Paper on Refining Oil

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THE PETROLEUM SOCIETY OF

el

PAPER

A Review

of

Underbalanced Drilling

of

Horizontal Wells in the Carbonate Reservoi

of

Southeastern Saskatchewan Case Stud

S J Springer

Springer Consulting Services

B.Lunan

Northland Wireline Service

A

Brown

BP Exploration UK)

D Sadal

Sadal Consulting Services

This paper is to be presented at the 46th Annual Technical Meeting of The Petroleum SOCiety of CIM in Banff, Alberta, Canada, Ma

17, 1995. Discussion of this paper is invited and may be presented at the meeting if filed

in

writing with the technical program cha

prior to the conclusion of the meeting. This paper and any discussion filed will be considered for publication in el journals. Publ

rights are reserved. This is a pre-print and

is

subject to correction.

BSTR CT

Underbalanced Drilling (UBD) of

Horizontal wells is gaining popularity in

Western Canada. The technique has been

used

to

drill more than 250 wells in the last

two years. Advances in drilling equipment

and drilling techniques are mainly

responsible for the progress

of

this new

evolving technology. The advent of

rotating BOP's and Closed-System

surface facilities to handle drilling fluids

and produced hydrocarbons has greatly

increased the safety and efficiency of the

drilling operation. Many of the short

term benefits of

UBD

such

as

increased

penetration

rate

and evaluation

of

the

productive zone while drilling, have been

well documented. However, some of the

long term benefits are not clearly

defined. This paper presents a study

conducted on

1

00 horizontal wells drilled in

the Midale, Frobisher-Alida, and Tilston

formations in SE Saskatchewan. About

half of the wells were drilled overbalanced

and the others were drilled underbalanced.

The objective of the study was to examine

the initial stabilized production

rate the

productivity index (PI) and the mechanical


2/14

skin. These parameters were used

to

provide a better appreciation of the long

term benefits of UBD. The results

indicated that the initial production rate

of

the UBD wells was generally superior to

wells drilled overbalanced. However, the

other two parameters did not clearly

indicate this superiority.

INTRODUCTION

Horizontal Well Technology (HWT) is

evolving at a brisk yet cautious pace.

During the last five years, about 2500

horizontal wells have been drilled in

Western Canada. The number of

horizontal wells drilled in clastic and

carbonate formations are almost evenly

divided. Typical true vertical depths (TVD)

drilled during the

1989-91

period

were

500

to

1000 meters. The horizontal lateral

of

these wells were also in the 500 to 1000

meter range, resulting in a measured well

depth

of

1200 to 2000 meters. Several.

horizontal wells, especially in Alberta and

British Columbia are now being drilled in

formations

t

depths of 2000 to 2500

meters.

Also, in the early stages of he technology

most wells were New Drills, using

conventional

mud

systems. Today, at least

25% of horizontal wells drilled may

be

classified as Advanced Technology HWT

projects. These include Underbti aneed

Drilling (UBD) using nitrogen, natural

(methane) gas or air; Re-Entries (RE)from

114.3

to

177.8mm production casing;

Short-Radius (SR); 0 to 90 degrees in

12

to

20 meters; and Multi-Laterals (ML); one or

more arms radiating from a main lateral.

(Maurer et aI1994). Some production data

is becoming available for wells using these

2

Advanced Technology, and we .are now

able to examine some of the long term

benefits

of

these techniques. In this paper,

we

will look

at

some

of

the short term

and

long term performance benefits of

UBD. In particular,

we

will discuss a

study conducted on horizontal wells drilled

in

S

Saskatchewan which examined the

performance of horizontal wells drilled in

the Midale, Frobisher/Alida, and Tilston

formations. More than 100 horizontal

UBD

wells have been drilled in these

formations). Table 1 is a sample

of

horizontal wells reviewed in the study. The

table indicates drill time, initial production

rates, actual productivity indices, total

calculated skin, and whether the wells

were

drilled overbalanced or underbalanced.

The paper is divided into two parts. In

part one, some of the important

characteristics of UBD are discussed.

Also, some of the short term benefits are

identified. In part two,

we

will discuss a

Case

Study conducted on about 100 wells.

About 50% of these wells were drilled

underbalanced. The major objective

of

the

study was to attempt to quantify some of

the long term benefits

of UBD

in the S

Saskatchewan area.

