WATER LIFTING OF GAS WELLS BY THE CWC METHOD

Behrokh Khoshnevis and Iraj Ershaghi, ProfessorsMahdi Yoozbashizadeh and Kasra Zamani, Graduate Students

Viterbi School of EngineeringUniversity of Southern California

1 Abstract

This paper describes a novel method for water unloading of natural gas wells in mature

reservoirs experiencing low reservoir pressures. Current methods for water unloading water

from gas wells have at least one of the drawbacks of restricting the gas production, requiring

external energy, using consumable surfactants and being labor intensive. The proposed design

offers a new approach to water unloading that does not restrict or interrupt gas production. It can

operate without external energy, and uses no consumables. Virtual and physical simulators have

been developed and the full-scale version of the concept has been studied in test wells to

demonstrate the feasibility and performance of the new water-unloading concept.

2 Introduction

Concurrent entry of water in gas wells because of natural influx or wet intervals can significantly

affect the economics of production. At the early stages of production the reservoir pressure is

sufficiently large to allow gas entering the well to lift the water entering the wellbore. Gas and

water mist flow to the surface where the water content can easily be separated from gas using

wellhead separation. As the production of the well continues, the reservoir pressure drops over

the years to the point where water can no longer be lifted to the surface by gas flow. This

eventually results in the accumulation of water in the bottom of the wellbore, sometimes

reaching a height of several thousand feet. In such situations well production stops completely

and the only remedy is water unloading by means of conventional pumping, which can be

prohibitively expensive. 1-4

1

It should be noted that even before the complete blockage of gas flow by the accumulated water

column, early stages of water formation in the tubing creates a water ring inside the wellbore that

reduces the effective diameter of the pipe hence restricting gas production. This effect is

especially significant in low-pressure reservoirs.

Besides pumping water, other methods to unload water from gas wells have also been devised

each based on a physical concept. These methods utilize the natural or artificial gas-pressure for

lifting water out of the wellbore while the well produces gas. The most popular methods are:

a) Velocity Strings: Reducing the diameter of well tubing to increase flow velocity and

hence lift water mist all the way to the surface. This method naturally reduces production

rate due to restricted flow area and increased friction and fails as soon as the gas pressure

drops again below some critical point.

b) Foaming Agents: Detergents are used to reduce the interfacial tension between gas and

water by creating foam and enhancing the lift. This method uses consumable material

and hence can be operationally expensive and labor intensive. Furthermore, interfacial

reduction materials can adversely reduce gas relative permeability around the wellbore.

c) Plunger Lift: Plunger lift is based on a method of intermittently shutting in the well to let

the gas pressure build up to a level making lifting of water impossible. This is followed,

by sudden opening of the wellhead to allow production of high-pressure gas and water

mix. To push the water column up, a solid cylinder, or “plunger”, is used. This cylinder

acts as a barrier between the gas and liquid and moves up and down with every opening

and closing of the well. Because this method works intermittently it requires frequent

shut-down of the well flow, resulting in reduced overall production.

d) Gas Lift: By injecting compressed gas from the wellhead through the annulus the

compressed gas enters the bottom of the tubing thereby resulting in higher overall gas

velocity enhancing the gas ability to lift the water. As the gas pressures drops over time

in mature fields, the gas lift method becomes less efficient.

Concerns for using above the methods include restriction of the production rate of the well,

expensive consumables, and high cost of external energy in form of electric power to drive gas

compressors.

2

The Water Loading Phenomenon:

Natural gas reservoirs generally produce associated water. As this mixture enters the bottom of

the well water easily flows upward because of a combination of gas lift and a relatively low

water viscosity at high bottomhole temperatures. The formation temperature decreases upward at

a rate of 30ºC/1000m (~2ºF/100ft), which is the typical geothermal gradient. At shallower depth,

water condensation begins because of heat losses to surrounding formation unlike the lower parts

of the well, at the upper parts of the tubing the condensate is cooler and its viscosity is higher,

therefore, at these elevations higher gas velocities would be required to lift and push the

condensate out of the wellbore. Adding to the problem is a higher rolling friction which cooler

condensate imposes against gas flow. The difference between the friction force against gas flow

in dry sections of tubing and this top section is analogous to the difference between rolling

friction for a wheel on pavement and on a glued surface. This physical analogy describes the

difference of condensate dynamic behavior in the upper and lower portions of the tubing.

The top portion of the tubing is the critical part where for older wells with declining pressure the

condensate begins to accumulate and drops to the well bottom creating a water column which

has to be removed artificially for gas production to continue.

