stable dual-string production has been achieved. Investigation will also show which

valves have been open on the short string, bringing understanding of when and why they closed, and if the

long string is multi-pointing.

Investigation of Valve Performance:

The convention in REVEAL for reporting gas lift valves is sequential, such that GLV1 corresponds to the first

unloading valve on the long-string, GLV2 is then the first unloading valve on the short-string (second tubing),

Page 6

CAS E STUDY

REVEAL Dual String Transient Gas Lift Wellbore Simulation

May 2017

Gas density in annulus to

deepest point of injection Well unloaded

with long-string

producing

Second string not producing

where GLV3 will be the second unloading valve on the long-string, etc.

The following figure shows the gas lift unloading valve’s gas injection rates, note that the second string gas

lift valves throttled and closed due to current valve attributes responding to changing tubing gradients:

The above figure also illustrates that the long-string is multi-pointing due to incorrect valve sizing and dome

pressure settings, and all the short string valves close due to incorrect valve characteristics to operational

tubing pressure that controls PPO valves.

Short String Valve Investigation:

To simplify the analysis, the focus will be on the first valve of the short string (GLV2), where the following

valve attributes can be plotted to understand why the PPO valve closed:

Gas injection rate

Dome Pressure

Valve inlet pressure ( or annulus pressure can be selected)

Outlet pressure (or second tubing pressure at depth from WellPipe results can be selected)

Page 7

CAS E STUDY

REVEAL Dual String Transient Gas Lift Wellbore Simulation

May 2017

As the well unloads, all valves are initially open, then the valves on the second string (GLV2, GLV4 and GLV6) throttle and close leaving the long-string multi-pointing through to the

deepest injection point ICV1

Valve attributes (or type) can now be changed until stable well production has been achieved.

REVEAL Gas Lift Valve Database:

The well Equipment Builder in REVEAL enables detailed well completion equipment to be described, as well

as containing a gas lift valve database, where valve attributes (and types) can be changed as required until

stable well production has been achieved.

Page 8

CAS E STUDY

REVEAL Dual String Transient Gas Lift Wellbore Simulation

May 2017

Valve throttling and closing due to change in valve

outlet pressure (increasing tubing pressure)

As the valve outlet pressure (tubing pressure) increases to

the valve inlet pressure (annuls pressure at depth) the

valve throttles and closes

Dual String Gas Lift Valve Re-design:

The design of PPO valves depend on the tubing pressure gradient in the well, which is less well known since

it is influenced by dynamic factors such as BS&W, inflow performance, fluctuating tubing pressures, etc. It

is clear from the above response of the second tubing that the production pressure was not fully

understood. For this case, IPO valves will also be designed and installed in the short string based on the

now known pressure gradients.

The following figure now shows all unloading valves have throttled and closed, with both strings operating at

the desired deepest point of injection from having reviewed the gas lift valve design:

The following figure shows details of GLV5, which is the last unloading valve on the long-string. Valve

throttling can be clearly seen due to changing casing pressure’s interaction with the valve dome pressure

that serves to throttle and close the valve:

Page 9

CAS E STUDY

REVEAL Dual String Transient Gas Lift Wellbore Simulation

May 2017

Successful unloading and closure of all gas lift valves, with the well

operating at each string’s deepest point of injection

Now that we have achieved well unloading and desired stable production, the simulation run-time can be

extended to evaluate well production stability and connect to the surface network in GAP through RESOLVE

enabling global optimization, flow assurance, etc., studies to be evaluated.

Summary:

Complex wells can easily be constructed directly in REVEAL, or using RESOLVE’s WellBuilder DataObject

then imported directly into REVEAL, enabling multi-string well performance to be investigated.

REVEAL’s transient wellbore modelling capabilities adds further value and capability enabling the complex

behaviour of dual gas lift completion performance to be evaluated. Well unloading strategies, including

understanding gas lift valve re-design requirements to obtain stable well production can be studied from

being able to evaluate the transient gas lift valve throttling behaviours with respect to changing well

gradients, etc. Valve modelling capabilities provide the basis to evaluate PPO and IPO valve combinations,

providing invaluable knowledge as to what valve type and or valve combinations deliver stable well

production performance.

Longer-term well stability from implementing the gas lift valve designs can be further studied from extended

forecasting to verify well stability prior to implementing wireline valve change-out operations. For extended

forecasting, REVEAL automatically changes from transient wellbore simulation to adaptive time-stepping

based on the Max dT value specified at the schedule level. RESOLVE integration enables implementation of

extended modelling approaches, linking the wells to GAP, addressing field management, global optimization,

flow assurance, etc., requirements. © Text and images contained herein are subject to copyright laws, and should not be reproduced without permission of Petroleum Experts. Petroleum Experts is a registered trademark.

Page 10

CAS E STUDY

REVEAL Dual String Transient Gas Lift Wellbore Simulation

May 2017

Valve inlet pressure reduction serving to throttle

the valve as it approaches the valve dome pressure

Valve throttles and closes as the inlet pressure no longer over

burdens the valve dome pressureDome

Pressure