HISTORICAL

Underbalanced drilling is a drilling

technique which involves a drilling fluid,

which imposes a hydrostatic head on the

formation less than the reservoir pressure

local to the well. The technique involves

drilling with suitable specialized rotary

equipment, or coiled tubing, and pumping

a fluid, which may be lightened by

nitrogen, natural gas, or air. The well


3/14

flows as the formation is drilled and the

produced fluids, drilling fluids, and

cuttings are handled

by

suitable surface

processing facilities. The underbalanced

drilling technique contrasts with

overbalanced drilling, which involves

sealing off the formation with an

impermeable filter cake after some degree

of formation invasion and damage has

taken place.

The difference between

UBD

today and

1992, when the first horizontal wells were

being drilled UBD in Canada, is the

operators experience

of drilling over 250

wells using new UBD technology. The

reason

for

the rapid increase in the number

of wells being drilled underbalanced is the

introduction

of

a true rotating BOP and a

closed sUrface control system which can

successfully separate hydrocarbons, drilling

fluids, solids, and gases during the drilling

process. This advanced technology can be

utilized to drill in pressure depleted

reservoirs where conventional

mud

systems

could result in

mud

losses, severe

formation impairment, and further

increased costs from expensive stimulation

practices

to

remove formation damage.

Underbalanced drilling techniques is an

alternative approach that

can be used by oil

and gas producers particularly in horizontal

well applications.

To oil and gas operators who have

correctly drilled underbalanced, the

technical

and

financial results can be most

rewarding. Moreover, underbalanced

drilling in gas or oil rich reservoirs can be

an exciting experience. Envision the scene

as the horizontal well intersects a

geological fracture system: the flare takes

off into the air or oil starts accumulating

3

on sUrface and additional production tanks

are required to store these produced fluids.

These immediate results at surface are

becoming common occurrences to operators

who successfully drill with UBD

technology .

Four basic UBD techniques have

emerged:

1 Stand Pipe

2) Parasite String

3) Concentric Casing Strings

4 Coiled Tubing

Stand pipe UBD is the mostpopular and

accounts for 90 of the

UBD

operatiQns to

date.

The

method simply involves mixing

gas with drilling fluid at surface and

injecting the mixture down the drillstring at

pressures varying from 5 to 20 MPa.

Traditionally, air drilling was used to

achieve UBD conditions. This poses a real

danger of a possible downhole fire in most

reservoirs except in

dry

gas reservoirs.

Moreover, as soon as liquids are

encountered, especially with ~ present,

air drilling systems cannot be used. UBD

with a closed system uses an inert gas

system, either nitrogen

N:J

or natural gas.

Neither can

bum

without oxygen.

The second technique parasite string UBD

is achieved

by

cementing in a

1

string

down the backside of the casing.

n

underbalanced state is created and

maintained by injecting gas through a side

entry sub

just

above the kick-of f point.

The major advantage of the system is that

an UBD state can be realized throughout

the drilling operation. Consequently, pure

fluid can be pumped down the drillstring,

preserving the mud devices functionality.


4/14

The downside, and the major reasons why

more parasite strings have not been run,

are

the extra costs involved, the perceived

difficulty

of

the operation, and the possible

problems come abandonment.

The third

UBD

technique developed

achieves effects similar

to

a parasite string

by

inserting a temporary inner casing

concentric with the drill string and outer

casing. Fluid purity is maintained within

the drill string allowing pulse-type

mud

signals. The UBD state is created

by

injecting gas down the inter-casing micro-

annulus to the foot

of

the well's vertical

portion where it mixes with returning fluids

and cuttings and is returned

to

the sUrface.

The fourth, and technically the ideal

system is a coiled

tubing

SJsIetn. This

permits the well to be underbalanced at all

times, allowing gas

to

be commingled at the

sUrface during continuous drilling and

pumping.

t

also saves tripping time.

Tools for coiled tubing are not fully

developed, but as experimentation continues

and improvements are made, it could prove

to be the ultimate UBD technology because

the well is always in an UBD state.

SURFACE SYSTEMS

The concept

of

a closed loop system

control drilling package has evolved from

the initial prototype first used and

positioned on a drilling job during the last

quarter

of

1992

The system initially

utilized

was

a Production Test separator

which

was

set up basically

to

keep back

pressure on the well and separate gas from

the drilling medium.

The

philosophy then

was to

employ a Production Testing setup

4

for

a drilling operation. Since that time,

over 250 jobs have been drilled using the

closed-loop concept.