For a case under study with a well depth of 9000 feet, the critical portion of tubing is in the top 1

km (~3300 ft) as shown in Figure 1. This is based on the assumption of ground surface

temperature ranging between 20ºC (68ºF) and 50ºC (122ºF) for this section.

3

gradient ~ 0.1 PSI/ft

gradient ~ 0.02 PSI/ft

gradient ~ 0.45 PSI/ft

Pressure (PSI)

Dep

th (f

t)

3000 ft

8900 ft

300 PSI

Water condensation zone

Dry gas zone

Water column zone

Figure 1. Water condensation, dry gas and water column zones in a typical gas well (Source:

Carthage field data)

The CWC Method:

The new method benefits from the fact that a great portion of the water which drops to the well

bottom is actually the result of the condensation of water vapor and consolidation of water mist

in form of larger droplets in the upper segment of the well (i.e., the upper 1 km segment) where

the temperature is much reduced. Traditional water unloading methods allow this condensed

water to return to the bottom of the well, thereby losing all the valuable potential energy that the

flowing gas has put into the water by lifting it to those shallower depths. Consequently, most of

the gas energy used for water lifting by these traditional methods must compensate for this loss

of potential energy that is a significant portion of the overall energy needed to lift the water to

the surface.

The CWC system attempts to capture the water at higher elevations, thereby preventing the loss

of potential energy put in the water to bring it to high elevations. The CWC system operates by

means of two types of modules: a) the Collection module, and b) the Water Push-Up Station

module. These modules have a small number of moving parts, do not use power, operate

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Figure 2. The Collector Module

automatically, and are expected to be maintenance free. Following are the description of these

modules and the integrated CWC system.

Collector Module: This module of which only one unit is used in the CWC system is positioned

at about 1 km from the well top. The module (see Figure 2) captures the returning water which

has condensed at higher elevations by directing it to a closed chamber (preferably made of

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stainless steel) positioned between the well casing and tubing. The funnel shown in Figure 2

directs the water that runs down from higher elevations into the Collector Module chamber. Note

that because of the upward flow of the gas in the tubing the returned water goes to the funnel

rather than into the opening of the lower tubing (attached to the lower section of the funnel).

Because the collector chamber can communicate with the production tubing (through the funnel

holes), its internal pressure is the same as the gas pressure inside the production tubing at the

Collector Module elevation.

As the water level rises in the collector chamber it lifts a float installed in the chamber. The float

in turn moves a lever that opens the valve at the bottom of the small tubing. The gas pressure

pushes the accumulated water upward and out of the collector chamber. The water accumulated

in the collector housing can be automatically pushed upward through the small tube if the

pressure at the upper end of the tubing is less than the pressure inside the collector chamber

minus the pressure caused by the weight of the water column in the small tubing. As will be

explained in the following section, the upper end of the tubing resides at ‘casing pressure’.

Therefore, if the production tubing at the collector module has 300 psi pressure and the casing

pressure is at atmospheric pressure, then water can be pushed up as high as almost 600 feet.

Note that the float-controlled valve only allows water and not gas to flow into the small tubing,

because the valve is open only when there is sufficient water accumulated to lift the float. Also

note that the small tubing has a one-way valve at its end which does not allow water to return

back to the collector chamber.

Water Push-Up Station (WPS) Module: The water pushed out of the Collector module enters

the chamber of the WPS module (see Figures 3 and 4) that is an intermediate station positioned

above the Collector module and slightly below the maximum elevation possible (e.g., in the

above example it would be at 590 feet above the Collector module). The WPS module receives,

accumulates and pushes water upward through the following stages:

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a) The WPS module starts with no fluid accumulated. The float mechanism allows the

module to equalize with the casing pressure by opening a valve (Casing Valve). This

Casing Valve allows the chamber to communication with the casing volume.

b) Water enters from below through the small tubing and accumulates inside the WPS

module until the float inside its chamber rises to the point where it closes the Casing

Valve and almost simultaneously opens another valve (Tubing Valve). The Tubing Valve

increases the pressure of the WPS chamber to that of the well tubing at the elevation

where the WPS module is positioned (e.g., 180 psi at 3000-590= 2310 feet below the

surface).

c) The tubing pressure pushes the water in the chamber upward into a section of Transport

Tubing which directs the water into another WPS module located at a higher elevation.

d) When the water level in the WPS module drops sufficiently, the Tubing Valve closes and

the Casing Valve opens. Note that all incoming and outgoing water Transport Tubes have

one way valves that prevent reverse (downward) water flow.

Note that in this configuration several WPS modules may be used to push the water in a stage-

wise fashion up to the surface. This action would be done entirely by the gas pressure (see Figure

5 - right) and without the use of external energy. Distances between WPS modules progressively

become smaller at higher elevations, as the gas pressure inside the production tubing drops at

these elevations.

