Note: Descriptions are shown in the official language in which they were submitted.
WO 2022/256485
PCT/US2022/031895
U.S.
PATENT
APPLICATION
TITLE: UNCONVENTIONAL WELL GAS TO OIL RATIO CHARACTERIZATION
INVENTORS: Yongshe LIU, Brian COFFMAN, Nathan B. MCMAHAN, Ali sdair FARTHING
ASSIGNEE(s): ConocoPhillips Company
CERTIFICATE OF ELECTRONIC FILING VIA EFS 37 CFR 1.8
I HEREBY CERTIFY THAT I HAVE A REASONABLE BASIS FOR BELIEF THAT THIS
CORRESPONDENCE IS BEING SUBMITTED TO THE UNITED STATES PATENT AND
TRADEMARK OFFICE VIA EFS (ELECTRONICALLY) ON THE DATE INDICATED BELOW,
AND IS ADDRESSED TO:
Commissioner for Patents
P.O. Box 1450
Alexandria, VA 22313-1450
Signature: / Jeanine Richardson/
DATE OF SUBMISSION: 02.06.2022
ELECTRONIC FILING (EFS)
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UNCONVENTIONAL WELL GAS TO OIL RATIO CHARACTERIZATION
PRIOR RELATED APPLICATIONS
[00001] This application claims priority to 63/196,648, filed June 3, 2021,
and incorporated by
reference in its entirety for all purposes.
FEDERALLY SPONSORED RESEARCH STATEMENT
[00002] Not applicable.
FIELD OF THE DISCLOSURE
[00003] The disclosed methods relate generally to the optimization of a
reservoir production, in
particular gas to oil levels.
BACKGROUND OF THE DISCLOSURE
[00004] With technically recoverable reserves estimated by United States
Geological Survey
(USGS) to be between 4.4 and 11.4 billion barrels, Bakken is one of the
earliest hybrid
unconventional plays to be developed into a mature asset by the industry. The
Williston Basin
contains a complete stratigraphic record from the Cambrian to Tertiary with
sediment thickness of
over 16,000 feet with multiple conventional and unconventional targets that
have been exploited
over the last 40 years.
[00005] The Bakken Formation within the Williston Basin has three main
reservoir targets and
two potential source rocks (FIG. 1A-B): the Upper Bakken Shale and the Lower
Bakken Shale
source rocks were deposited in a sub-oxic to anoxic offshore marine
depositional environment
with a stratified water column, whereas the Middle Bakken (MB) member was
deposited in a
marine to marginal setting under oxic conditions. The units of the Three Fork
members like the
Upper Three Forks (UTF) and the Middle Three Forks (MTF) are mainly cyclical
deposits of wind-
blown silts deposited in shallow wet lacustrine environment that are
interbedded with green silty
dolomitic claystones.
[00006] The Williston basin and the Bakken formation horizontal development
started in the
early 2000's and ramped up into the latter part of the decade. Many of these
wells were drilled in
single well drilling space units (DSUs) with smaller intensity, sleeve
completion designs. The
most common artificial lift (AL) used early on was rod pump. As field oil
production ramped up
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in 2015, the associated gas also peaked, but the gas to oil ratio (GOR) was
relatively stable or
slowly increasing. See FIG. 2.
1000071 As more development and investment was made in 2017/2018, we also saw
more
intense completion designs, tighter well spacing, AL optimization, and
depletion effects with it.
As the oil peaked again in 2019, this time the fi el dwi de GOR was much
higher and with that came
record high gas production. Many operators and gas gatherers capacity were
overwhelmed, and
higher flaring was seen throughout the field. This in conjunction with the
higher attention on the
environment created a greater focus and collaboration in Bakken to handle the
gas. As more
offtake capacity was created, operators also came forward with innovative
solutions and uses for
the excess gas.
1000081 We continue to see ever increasing GOR and gas rates throughout the
field, highlighting
the importance of accurate forecasting and prediction of GOR. The proper
forecasting will allow
us to maximize production, increase economics, and do so in an environmentally
friendly manner.
This invention addresses one or more of these needs.
