downhole pump cards petsoc-95-89

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MONITORING AND CONTROLLING ROD PUMPED WELLS

USING DOWNHOLE PUMP CARDS

S.G. GIBBS K.B. NOLEN F.E. MORROW W.C. LYNCH

this article begins on the next page FF

THE PETROLEUM SOCIETY OF CIM PAPER 95-89 Monitoring and Controlling Rod Pumped Wells Using Downhole Pump Cards S.G. Gibbs K.B. Nolen F.E. Morrow W.C. Lynch Nabla Corporation This paper is to be presented at the 46th Annual Technical Meeting of The Petroleum Society of CIM in Banff, Alberta, Canada, May 14 - 17, 1995. Discussi on of this paper is invited and may be presented at the meeting if filed in writing with the technical program chairman prior to the conclusion of the meeting. This paper and any discussion filed will be considered for publication in CIM journals. Publicati onrights are reserved. This is a pre-print and is subject to correction. ABSTRACT Pump off controllers ha-,,e earned a place in the technology of rod pumping. Many methods exist for sensing when a well has pumped off. Most widely used are techniques based on the surface dynamometer card or motor speed or production rate. This paper describes several methods for sensing pump off using the downhole pump dynamometer card. These include areas inside of the pump card, areas outside of the pump card, set point and liquid fillage, among others. Procedures for

calibrating the controllers are described together with provisions for high fluid level recovery. Combining the liquid tillage method with variable frequency drives and eddy current drives is presented as a way of perforirning variable speed - no stop control. Pump card monitors (PCMS) hold promise of being useful devices which are easy to apply and comprehend. INTRODUCTION Application of pump off controllers (POCS) began about 25 years ago. These controllers have proven useful in saving power and minimizing wear and tear on rod pumping equipment. Thes e savings are

realized by stopping the unit when the pump is not filling sa tisfactorily. The POC has gradually evolved into more than a device to sens e pump off and to stop the unit. 1 Rod load based controllers can also sense abnorm al rod loads and shut the unit down before the pump and/or rods are damaged . Controllers that monitor motor speed can also prev ent damage to the prime mover and gearbox. Controllers that monitor liquid rate ca n also prevent dimage to the surface stuffing box packing . P OCs sometimes increase production slig htly by detecting malfunctions sooner than ord inary surveillance

techniques. At this writing, about 35000 wells worldwide have been equipped,with POCS. t'he purpose of this paper is to share ideas concerning how the downhole pump card can be used to sense pump off. While creati on of the downhole card is not simple, use of the pump card to sense fluid pound is. The complexit y of computing tlie pump card can be hidden in software and hardware with troubling the user. The result is a pump off control concept that is more easily understood and applied.


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Monitoring and Controlling Rod Pumped

Wells Using Downhole Pump Cards

.

THE PETROLEUM SOCIETY OF CIM

PAPER 95 89

.

.

.

S.G. Gibbs

K-B. Nolen

F.E. Morrow

w.e. Lynch

Nabla Corporation

This

paper

is to be presented at the 46th Annual

Technical

Meeting

of

The Petroleum Society of

elM

in

8anff

Alberta

Canada May

14

-

17 1995.

Discussion of this

paper is invited and may be presented at the meeting

filed

in writing with the technical program chairman

prior to the

conclusion of

the meeting.

This

paper and any discussion

filed

will be considered for

publication

in

elM

journals. Publication

rights

are

reserved. This is

a

pre print and is

sUbjecl

to

correction.

-

.- .

r

ABSTRACT

Pump off controllers have earned a place in the

technology

o

rod pnmping. Many methods exist for sensing

when a well has pumped o Most widely used are

techniques based on the surface dynamometer card or motor

speed or production rate.

This paper describes several methods for sensing

pump off using the downhole pump dynamometer card.

These include areas inside o the pump card, areas outside of

the pump card, set point and liquid lillage, among others.

Procedures for calibrating the controllers are described

together with provisions for high fluid level recovery.

Combining the liquid lillage method with variable frequency

drives and eddy current drives is presented as a way of

perfonning variable speed - no stop control.

Pump card monitors PCMs) hold promise of being

nseful devices which are easy to apply and comprehend.

INTRODU TION

Application of pump off controllers POCs) began

equipment. These savings are realized by stopping the unit

when the pump

is

not filling satisfactorily. The POC has

gradually evolved into more than a device to sense pump off

and to stop the nnit. I Rod load based controllers can also

sense abnormal rod loads and shut the unit down before the

pump andlor rods are damaged. Controllers that monitor

motor speed can also prevent damage the prime mover

and gearbox. Controllers that monitor liquid rate can also

prevent damage to the surface stuffing box packing. POCs

sometimes increase prodnction slightly by detecting

malfunctions sooner than ordinary surveillance techniques

At this writing, about 35000 wells worldwide have been

equipped with POCs.