When gas pressure drops over time, the Collector and WPS modules may not be able to elevate

the water far enough to reach the next station. The CWC system may be assisted in such

situations by means of one of the following methods:

1. The well pressure profile may be elevated along the tubing by restricting the out-flow rate

of gas at the wellhead (via a proportional valve). This is similar to the action during the

off-cycle of the plunger lift method. The advantage in CWC system is that, unlike the

plunger lift case, the well production is not completely interrupted and produces

continuously even at its low pressure conditions.

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Figure 3. The Collector Module pushing water into the first Water Push-Up Station Module.

Figure 4. A Water Push-Up Station

Well Casing

8

2. A small suction pump which can reduce the pressure in the volume inside the well casing

may be used, as shown in Figure 5 – left. Note that, it is the differential pressure between

the tubing and the casing that results in pushing the water up from one WPS to another.

The lower the casing pressure, the bigger the pressure differential. Such a pump of course

my not reduce the pressure below the absolute vacuum. In other words, such a pump can

maximally reduce the casing pressure by about 15 psi.

As the name implies, the CWC system unloads water concurrent with gas production, and it does

not require periodic well shut downs. Also, unlike the plunger lift system in which high impact

and high friction frequently damage the plunger and other components that come in contact with

it (downhole and at surface), the moving parts in CWC have small and low impact motions.

Note: It is possible to use the CWC system in conjunction with the plunger lift system, if so

desired. This may be useful in certain situations where down-hole water entrance is significant. It

is anticipated, however, that the use of CWC over time would keep the well significantly dryer

than it would be otherwise. In summary, the CWC approach has the following advantages:

Use of available gas pressure (however low) as the water lifting force

Independence from external power; no power bills; no power loss impact

No obstructions or moving pieces inside the tubing,

Continuous gas production without any interruption, and

Simplicity of hardware

The CWC system components, especially the valves, must be reliable, as the repair and

maintenance would require the removal of up to a kilometer of tubing.

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Suction Pump

Well Top Packer

Unloaded Water

Water Push-Up Stations

Figure 5. The Complete CWC System

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3 Analytical ModelingIn designing the CWC system the main challenge is finding the preferred distance between water

accumulation stations to assure the unloading of all produced water while using the smallest

number of Push-Up stations. For water flow though pipes, if the friction is ignored, then the

maximum distance between every two consecutive stations is simply given by the following

equation:

Δh=P1−P2

ρ .g (1)

Where:

P1 : Inlet pressure (pressure in the lower station)

P2 : Outlet pressure (pressure in the upper station)

For a given pipe diameter there would be a specific flow rate associated with the above inter-

station distance. In actual conditions, however, there is friction against the water flow in the pipe

which results in pressure head loss and hence a reduced flow rate. To maintain the same

unloading rate as in the friction-free case, either the distance between the two stations have to be

reduced or the diameter of the water pipe has to be increased. Assuming a fixed (selected) water

pipe diameter, the only means for maintaining the higher flow rate will be reduction of inter-

station distance.

The total volume of water accumulated in the Collector station before this water is pushed to the

next Push-Up station isVolc

, the effective volume of the Collector station (station housing

volume less the volume occupied by gas and water tube as well as the float, valve, etc.).

In the CWC module water is accumulated in the Collector station with a rate equal to the water

production rate of the gas well .Assuming the rate of water produced by the well is Q

(barrels/day) the following equations are derived:

q1=0. 084 Q (2)

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Volc=q1 . t1 Or t1=

Volc

q1 (3)

Where:

q1 : Rate of water produced by the gas well (in .3

sec . )

t1

: The time it takes for the water accumulated in the Collector station to reach a level to open

the Collector station outlet valve (sec.)

When water reaches its maximum level in the Collector station the outlet valve in the collector

station opens and water starts moving to the Push-Up station above. During this time water

continues to flow into the Collector station. The flow of water out of Collector station stops

when the float is lowered enough to close the outlet valve. The total volume of water pushed out

of the collector valve in each cycle is, therefore, larger than the effective volume of the Collector

station. The equation bellow shows the total volume pushed from the Collector station to the

Push-Up station in each cycle:

q2 .t2=q1 . t1+q1 .t2 Or q2=

q1( t 1+t2)t 2 (4)

Where:

q2 : Flow rate of water pushed out of the collector station (in .3

sec . )

t2 : The time it takes to empty the Collector station into the Push-Up station above it (sec.)

