paper on refining oil
TRANSCRIPT
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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
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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
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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.
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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:
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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
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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
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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
Frobisher/Alida formations.
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
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underbalanced total skin factors for the
Frobisher/Alida formations.
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
Frobisher/Alida formations.
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.
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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.
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.
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
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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
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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
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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
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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