During the past three years, the

Production Testing equipment has been

replaced with a true Underbalanced

Drilling Surface Control Package

especially designed

to

handle and separate

four

phases (drilling fluid, liquid

hydrocarbons, gases, and solids). The

system has become more sophisticated, and

a unique set-up is employed

for

each

particular job.

The introduction

of

an improved manifold

sampling system with a continuous solids

transfer pump eliminated many

of the

initial surface handling problems and

improved on the overall efficiency

of

the

system. Input from the operator and an

understanding of their particular needs

have seen many minor improvements

to

the

overall system. Constant monitoring of the

fluids and gases with real-time electronic

equipment has realized more accurate data.

Better sample catcher design has led

to

better trajectory control for the geologist.

In conjunction with the command centre

in the field, the system has the advantage

of excellent communication between the

driller, the pumpers, the

sUrface

control

personnel, the directional people, and the

geologist. This team approach has resulted

in an excellent safety record for all

UBD

operators using a closed system.

SHORT TERM BENEFITS

Some of he attractive short term benefits,

most

of

which occurs while drilling

are:


5/14

An

improved rate

of

penetration

ROP)

- Elimination

of

drilling mud losses

- The ability to produce hydrocarbons,

evaluate the formation and size the

necessary equipment

An increased ability

to

detect

and

mitigate

kicks

- Reduced rate o f helical and differential

sticking

Reduced formation damage and

stimulation cost

CASE STUDY

The UBD study was conducted on about

100 horizontal wells drilled in

SE

Saskatchewan (Williston Basin).

STUDY OBJECTIVE

The objective of the study was to attempt

to identify/quantify the long term benefits

of UBD. The Mississippian carbonate

formations of SE Saskatchewan (Williston

Basin) provided this unique opportunity,

since more than 500 horizontal wells have

been drilled in various formations in the

basin during the 1993 94 period.

At

least

one hundred of these wells were UBD.

MEmO OLOGY

The first step was to review historical data

from

a large number

of

pools in SE

Saskatchewan in which horizontal wells

were drilled. (This exercise was conducted

at the Saskatchewan Energy and Mines

(SEM) offices in Regina). 100 wells from

about 20 pools were selected. Some of the

selection criteria were adequate available

data on rock and fluid properties. Pools

with strong aquifers, or intense fracture

5

systems which impacted on the well

performance were generally omitted. An

attempt was made to select wells drilled

parallel to the natural fracture system; this

was assumed to be SW-NE;

and

especially

in the case of the UBD, wells were selected

that were drilled using similar drilling

practices.

Once the wells were selected, an attempt

was made to obtain more detail information

on them. A questionnaire was developed

and sent to the operators requesting specific

information on the wells. The data

requested included general well data such

as spud dates, elevations, drilling data,

including mud systems, kick off point,

intermediate casing size, shoe depth, etc.,

length (gross and net)

of

the horizontal

lateral. Reservoir data include rock and

fluid

properties, pay thickness, reservoir

pressures, etc.. This data was used in

conjunction with the historical data

obtained for the pools. The final

and

perhaps most important data was the

operation

and

production performance

of

the wells. This consists of production

rates, oil and water fluid levels, casing and

tubing well head pressures, location of

subsurface pump etc ..

The data was all compiled in a spread

sheet. This permitted calculations

of weight

ofhydrostatic columns, flowing bottom hole

pressures, and productivity indices. In

addition, parameters such as drilling time

were examined.

A final selection of 34 wells were used for

the calculation

of

mechanical skin. The

formation in which the wells which were

finally selected, were drilled are as follows:

Midale 19 underbalanced

6 overbalanced


6/14

Frobisher/Alida

5 underbalanced

4

overbalanced

RESERVOIR DESCRIPTION

The

Midale Fonnation

has two normally

pressured units. The lower unit is a vuggy

limestone (named the Vuggy)

of

about

50md permeability. This is overlain by a

marly dolomite

of

about 1

Omd

permeability

(named the

Marly).

The two units are in

pressure communication. Most

of

the

unswept oil in these mature pools is

believed to be in the Marly, and it is the

main target for the horizontal wells. The

Vuggy is naturally fractured

and

in most

waterflood areas are almost totally swept.

The Marly is

not as extensively fractured.

Horizontal wells in the Marly drilled

parallel to the fractures produce less water

than those drilled normal to the fractures.

