Reservoir Characterization
Using Production Performance
Case History: North Robertson Unit (NRU)
(Clear Fork Formation, West Texas)
Tom Blasingame
Department of Petroleum Engineering
Texas A&M University
College Station, TX 77843-3116
(979) 845-2292 — t-blasingame@tamu.edu
Reservoir Characterization — NRU (West Texas) — Slide 1
NRU Example: Introduction
 The NRU (Clear Fork) is a heterogeneous, low
permeability dolomite sequence. This particu-lar
unit includes approximately 150 production wells
and 110 injection wells over an area of about 5500
acres. The unit is currently under waterflood on a
20-acre spacing (5-spot).
 Justification for the study—poor pressure and
fluid communication appear to inhibit maximum
waterflood response. Some portions of the unit
are clearly worse than others—why?
 Work performed:
 Reserves estimated using well logs
 Pressure transient analysis
 EUR analysis: (q/p) versus Np
 Decline type curve analysis
Reservoir Characterization — NRU (West Texas) — Slide 2
 Mapping of reservoir properties
Reservoir Characterization — NRU (West Texas) — Slide 3
NRU Example: Description of Field Work
 In this study extensive efforts were made to
properly allocate (or account) for all production
data. Although no measured pressure data were
available for the production wells, esti-mates of
bottomhole pressure were made using pump
records.
 In addition to the production data, all injection data
were analyzed using the decline type curve
analysis approach. This provided assist-ance in
designing well stimulation and in the confirmation
of reservoir properties.
 Numerous pressure transient tests were run on
production and injection wells in order to con-firm
reservoir pressure trends.
Reservoir Characterization — NRU (West Texas) — Slide 4
 After this study, 2 separate 10-acre infill pilots
were developed, both were targeted using the
results of this study.
Reservoir Characterization — NRU (West Texas) — Slide 5
NRU Map:
Production and Injection Well Configurations
 The unit is uniformly developed (except along some lease
lines), and the comparative focus will be on the trends of
computed reservoir properties.
Reservoir Characterization — NRU (West Texas) — Slide 6
NRU Example: Typical Production History
Plot of Long-Term Production Rate—NRU Clearfork Well
102
q, STB/D
Time Reinitializ ed - t 0 = 336 days
Well Produc ed at Cons tant Rate
101
Completion/Workover Reduces Dec line Rate
Primary Production Only
100
0
2000
4000
t, Days
6000
8000
 Flowrate decline is reasonably well behaved, but the change
of trend suggests that "conventional" extrapolations may not
be representative of the in-situ processes.
Reservoir Characterization — NRU (West Texas) — Slide 7
NRU Example: WPA Analysis (Production)
Type Curve Match of Production—NRU Clearfork Well
10-4
102
10-3
12
4
28
qDd, qDdi and qDdid
qDdi
qDd
101
80
qDd
28
80
160
100
160
100
80
12
103
102
101
Depletion "Stems"
(Boundary-Dominated Flow)
4
100
qDdid
800
qDdi Arps Equations
qDdid
reD=re/rwa=1x104
10-1
102
Results Legend: NRU W ell 1004 (Clearfork)
(q/p)MP /(qDd)MP = 0.013 STB/D/psi
(tbar)MP /(tDd)MP = 2000 Days
bpss= (qDd)MP /(q/p)MP = 1/0.013= 76.92 psi/STB/D
kh = 18.41 md-ft
s = -0.6
Nct = 26.0 STB/psi
Np,mov = 105,000 STB
800
1x104
28
101
Model Legend : Fetkovich/McCray Type Curve
Radial Flow, Circular Reservoir Solutions
Transient "Stems"
(Transient Flow)
12
160
800
1x104
10-1
Data Legend:
(q/p), STB/D/psi
(q/p)i, STB/D/psi
(q/p)id, STB/D/psi
qDdi
reD=re/rwa=4
10-2
10-1
qDdid Arps Equations
b=0
0.2
0.4
-2
10
0.6
qDdArps Equations b=0 0.2 0.4 0.6 0.8 1.0
-4
10
-3
10
-2
10
-1
10
0
tDd
10
1
10
2
10
0.8 b=1.0
10-2
3
10
 Note the excellent match of the production rate perfor-mance.