The purpose o this paper is to share ideas

concerning how the downhole pump card can be used to

sense pump

off

While creation of the downhole card

is

not

simple, use o the pump card to sense fluid pound is. The

comple> ity o computing the pump card c an be hidden i n

software and hardware without troubling the user. The

result is a pump off control concept t ha t

more easily

understood and applied.

:.

-, .


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The downhole pump card is computed by solving

the one dimensional wave equation

u x,t a-u x,t

8u x,t

a

-e

g ....1

2

2

iii:

Ox

t

subject to measured boundary conditions cornprismg time

histories surface rod load L t) and surface rod position

P t .

The

result ing solutions are used to construct pump

dyno cards

utility in well analysis and control.

Oil producers began

to

be familiar with computed

downhole pump dynamometer cards 2 about 30 years ago.

These cards have the advantage of simplicity. Only two

types of cards have to be comprehended

in

calibrating a

pCM. These are full liquid fillage and incomplete liquid

fillage fluid pound or gas interference .

With anchored tubing, pump cards that indicate full

liquid fillage no fluid pound

or

gas iuterference have

roughly a rectangular shape Figure I . At top and bottom

of

the stroke. the pump is virtually statIonary as load

is

transferred

from

rods

to

tubing

and

vice versa. On the

upstroke, fluid is lifted while liquid completely fills the

chamber below the rising plunger.

On

the downstroke, only

buoyaucy and fluid friction forces act.

The incomplete fillagc case

is

shown in Figure

II

differs from the full fillage case in that the pump does not

fill completely on the lift iug portion

of

the stroke. Liquid

oil and water only part ially fills the chamber below the

rising plunger. Free gas occupies the remainder the

volume. On

the

downstroke. free gas must

be

compressed

before the traveling valve opens. The shape of the

compression curve is determined by the pressure the free

gas. For high pressure gas the compression takes place

gently as fluid load is transferred from rods to tubing so

called gas interference .

If

low pressure gas enters the

chamber during the upstroke. the compression takes place

rapidly fluid pound as load is suddenly transferred from

rods to tubing.

USE

OF THE

PUMP CARD TO SENSE PUMP

OFF

Several ways exist to sense incomplete f llage and

the following presentations show how features

of

the

downhole card change as fillage ceases to be complete.

Inside rea

Pump

Power

Figure 3-a shows the area within the pump card while the

pump is filling completely but is on the verge

of

pump alI.

Figure 3-b shows that area

of

the card decreases when pump

off occurs. The pCM which contains a digital computer

can be taught

to

compute pump power and

to

stop the unit

when power drops below a preselected amount. The pump

power method has problems with high fluid level recovery.

An ambiguity arises because pump power a so decrcflses

when fluid level rises. This is because the high fluid level

helps lift the well thereby decreasing pump power required.

Thus a PCM based on pump power might incorrectly shut

the well down with a h igh fluid level.

t

therefore needs

help in differentiating between pump off and high fluid

level. TillS is discussed later.

Inside

rea

to Left

and

Right of a Vertical Line

Area inside the pump card can be llsed in other

ways. An arbitrary vertical line can be drawn at some point

in the pump card, sa} at mid-stroke. Pump al I can be

deelared when area to the left of the line differs from area to

the right of the line by a preset amount see Figure 4 .

Outside Area Between Don nstrokc Trace and Horizontal

Line

Areas outside

the pump card trace also have

meaning with respect to pump off. Figure 5-a shows a

horizontal line drawn beneath a pump card indicating full

liquid fillage. The shaded area outside of the card between

the horizontal line and the downstroke trace is singled out

for study. Figure 5-b shows how the shaded area increases

as the well pumps off

The

PCM can be taught to calculate

the area and shut the unit

when the area rcaches a

specified amount.

Set Point Specifying Reference Load

and

Position

A reference point on the surface card has been

successfully used

to

deelare pump off. A similar idea can be

used with respect to the downhole card. Figure 6-a shows a

reference point specified pump load and pump position

superimposed on a full liquid fillage pump card. As long as

pump load at the reference position is less than the reference

load, the well is considered not to be pumped off and

pumping is continued. Figure 6-b shows the same reference

point on a fluid pound pump card. Pump al I is declared

when pump load at the reference position is greater than the

reference load.


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