The Frosbisher/Alida is a tight Vuggy

carbonate with permeability in the 5

to

20md range. Fracture trend is again

generally in the South West to North East.

PERFORMANCE PARAMETERS

The following is a discussion on the

parameters used in our evaluation

of

short

term

and

long term benefits.

DRHLlNG TIME

The spud date and date drilling was

completed, which is provided by the SEM,

gave the total drilling time. However,

tour reports were

not

examined to

establish the time spent drilling the

horizontal lateral,

or

to

obtain actual rates

of

penetration ROP). The impact

of U D

on drilling time is qualitative. Drilling

times are indicated in Table 1.

INITIAL RATES

The initial production rate

of

the

horizontal well generally is a good indicator

of

the well performance and success

of

the

drilling program. Moreover,

it

has a direct

impact on the economics

of

the project. In

conventional overbalanced drilling a

mud

system is selected which will

do

least

damage to the productive formation since

stimulation

of

the wells are generally

costly. The objective

of

underbalanced

drilling is to totally eliminate this damage.

The average rate

of

the first three months

of

production was used as the initial rate.

Initial rates are shown in Table 1

ACTUAL PRODUCTIVITY INDEX (PIac)

The

actual

productivity index is the

ratio

of

the producing rate

and

the

difference between the average reservoir

pressure and the flowing bottom hole

pressure. In most cases the average

reservoir pressures were supplied by the

operators. However, most

of

the wells

considered are on pump . This makes

direct measurement

of

the flowing bottom

pressure inconvenient. This pressure was

calculated

from the fluid

level

measurements. Table 1 shows the results

of

the Plac calculation.

THEORETICAL PRODUCTIVITYINDEX

(PIth)

The two phase equations used to calculate

the PIth are as follows:

PItotal

=

Ploil PIwat

er

Eqn Ia


7/14

P o i l = 0. 00707kromax*kav*h)

l(p.o*Bo*ln(reholrw'oil) Eqn. Ib

PIwater=(0.00707krwmax*kav*h)1

(p.w*Bw*ln(rehwlrw water) Eqn. Ic

TOTAL MECHANICAL SKIN

The

two

phase equations used to estimate

the Skin are as follows:

St=So*Lo)+(Sw*LwI(Lo*Lw) Eqn. 2a

So =[(0.00707*kromax*kav*h)1

(Ploil*p.o*Bo)]- Eqn. 2b

In(reholrw-oil)

Sw =[(0.OO707*krwmax*kaw*h)1

(PIwater*p.w*Bw ]- Eqn. 2c

In(rehwlrw'water)

Lo = Lt*[(krwmax*p.o*Qo)1

(kromax*p.w*Qw ]

Eqn.

2d

Lw = Lt - Lo

Eqn.2e

Appendix provides additional equations

and definitions

RESULTS

Total skin factor for the underbalanced

and overbalanced drilled wells in the

Midale and Frobisher/Alida formations are

shown in Tables

1

Minimum, average, and maximum total

skin factors for the underbalanced and

overbalanced drilled wells in the Midale

and Frobisher/Alida were calculated by

assuming that:

the average total skin is equated to the

7

statistical mean total skin factor

the maximum and minimum total skin

factors equate to one standard deviation

above and below the statistical mean

respectively. These results are presented

in Table 2 The total skin factors are also

compared in Figures 1.1 and 1.2

Table 1 also shows the three other

parameters used in qualitatively comparing

underbalanced and overbalanced drilled

wells. The data

was

not analyzed

statistically. However, it does provide a

''feel'' for the performance of these wells.

Underbalanced wells drilled in the Midale

formation appear

to

perform better than

overbalanced wells, both in initial rate and

rate of penetration categories. The

difference is not as obvious in the

Frobisher/Alida formations.

DISCUSSIONS OF RESULTS

Due

to

the many assumptions made in

calculating the total skin factors, the total

skin factors are best used

to

compare

underbalanced wells relative to

overbalanced drilled wells.

Table 1 and Figures 1.1 and 1.2 indicate

there is no obvious reduction in total skin

factor in the underbalanced drilled wells

when compared to the overbalanced drilled

wells in either the Midale or


Table also indicates that the skin factors

are lower in the higher permeability

Frobiser/Alida formation. Also, there is

evidence that overbalanced total skin

factors are slightly lower than


8/14

underbalanced total skin factors for the


Figure

1.1

and 1 2 show that

overbalanced and underbalanced total skin

factor probability distributions have a

similar character both in the Midale and

Frobisher/Alida formations. The

probability distribution bell for the

underbalanced will be well to the left if the

skin factor was significantly less

The study indicates that for the wells

investigated underbalanced drilling did not

result in a reduction in skin in either the

Midale or Frobisher/Alida formations.