SUMMARY OF THE DISCLOSURE
1000091 With the increased focus on the environment, the oil and gas industry
is taking
aggressive action to reduce greenhouse gas (GHG) and flare emissions. As an
important part,
characterizing the GOR from Unconventional Resources (UR) wells will help
predict long-term
gas production trends and develop high GOR mitigation strategies. In this
disclosure, the short-
and intermediate-term GOR behaviors are discussed based on Bakken wells with
different
completion designs and the key drivers and mechanisms dictating the GOR trends
are investigated.
A novel modeling workflow is developed to match GOR & other observed data and
forecast long-
term GOR trends. Potential techniques to mitigate rising GOR are also
proposed.
1000101 The GOR data from a large number of wells were analyzed to identify
GOR correlations
with fluid properties, completion design, depletion, drawdown strategy and
artificial lift. Multiple
types of data were collected in this study in conjunction with production
data, including long-term
bottom hole pressure (BHP), pressure volume, temperature data (PVT),
interference test and shut-
in data. Data driven reservoir models were built to match the GOR behaviors
for both old sliding
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sleeve completion wells and modern plug-and-perf completion wells. Long-term
GOR trends were
predicted with the calibrated reservoir model.
1000111 The key insights and conclusions from this work included: 1) Bakken
unconventional
GOR shows conventional black oil reservoir GOR behaviors, but it also has many
unconventional
characteristics due to the low matrix permeability and different types of
hydraulic fracture systems
created. 2) GOR trends are sensitive to pressure drawdown. Strong correlations
between GOR
and BHP are observed. 3) Old sliding sleeve completion well GOR shows cyclical
rise-and-fall
patterns with a gradually increasing long-term trend, whereas modern
completion wells show
rising GOR trend after their BHPs drop below bubble point pressure (Pb) and
then reach an
intermediate-term plateau. 4) Offset parent well depletion drives child well
GOR to rise faster and
to a higher level. 5) Less aggressive drawdown, recharging due to frac hits
and long shut-in can
delay/mitigate GOR rising. Finally, 6) Bakken well GOR behaviors can be
accurately modeled
using the proposed approach.
1000121 The learnings presented herein improve our understanding of
unconventional well GOR
trends in the industry. Better GOR characterization will improve forecasting
for offtake capacity
and flare emission reduction and help the industry to meet environmental
standards. The analysis
and modeling approaches proposed herein can also spark further research and
development
activities.
1000131 Although GOR is specifically exemplified herein, the method is not so
limited and can
be used to optimize other well parameters, as desired by the operator.
1000141 The present methods include any of the following embodiments in any
combination(s)
of one or more thereof:
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A method of optimizing oil production from a well in a reservoir, said method
comprising:
a) providing a model of said well;
b) inputting well data for a plurality of wells into said model, said well
data selected from
geological layers, reservoir properties, fracturing data, completion data,
permeability data,
geochemistry, and combinations thereof;
c) inputting historical production data from said plurality of wells into
said model, said historical
data selected from PVT data, BHP, oil production rates, gas production rates
and water
production rates, or combinations thereof;
d) controlling said model to match one or more parameters selected from
production rates,
gas to oil ratio (GOR), bottom hole pressure (BHP), cumulative oil production
(COP), or a
combination thereof in a probabilistic manner to obtain a plurality of
historical models;
e) verifying one or more test models against said historical models to
identify an optimal model
with minimum error;
f) using said optimal model to predict one or more parameters selected from
production rates,
gas to oil ratio (GOR), bottom hole pressure (BHP), cumulative oil production
(COP), or a
combination thereof from said well into a future;
g) optimizing a production plan using said predicted parameters;
h) implementing said optimized production plan in said well, whereby oil
production is
optimized as compared to similar well produced without said optimized
production plan.
A method of reducing GOR in oil production from one or more well(s) in a
reservoir, said method
comprising:
a) providing a model of said one or more well(s);
b) inputting well data from one or more well(s) into said model, said well
data selected from
geological layers, reservoir properties, fracturing data, completion data,
permeability data,
geochemistry, and combinations thereof;
c) inputting historical production data from said one or more well(s) into
said model, said
historical data selected from PVT data, BHP, oil production rates, gas
production rates and
water production rates, and combinations thereof;
d) controlling said model to match one or more parameters selected from
production rates,
gas to oil ratio (GOR), bottom hole pressure (BHP), cumulative oil production
(COP), or a
combination thereof in a probabilistic manner to obtain a plurality of
historical models;
e) verifying one or more test models against said historical models to
identify an optimal model
with minimum error;
f) using said optimal model to predict gas to oil ratio (GOR) from said one
or more well(s) into
a future;
g) optimizing a production plan using said predicted GOR to reduce gas
production;
h) implementing said optimized production plan in said well, whereby GOR is
reduced as
compared to said predicted GOR.