Reservoir Characterization — NRU (West Texas) — Slide 8
NRU Example: Typical Injection History
Injection Rate and Bottomhole Pressure—NRU Clearfork Well
 Injection rate decline and bottomhole pressure increases are
fairly well correlated, but how do we estimate reservoir
properties?
Reservoir Characterization — NRU (West Texas) — Slide 9
NRU Example: WPA Analysis (Injection)
Type Curve Match: Injection Rate/Pressure—NRU Clearfork Well
10-4
102
10-3
qDdi
10-2
Tran sien t "Ste ms"
r eD=r e/xf = 1 (Tran sien t Fl ow Regio n--
Ana lytical So luti ons)
1
qDd
2
qDd , qDdi , and qDdid
101
2
10-1
100
Leg end: Data Fu nctions
(qwi /p) Data
(qwi /p)i Data
(qwi /p)id Data
qDd
5
5
10
103
102
Resul ts L egen d: Injectio n Data Ana lysis Exa mple
reD - 6 , (q/p)MP /( qDd)MP = 0.27 STBW/D/psi
(t bar)MP /( t Dd)MP = 1600 Days
Wtot cti = (0.27 STBW/D/psi)(1 600 Da ys )
or Wtot cti = 432.0 STBW/psi (Wmov - 1 .7 MMBW)
101
-2
bDps s = ln(6) - 0.0 4929 8 + 0.43 4645 (6) = 1.75 453
20
20
102
bps s = (qDd)MP /(q/  p)MP = 1 /0.2 7 = 3.70 37 p si/STBW/D
qDdi
10
101
k wh = 141.2 Bw w (bDpss / bpss ) = 56.7 md -ft
50
50
100
100
1x10 3
1x10 3
0
100
10
qDdid
20
qDdid
r eD=r e/xf = 1x10
10-1
2
5
1
Deple tion "Stem s"
(Boun dary-Domin ated Flo w
Regi on--Vo lume tric Reservoi r
Beh avior)
10
50
100
3
10-1
Fractured Well in
a Bounded Circular
Reservoir
b=0
Model Leg end: Fetkovich /McCray Type Curve -Fracture d We ll Cen tered in a Bou nded Circu lar Res ervior,
Infinite Cond uctivity Ve rtical Fracture Cas e
0.4
0.6
b=0 0.2
-2
10
0.2
-4
10
-3
10
-2
10
-1
10
0
t Dd
10
0.4 0.6
1
10
0.8
1.0
0.8
b=1. 0
2
10
10-2
3
10
 Note the excellent match of the injection performance.
Reservoir Characterization — NRU (West Texas) — Slide 10
NRU Example: Pressure Transient Test Results
Map of average reservoir pressure in 1988
- from pressure transient tests (Contour Interval = 200 psia).
psia
 Note the poor communication of pressure, especially in the
central part of the unit (high pressures compared to the better
producing regions). NO waterflood influence at this time—
depletion performance only.
Reservoir Characterization — NRU (West Texas) — Slide 11
Reservoir Characterization — NRU (West Texas) — Slide 12
NRU Example: Pressure Transient Test Results
Map of average reservoir pressure in 1995
- from pressure transient tests (Contour Interval = 250 psia).
psia
 After waterflood, pressures are again higher in the central part
of the unit—suggests significant reservoir hetero-geneity. The
central portion of the unit should NOT be considered for the
initial 10-acre infill program.
Reservoir Characterization — NRU (West Texas) — Slide 13
NRU Example: Decline Curve Analysis Results
Map of flow capacity (koh) for 40-acre wells,
decline type curve analysis (Contour Interval = 10 md-ft)
md-ft
 Note the "fairways" of high koh in the northwest and southern
portions of NRU.