Some of the reasons for the results

are as

follows:

ethe theoretical equations developed nd

the quality of the pressure d t used was

not accurate enough to calculate the

variation

in total skin in the clean

relatively

medium-high penneabiIity/high

pressure carbonate reservoirs .

eduring the drilling operations

it

is still

difficult

to maintain

the underbalanced

conditions especially when connections are

made

Fluid invasion when this occurs

may be resulting in some damage since the

drilling fluid generally has no material to

fonn a protective filter cake

ethe underbaIanced pressure differential

h s reduced to a level at which

countercurrent spontaneous imbibition

(Benion et

l 1994

is possible.

This will

result in a degree of ormation damage

ethe formations are not significantly

damaged

by

the present overbalanced

drilling techniques

8

CONCULSIONS

1. The study confirms, qualitatively, that

in the S Saskatchewn Mississippian

formation the drilling time for wells drilled

underbalanced

was

generally less than the

time required to drill overbalanced.

2. The initial production rate of wells

drilled underbalanced in the Midale

formation is two to three times greater than

the wells drilled overballanced.

3. The actual Productivity Index of

underbalanced wells drilled in the Midale

formation was generally superior to the

overbalanced drilled wells. This was not as

evident in the Frobisher/Alida formation.

4. The mechanical skin calculation did not

clearly indicate that underbalanced drilled

wells had lower skin factors than wells

drilled overbalanced in either the Midale or


5. Possible reasons for the results of the

skin factor are as follows:

Sa) The theoretical/analytical 2-phase

equations developed are not accurate

enough to properly calculate the

underbalanced and overbalanced skin

factors in the Mississippian formations in

S Saskatchewan.

Sb) The data available

for

calculating

flowing bottom hole pressures, and the

available data for reservoir pressure may

require refinement.

Sc) Underbalanced wells evaluated in the

study may be experiencing fluid loss during

the drilling operation which is resulting in

formation damage.


9/14

5d) The overbalanced drilled horizontal

wells analyzed in the study are not severely

damaged

by

the drilling fluids used.

ACKNOWLEDGMENTS

The authors sincerely thank the

individuals and companies who responded

to the questionnaires with information on

their projects.

We also thank our

employees BP Exploration, (UK),

Northland Wireline Service for giving us

the time and resources

to

work on this

paper. Finally,

we

would like

to

thank

Zelda Blanchard and Erwine Springer who

handled the typing and drafting

of

the

paper.

REFERENCES

Deis,

P

et al. Infill Drilling in the

Mississippian Midale Beds

of

the Weyburn

Field Using Underbalanced Horizontal

Drilling Techniques CADE CAODC Paper

No: 93-1105 presented at the CADE

CAODC Spring Drilling Conference April

14,15,16, 1993,

Calgary, Alberta

Bennion,

D.

B. et

ale

Underbalanced

Drilling of Horizontal Wells - Does It

Totally Eliminate Formation Damage?

Paper HWC 94-95, presented at the

Canadian

SPE CIM

C NMET

International Conference on Recent

Advances in Horizontal Well Applications,

March 20-23, 1994.

Joshi,

S D

Horizontal Well

Technology Pennwell Books 1991

Lunan. B Underbalanced Drilling -

Surface Control System. Paper HWC 94-

20 presented at the Canadian SPE

el

CANMET International Conference on

Recent Advances in Horizontal Well

Application, March 20-23, 1994.


10/14

.