Any method herein described, wherein said reservoir is an unconventional
reservoir.
Any method herein described, wherein said reservoir is a hybrid shale and
limestone reservoir.
Any method herein described, wherein said model in a) has finer gridding near
said well.
Any method herein described, wherein said finer gridding is Tartan gridding.
Any method herein described, wherein said optimized reservoir plan includes
reduced drawdown to
reduce GOR.
Any method herein described, wherein said optimized reservoir plan includes
widening fracture
cluster spacing to reduce GOR.
Any method herein described, wherein said optimized reservoir plan includes
recharging high GOR
parent wells with offset child well fracture hits to reduce GOR.
Any method herein described, wherein said optimized reservoir plan includes
extending well shut-in
to reduce GOR.
1000151 As used herein, a regular grid is a network of crossing right angle
lines, such as is seen
on graph paper. In reservoir modelling, grids may be two dimensional and need
not be at right
angles or regular. A common requirement in reservoir simulation is an
increased level of detail
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around an item of interest such as a well. A "tartan" grid has variable height
and/or width of the
lines and is a gridding style available in some reservoir modelling programs.
1000161 As used herein, a "reservoir" is a formation or a portion of a
formation that includes
sufficient permeability and porosity to hold and transmit fluids, such as
hydrocarbons or water or
natural gas, and the like.
1000171 A reservoir can have a plurality of chemically distinct "zones"
therein, particularly in
very tight rock, where mixing is almost non-existent. The data herein can be
catalogued by zone,
allowing that portion of the data to be used for other zones, even in other
wells, as long as the zone
has similar fingerprints.
1000181 A "production plan" can include placement of wells, length of well,
depth of well,
completion details, enhanced oil production methods, stimulation methods,
fracking methods,
order of completion, production rate, and the like. Production plans can also
include well stacking,
well spacing, completion designs (frac job types, job size, number of stages,
number of clusters
per stage, etc.) and strategies (e.g., at what sequence to frac different
target zones, how to
synchronize/coordinate with nearby wells, alternating or zipper fracking,
etc.), production well
pressure management, enhanced oil recovery strategies, and the like.
1000191 An "optimized- production plan is generated using well predictions and
modeling to
improve the simulated production from a well. Once a well plan is optimized,
it may then be
implemented at the well, at a well pad with multiple wells, or in an area
penetrating one or more
reservoirs and used to produce hydrocarbons or other reservoir fluids.
1000201 To "implement" an optimized plan means to actually drill and/or
complete a well or
wells according to the plan and then produce hydrocarbons from that well.
1000211 Reservoir performance during primary depletion is controlled largely
by the natural
drive mechanisms present. bOnce the drive mechanisms are known, material
balance methods may
be used to analyze and predict reservoir performance. Drive mechanisms are the
natural sources
of reservoir energy which cause oil and gas to flow into a wellbore. The three
primary reservoir
drive mechanisms are solution gas drive, gas cap drive, and water drive.
Gravity drainage is a
secondary drive mechanism capable of improving recovery in steeply dipping or
high permeability
reservoirs. The active drive mechanisms can often be identified from a
reservoir's gas/oil ratio,
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reservoir pressure, and production rate histories. Early identification of the
active drive
mechanisms may be important to optimize a reservoir's performance.
[00022] As used herein, "cumulative oil" or "Cumulative Oil Produced" (COP) is
the total
amount of oil produced over time.
[00023] As used herein, "cumulative gas" is the total amount of gas produced
over time.
[00024] As used herein, "water cut" is the ratio of water produced compared to
the volume of
total liquids produced.
[00025] As used herein, "production rate" is the rate of fluid production from
the well.
Production rates can be adjusted by changing the amount of fluid produced and
are dependent
upon the reservoirs rate of inflow and bottom hole pressure. Inflow
performance relationship is
controlled by the ratio of bottom hole pressure to production rate.