Reservoir Characterization — NRU (West Texas) — Slide 14
NRU Example: Decline Curve Analysis Results
Map of contacted oil-in-place (OOIP) for 40-acre wells,
decline type curve analysis (Contour Interval = 1.0 million bbls)
MMSTB
 Note the "fairways" of calculated oil-in-place (OOIP) trends in
the northwest and southeast portions of NRU, similar to the
calculated koh trends.
Reservoir Characterization — NRU (West Texas) — Slide 15
NRU Example: Decline Curve Analysis Results
Map of movable oil-in-place (OOIP) for 40-acre wells,
EUR analysis (Contour Interval = 70,000 bbls)
MSTB
 Note the very strong correlation of EUR results (which are
essentially empirical) with the results of the decline curve
analysis (which are semi-analytical). Central portion of the unit
Reservoir Characterization — NRU (West Texas) — Slide 16
appears to be of poor reservoir quality—should be avoided for
infill drilling programs.
Reservoir Characterization — NRU (West Texas) — Slide 17
NRU Example: Decline Curve Analysis Results
Map of flow capacity (koh) for 20-acre wells,
decline type curve analysis (Contour Interval = 10 md-ft)
md-ft
 Note the excellent comparison of the koh trends with the
calculated oil-in-place (OOIP) trends for the 20-acre wells.
Also note the reappear-ance of the "fairways" in the Northwest
Reservoir Characterization — NRU (West Texas) — Slide 18
and Southern portions of the unit, reiterating the control of koh
of reservoir performance.
Reservoir Characterization — NRU (West Texas) — Slide 19
NRU Example: Decline Curve Analysis Results
Map of contacted oil-in-place (OOIP) for 20-acre wells,
decline type curve analysis (Contour Interval = 1.0 million bbls)
MMSTB
 Excellent comparison of the koh and OOIP trends. Fairly
strong correlation of OOIP results for the 40 and 20-acre cases,
which implicitly indicates the control of koh on rate and
pressure performance—even for secondary recovery!
Reservoir Characterization — NRU (West Texas) — Slide 20
NRU Example: Decline Curve Analysis Results
Map of movable oil-in-place (OOIP) for 20-acre wells,
EUR analysis (Contour Interval = 70,000 bbls)
MSTB
 EUR results again correlate well with the OOIP results from
decline type curve analysis. Note the excellent comparison of
these results with the koh trends for the 40-acre wells—
conclusion, koh controls performance.
Reservoir Characterization — NRU (West Texas) — Slide 21
NRU Example: Results of 10-Acre Infill Program
Preliminary Results — Ten-Acre Infill Wells (November 1996)
2500
15
2000
12
1500
9
Wells on Production
Production Rate, STB/D or BW/D
North Robertson Unit - Field Demonstration
Preliminary Results of 10-Acre Infill Drilling Program
Nov. 1, 1996
1000
6
~ 900 ST B/D
Incremental Production
500
3
inc remental oil
inc remental water
wells on production
May 8, 1996
0
0
0
30
60
90
120
150
180
210
Time, days
 Average "incremental" oil production is ≈ 70 STB/D for the last
4 months. This performance is on par with the initial well
completions made in 1957!
Reservoir Characterization — NRU (West Texas) — Slide 22
Summary:
 All of the maps of computed properties (both 20
and 40-acre well data) confirm the major fea-tures
in the NRU reservoir sequence. Further, the maps
of the 1991 and 1995 pressure distri-butions also
confirm the large-scale hetero-geneities in the
reservoir.
 This study was a case history of "what went
right"—the operator made every effort to ac-quire
all data necessary to characterize the reservoir,
and the integration of these analyses led to a
comprehensive
"roadmap"
for
con-tinued
development and management of the NRU
reservoir sequence.
 Wells in the 10-acre targeted infill program (based
largely on the performance-based characterization)
Reservoir Characterization — NRU (West Texas) — Slide 23
show initial potential similar to wells completed
over 40 years ago.
Reservoir Characterization — NRU (West Texas) — Slide 24