W

W

W

W

W

W

W

W

W

W

W

W

W

W

W

W

W

W

WI

WI

WI

W

WI

W

W

SAMPLE

OF

HORIZONTAL WElLS

DRIlLED

IN SE SASKATCHEWAN

MIDALE FROBISHER/ALIDA FORMATIONS TABLE 1

SPUD FINAL

DRIlL LENGm

FORM. INIT. PROD Plac SKIN

DATE DRIlL TIME

HORIZ. OB/DB RATE M/D

M/D/kPa

DATE DAYS LAT M) OIL- WATER

93-09-14

93-09-23

9

525 MIDALE *UB

74.2 17.6

0.0132

1

93-08-22

93-09-08

7

N/A

MIDALE *UB

40.2-52.3

0.0210 -0.88

93-03-18 93-03-24 6 1692 MIDALE *UB 68.9-70.4 0.0153 -0.76

93-08-03

93-08-23

20

974 MIDALE *UB

88.5 14.7 0.0253 -0.70

93-08-03

93-09-07

3 963

MIDALE

*UB 49.7 34.2

0.0169 -0.66

93-03-26

93-04-02

7

727 MlDALE

*UB

43.7 56.2

0.0085

-0.31

93-05-30

93-06-10

2

667

MlDALE

*UB

3.4-100.6 0.0276 -0.09

92-10-29

92-11-06

8

725 MlDALE

*UB

9.3 34.6

0.0121

0.25

93-06-07

93-06-21

4 1000 MIDALE *UB

79.9 18.2 0.0151 0.28

92 11 22

92 12 12 20 1018

MIDALE *UB

10.3-10.4

0.0033

1.07

93-09-08

93-09-18

10

1006

MIDALE *UB

37.8 26.1

0.0055

1.71

93-02-07

93-02-22

5

212

MIDALE *OB

2.2 33.2

0.0004

1.14

91-10-09

91-10-22 3

574

MlDALE

*OB

10.1-15.2

0.0091 -0.55

92-03-15

92-03-30

5

1294

MIDALE *OB

86.8-10.5

0.0242

-0.40

91-05-10

91-05-22

2

500

MlDALE

*OB

22.4-0.6

0.0007

0.16

92 11 21

92 12 11

20

N/A

MlDALE *OB

33.4-25.0

0.0137

0.27

92 0 25 92-10-13 8

495

MIDALE *OB

23.5 3.5

0.0051 1.40

93-02-22

93-03-01

7

N/A

FROB/ALlD *UB

9.5 3.1

0.0067 -0.88

93-09-14

93-09-25

11 334

FROB/ALID *UB

52.8-0

0.0133 -0.65

93-10-17

93-10-26

9

575 FROB/ALlD *UB

1.6 7.2

0.0673 0.15

93-07-11

93-07-19 8

405 FROB/ALlD *UB

48.6-0.5

0.0163

0.50

93 11 28

93-12-08

10 279 FROB/ALlD *OB

63 2.5

0.0172

1.68

93-01-23

93-02-05 3

748 FROB/ALlD *OB

38.7-37.0

N/A

-0.34

93-12-20

94-01-08 9

569

FROB/ALlD *OB

28.8 141.6

0.0388 -0.24

93-12-03

93 12 18

5

3 FROB/ALID *OB

48.9 83.2

0.0189 -0.05

10


11/14

TOTAL SKIN R NGES

MID LE ND FROBISHER/ALIDA

TABLE 2

FORM TION

TECHNIQUE

MIN. SKIN VER GE SKIN M XIMUM SKIN

MID LE

FROBISHER/ALIDA

UNDERBALANCED

OVERBALANCED

UNDERBALANCED

OVERBALANCED

0.69

-1.06

-1.08

-1.22

0.12

-0.26

0.43

0.58

PPENDIX 1.1

CTU L

PI CALCULATION

Pwf BOVE

BUBBLE POINT

PItotal ac) = Qtotal/ Pres. - Pwf)

WHERE

Qtotal

=

Qoil Qwater

Pres.

=

Static Reservoir Pressure

Pwl = Flowing

Bottomhole Pressure

Pwf B E WW

BUBBLE POINT

Pltotal ac) = Qtotal/[ Pres. - Pb) + PIT - Pw.f)/ 2*Pb)]

ND

Ploil ac) = Pltotal il Cut Fraction)

Plwater ac) = Pltotal Water Cut Fraction)

11

0.93

0.54

0.22

0.06


12/14

TOTAL SKIN CALCULATION**

St

=So*Lo +

(Sw*Lw))I(Lo+Lw)

WHERE

APPENDIX 1.2

Sw

=

[0.00707*krwmax*kav hl(Plwater*llw*Bw)]-ln(rehw/rw water)

So = [0.00707*kromax*kav hl(Ploil

11

Bo)]-ln (reho/rw oil)

o =

Lt [(krwmax*llo*Qo)l(kromax IlW

Qw)]

Lw = Lt - Lo

Additional equations, definitions, units, assumptions and the derivation of each parameter are discussed below:

ADDITIONAL EQUATIONS

reho

= [Lo +

660) 1320)1Jf/.5

rehw

= [Lw +

660)

1320 1JT/5

rw oil = (reho LoI2)I[ao(1

+1

(1-(LoI2ao)2)(JJhI2r

w

)(jJhILo)]

rw water

=

(rehw LwI2)/[aw(1 +J(1-(LwI2awf)( fJh/2r

w

)(jJhILW)j

ao

=

(LoI2) [0.5

+

(0.25

+

2

rehoILo/)0.5t.5

aw

=

(LwI2)

[0.5 +

(0.25

+

(2rehwlLw )0.51.5

kh

= kx +

ky)12

kav

= (kv kh 0.5

jJ

= (khlkv

0.5

**(Development in imperial units)

2


13/14

ao

=

aw

Bo

Bw

h

kav =

kh

kromax

krwmax =

kv

kx

ky

Lt

Lo

Lw

Pb =

Pres

Pwf

PI

PItotal =

PIoil

PIwater

Qt

=

Qo

Qw

reho

rehw

rw

re oil

rw water =

St

So

Sw

f l=

p o

p w =

APPENDIX 2

DEFINITIONS

AND

UNITS

Half of

major axis

of

drainage ellipse round the oil producting section (ft) (m)

Half of

major axis

of

drainage ellipse roung the water producting section(ft) (m)

Formation volume factor of oil (reservoir bblslstock tank bbls) (reservoir

m

3

1stock tank m

3

)

Formation volume factor

of

water(reservoir bblslstock tank bbls)(reservoir

m

3

1stock aux m

3

)

Stratigraphical thickness of unit (ft) (m)

Average permeability (millidarcy)

Average horizontal permeability (millidarcy)

Maximum (end point) relative permeability to oil

Maximum (end point) relative permeability to water

Vertical permeability (millidarcy)

Maximum horizontal permeability (millidarcy)

Horizontal permeability perpendicular to maximum horizontal permeability

(millidarcy)

Length of horizontal section through the payzone (ft) (m)

Length of horizontal section producing oil (ft) (m)

Length of horizontal section producing water (ft) (m)

Bubble point pressure (psia) (kPa)

Reservoir pressure (psia) (kPa)

Bottomhole flowing pressure (psia) (kPa)

Productivity Index (stock tank bbls/daylpsi)(stock tank m

3

Iday(kPa)

Total Productivity Index (stock tank bblsldaylpsi)(stock tank MldaylkPa)

Oil Productivity Index (stock tank bblsldaylpsi)(stock tank m

3

ldaylkPa)

Water Productivity Index (stock tank bblsldaylpsi)(stock tank m

3

ldaylkPa)

Totalflowrate (stock tank barrels per day) (stock tank ~ / d a y

Oil flowrate (stock tank barrels per day) (stock tank /day)

Water flowrate (stock tank barrels per day)

Effective horizontal drainage radius (oil) (ft) (m)

Effective horizontal drainage radius (water) (ft) (m)

Wellbore radius (ft) (m)

Effective wellbore radius (oil) (ft) (m)

Effective wellbore radius (water) (ft) (m)

Total mechanical skin

Mechanical skin due to oil production

Mechanical skin due to water production

Square root of horizontal permeability divided by vertical permeability

Oil viscosity at reservoir permeability divided by vertical permeability

Water viscosity at reservoir conditions (centipoise)

3


14/14

TOTAL SKIN PROBABILITY DISTRIBUTION

MIDALE FORMATION

UNDERBALANCE AND OVERBALANCE DRILLED WELLS

0 35 r ____.

.. ...

C

o

0 3

:P

0.25

u..

........ 0 2

1 0 15

D

o 0.1

a:

CL

0 05

UNDER

BALANCE

~

/ \

I

I

,

,

a

o ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~

3

2

1

o

2

FIG

1.1

TOTAL SKIN

TOTAL SKIN PROBABILITY DISTRIBUTION

FROBISHER/ ALIDA FORMATION

UNDERBALANCE AND OVERBALANCE DRILLED WELLS

3

0 8

r-----------------------------

.. ...

C

o

0 6

~ 0 4

:::i

D

D

o

0::

0 2

CL

BALANCE

o ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~

2

-1.5

1

o 0 5 1 5

2

FIG

1.2

TOTAL

SKIN

Documents

Paper on Refining Oil