[00026] As used herein, "Gas Oil Ratio" or "GOR" is the volume of gas that is
produced from
crude oil when the oil is being extracted from the reservoir to the earth's
surface through production
tubing. This is generally related to associated gas or saturated gas in the
oil reservoir. It is
represented as standard cubic feet per stock tank barrel (scf/stb).
[00027] The "associated gas" is natural gas that is dissolved in the oil and
is produced along with
the crude oil. Heavy crude oil has low API gravity and low capacities of
dissolved gas as compared
to lighter crude oil.
[00028] "Steam to Oil Ratio" or "SOR" is a measure used to quantify the
efficiency of
production of oil from a reservoir based on steam injection into the
reservoir. It can be defined as
the amount of steam injected to produce one unit volume of crude oil The steam
is quantified by
barrels of water used to make the steam, however. For example, a steam-oil
ratio is 4.5 means that
4.5 barrels of water¨converted into steam and injected into the well¨were
required to extract a
single barrel of oil.
[00029] "API gravity" measures the relative density of petroleum liquid and
water and has no
dimensions. To derive the API gravity, the specific gravity (SG) is first
measured using either the
hydrometer, detailed in ASTM D1298 or with the oscillating U-tube method
detailed in ASTM
D4052. The official formula used to derive the gravity of petroleum liquids
from the specific
gravity (SG), as follows: API gravity = 141.5/SG ¨ 131.5.
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[00030] A "core" or "rock core" is a sample of rock, typically in the shape of
a cylinder. Taken
from the side of a drilled oil or gas well, a core is then dissected into
multiple core plugs, or small
cylindrical samples measuring about 1 inch in diameter and 3 inches long.
[00031] "Drilling cuttings" or "cutting samples" are the small irregular rock
samples generated
during drilling and returned with the drilling mud.
[00032] As used herein, the term "fracture hit" was initially coined to refer
to the phenomenon
of an infill-well fracture interacting with an adjacent well during the
hydraulic-fracturing process.
However, over time, its use has been extended to any type of well interference
or interaction in
unconventional reservoirs.
[00033] By "obtaining" a sample herein we do not necessarily imply
contemporaneous sampling
procedures because existing samples can be used where available.
However, often
contemporaneous sample collection will be needed, except for core or cutting
samples, which may
already be available.
[00034] By generating a reservoir "map" we mean that the reservoir is
characterized in the three
directional axes as well as the fourth time axis, but we do not necessarily
imply a graphical
representation thereof, as data can be maintained and accessed in many forms,
including in tables.
The map may be segmented into zones, where the fingerprinting data is very
similar.
1000351 The use of the word -a" or "an" when used in conjunction with the term
"comprising"
in the claims or the specification means one or more than one, unless the
context dictates otherwise
[00036] The term "about" means the stated value plus or minus the margin of
error of
measurement or plus or minus 10% if no method of measurement is indicated.
[00037] The use of the term "or" in the claims is used to mean "and/or" unless
explicitly
indicated to refer to alternatives only or if the alternatives are mutually
exclusive.
[00038] The terms "comprise-, -have-, "include- and "contain- (and their
variants) are open-
ended linking verbs and allow the addition of other elements when used in a
claim.
[00039] The phrase "consisting of' is closed, and excludes all additional
elements.
1000401 The phrase "consisting essentially of' excludes additional material
elements, but allows
the inclusions of non-material elements that do not substantially change the
nature of the invention.
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Any claim or claim element introduced with the open transition term
"comprising," may also be
narrowed to use the phrases "consisting essentially of' or "consisting of,"
and vice versa.
However, the entirety of claim language is not repeated verbatim in the
interest of brevity herein.
1000411 The following abbreviations may be used herein.
ABBREVIATION TERM
AL ARTIFICIAL LIFT
API AMERICAN PETROLEUM INSTITUTE, ALSO API GRAVITY, IS A
MEASURE OF
HOW HEAVY OR LIGHT A PETROLEUM LIQUID IS COMPARED TO WATER: IF
ITS API GRAVITY IS GREATER THAN 10, IT IS LIGHTER AND FLOATS ON
WATER; IF LESS THAN 10, IT IS HEAVIER AND SINKS.