Note, therefore, the following facts:

The effective volume of Push-Up stations must be at least equal to q2.t2. Push-Up station housing volume will be always larger than the Collector station volume.

In order to avoid overflow in the collector station the inequality q2≥q1 must be satisfied.

The number of times the stations open and close during one day (i.e., frequency) is derived from

the following equation:

freq=24∗3600t1+t2 (5)

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In Equations 4 and 5 the value of t1 is fixed and depends on the effective volume of the Collector

station and inflow rate of water and the value of t2 depend on the flow rate of water out of the

stations. This outflow rate of course would depend on the station distances (if the water pipe

diameter is fixed). If the design engineer decides on the desired frequency of the system then t2,

outflow rate and station distances could be determined. Note that in order to increase the

reliability of the system the frequency of valve opening/closure should be kept as low as

possible.

The distance between two consecutive stations under friction may be modeled as follows. From

the Bernoulli equation we have:

V12

2. g+

P1

ρ . g+Z1+hp=

V22

2 .g+

P2

ρ . g+Z2+hf

(6) Where:

P1 : Inlet pressure

P2 : Outlet pressure

V 1 : Inlet average velocity

V 2 : Outlet average velocity

Z1 : Inlet elevation

Z2 : Outlet elevation

h p : Head pump

hl : Head loss ρ : Fluid density g : Gravity

Since water is an incompressible fluid,V 1=V 2 . Then equation 6 can be written as:

ΔZ=

P1−P2

ρ . g−hf +h p

(7)For a pipe flow the Darcy-Weisbach pipe friction head loss for a Newtonian incompressible fluid

is given below:

h f=f . LD

. V 2

2. g (8)

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Where:

h f : Head loss due to friction

f: Darcy-Weisbach friction factor

L: Pipe length

V: Average flow velocity derived from

4 . q2

π . dw2

g: gravity

D: Pipe diameter (In here is equal todw )

From equation (8) we can derive the following:

ΔP los=ρ .g . hf =f . LD

. ρ .V 2

2 (9)

Where the friction factor is a function of the Reynold’s number (Re ) shown below:

Re=ρ . V . D

μ (10)Where:

μ : Fluid absolute viscosity

For Re <2000 or in other words for a laminar flow we have:

f =64Re (11)

For 4000<Re <100,000 the friction factor can be derived from the following equation:

1√ f

=1 .14−2 log( eD

+ 9 .35Re .√ f

) (12)

Where:

e: Roughness factor of the pipe

If 4000<Re <100,000 for a hydraulically smooth pipe such as the pipes used in the CWC model

the friction factor can be derived from the following equation:

f = 0 .3164

Re0 . 25 (13)

From Equation 10 we can derive the following:

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Re=77 .145 (

Q2

dw2 ) Where Q2=q2/0 . 084 (14)

As dw

value is about .4 in. and Q2

is about 25 Barrels/day we conclude thatRe

is greater than

4000 thus we use equation 13 for defining the friction factor. Since in the CWC system there is

no head pump pressure between two consecutive stations, therefore, hp=0.

From equations 7, 8, 9, 10 and 13 we derive the following equation which yields the distance

between two consecutive stations in the CWC system (P1 and P2 in psi, Q in Barrels/day, dw in in.,

V in in./sec. and Δh in ft.):

Δh=( P1−P2 )

ρ⋅g⋅[1+0. 1582( μρ).25( V 1.75

d w1 .25 )]

(15)

Therefore the distance between every two adjacent stations can be determined by using the water

unloading rate and the difference between the pressures of previous station and casing pressure.

Note that in the above analysis the collector station volume and water pipe diameter were

assumed to be given. Of course each of these could be decision variables as well for which the

design engineer may tray different values. Besides the low frequency of valve opening and

closure cycle, it is also desirable to have the smallest possible number of Push-Up stations to

minimize the system cost and deployment expenses.

4 Simulation ModelingFor design and construction of actual full-scale CWC systems to be implemented in real gas

wells a virtual simulator which can be configured to the given well condition (pressure profile,

depth, etc.) has been developed. The simulator uses the System Dynamic concept where

difference equations and pipeline delays are used to trace the fluid motion from one

accumulation point to the next. In computation of the fluid velocity the analytical equations

presented in Section 3 have been used in the simulation model. Using this simulator the total

number of pushup stations required, length (and thus capacity) of the collection and pushup

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stations as well as the appropriate distances between the stations can be configured. The

simulation software also provides an animation of water flow and accumulation in all modules of

the CWC system, as well as a close-up animation for any specified module. Figure 6 shows a

screen shot of the simulator input values and a snap-shot of the real-time animation of upward

water displacement from one stage to the upper one.

The input values of the simulator include the gas well properties and the CWC system physical

geometries. The set characterizing the gas well includes the following properties:

1) Pressure at water separation point (i.e., the CWC Collector station)

2) Pressure at well top

3) Atmospheric pressure

4) Depth the water separation point in the gas well

The CWC physical properties consist of:

1) Water transfer tube diameter

2) Gas tube diameter

3) Casing diameter

4) Height of water collection station and push up stations

After receiving the input values in the simulator the software uses equation 15 to determine the

number of stations. The simulation dynamically monitors the amount of water extracted from the

gas well and the status of every station is scanned and updated. Table I shows a typical output of

the CWC software.

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Figure 6. CWC Simulation developed in VB.Net(left: Animation, right: input data)

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Table I. CWC simulation output table for values shown in the input screenCWC Simulation Results

Input data ( Gas well properties) Input data (CWC physical property)Pressure at separation

point( psi)

Well head

pressure( psi)

Atmospheric pressure

(psi)

Depth of collector station

(ft.)

Water production

rate(Barrels/

day)

Inside diameter of

water transfer tube(in.)

Outside diameter

of gas tube(in.)

Effective volume of Collector

station

( in .3 )

Frequency of collector

station per day

350 50 14 3000 25 0.5 2.4 121.8 800

Output resultsNumber of

stations neededVolume of

pushup stationin .3

Water discharge rate from the

stations in .3 /sec

Water discharge time from the

stations(sec.)

Water accumulation

rate in collector stationin .3 /sec

Water accumulation

time in collector station(sec.)

12 226.8 4.5 49 2.1 58Station # station depth (ft.) Station pressure(psi)

1 3000 3502 2438 2943 1970 2474 1581 2085 1256 1766 986 1497 761 1268 574 1079 417 9210 288 7911 179 6812 89 59

5 CWC physical simulator

The feasibility of the CWC concept has been demonstrated using a laboratory scaled physical

simulator. It has been shown, using the simulator, that the funnel in the lowest CWC module

does effectively collect water and that the tubing pressure can effectively push the collected

water upward to the maximum height linearly proportional to the pressure at the water collection

point. A push-up module is also included in the simulator and it has been demonstrated that the

module effectively pushes the water upward using the tubing pressure at its level. The simulator

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is built at approximately half the diameter scale of the real system and is mainly comprised of

four types of polycarbonate tubes with different diameters for well casing 2” OD), module

housing (1.75” OD), gas tubing (1” OD) and water tubing (0.25” OD). A rapid prototyping

machine for building 3D parts and a laser cutter for building 2.5D parts are used. An accurate

metering pump has been used to feed water into the gas tubing and air is metered in through the

lower opening of the gas tubing. The physical simulator has demonstrated the success of the

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concept in a variety of situations and has provided invaluable insight regarding the operation of

CWC. Figure 7 shows sections of the physical simulator.

6 Conclusion

The concept presented in this paper, Concurrent Water Collection (CWC), addresses the

perennial and difficult problem of water accumulation that slows and finally cripples low-

pressure gas wells.

The currently used methods can restrict gas production, use energy, or use chemicals that are

expensive and harmful to the environment. The CWC method benefits from the fact that a great

portion of the water which returns to the well bottom is actually the result of the condensation of

water vapor and consolidation of water mist in form of larger droplets in the upper segment of

the well where the temperature is much reduced. Traditional methods allow for return of this

condensed water to the bottom of the well, thereby losing all the valuable potential energy that

has already been put into the water by gas-lifting it to those higher elevations. Consequently,

most of the gas energy used for water lifting by these traditional methods must compensate for

this loss of potential energy. The CWC concept was first validated by computer simulations that

were followed by the construction and testing of several scaled physical prototypes.

A team of experts was then assigned by a producing company to the project to develop and test a

full-scale version of the CWC system. The full-scale system was built and the preliminary tests

of the system in shallow test wells again showed very encouraging results. The new method has

potential to significantly improve the overall production throughput over the producing lives of

wells. The amount of the production increase would depend on the condition of candidate wells.

The lower the well pressure, the more significant the comparative impact of the new method will

be. In the extreme case some gas wells that were abandoned because of low pressure may be

brought back to life.

With the emphasis on natural gas as fuel of choice for commercial, residential and transportation,

extending the economic life of mature gas fields requires creative solution for water unloading.

CWC has shown a promising potential for use in increasing number of low-pressure gas fields.

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Acknowledgement

This study was supported by CiSoft, Center for Interactive Smart Oilfield Technologies

sponsored by Chevron at the Viterbi School of Engineering at USC.

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