ASTM AMERICAN SOCIETY FOR TESTING AND MATERIALS
BBL BARREL
BHP BOTTOMHOLE PRESSURE, PSIA
BVO BULK VOLUME OIL
CPU CENTRAL PROCESSING UNIT
CGR CONDENSATE GAS RATIO¨CGR GIVES A MEASURE OF THE LIQUID
CONTENT TO THE VOLUME OF GAS. IT IS MEASURED IN BARRELS PER
MILLIONS OF STANDARD CUBIC FEET (BARRELS/MMSCF).
COP CUMULATIVE OIL PRODUCTION
CUM CUMULATIVE
DRV DRAINED ROCK VOLUMES
DSU DRILLING SPACING UNIT
EOS EQUATIONS OF STATE
ESP ELECTRICAL SUBMERSIBLE PUMP
GHG GREENHOUSE GAS
GOR GAS TO OIL RATIO¨WHEN OIL IS PRODUCED TO SURFACE
TEMPERATURE
AND PRESSURE IT IS USUAL FOR SOME NATURAL GAS TO COME OUT OF
SOLUTION. THE GAS/OIL RATIO (GOR) IS THE RATIO OF THE VOLUME OF
GAS THAT COMES OUT OF SOLUTION TO THE VOLUME OF OIL AT
STANDARD CONDITIONS (TEMPERATURE = 273.15K, PRESSURE = 1 BAR)
GPU GRAPHICS PROCESSING UNIT
GUI GRAPHICAL USER INTERFACE
LB POUNDS
LBS LOWER BAKKEN SHALE
MB MIDDLE BAKKEN
MBO THOUSAND BARREL OF CRUDE OIL
MTF MIDDLE THREE FORKS
NDIC NORTH DAKOTA INDUSTRIAL COMMISSION
PB BUBBLE POINT PRESSURE, PSIA
PI INITIAL RESERVOIR PRESSURE, PSIA
PV PRESSURE VOLUME
PVT PRESSURE VOLUME TEMPERATURE
PWF FLOWING BOTTOM HOLE PRESSURE, PSIA
RAM RANDOM ACCESS MEMORY
ROM READ-ONLY MEMORY
Rs/ INITIAL SOLUTION GOR, SCF/STB
SCF STANDARD CUBIC FOOT
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ABBREVIATION TERM
SG GAS SATURATION, FRACTION
SGC CRITICAL GAS SATURATION, FRACTION
SOR STEAM TO OIL RATIO
SRV STIMULATED ROCK VOLUMES
TLG TIME LAPSE GEOCHEMISTRY¨GEOCHEMICAL FINGERPRINTS TAKEN
FROM A PLURALITY OF SAMPLES COLLECTED OVER TIME
UBS UPPER BAKKEN SHALE
UR UNCONVENTIONAL RESOURCES
USGS US GEOLOGICAL SURVEY
UTF UPPER THREE FORKS
BRIEF DESCRIPTION OF THE DRAWINGS
1000421 FIG. 1A. Map showing the study area around Nesson anticline.
1000431 FIG. 1B Main reservoir formations in the Bakken.
1000441 FIG. 2. Historical Bakken oil, gas and flaring rates from NDIC.
1000451 FIG. 3. Old completion well GOR trends as a function of oil rate and
downhole
measured BHP (1st year data).
1000461 FIG. 4. Old completion well longer term GOR trends as a function of
oil rate and
downhole measured BHP (-5-year data).
1000471 FIG. 5. Modern completion well GOR trends as a function of oil rate
and downhole
measured BHP.
1000481 FIG. 6A. GOR trends by fluid property areas.
1000491 FIG. 6B. GOR rising magnitude by fluid property areas.
1000501 FIG. 7. Comparison of GOR for 2 wells on gas lift versus rod pump.
1000511 FIG. 8A. GOR rising magnitude vs. production months comparison between
parent and
child wells.
1000521 FIG. 8B. GOR rising magnitude vs. cumulative oil production comparison
between
parent and child wells.
1000531 FIG. 9A. schematic of parent vs. child well placement.
1000541 FIG. 9B. Impact of child well fracture hits on parent well GOR and oil
rate uplift, tubing
pressure and GOR pre- and post-fracture vs. time.
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[00055] FIG. 10A. GOR rising magnitude vs. oil production rate.
[00056] FIG. 10B. GOR rising magnitude vs. gas production rate.
[00057] FIG. 11A. Bi-wing hydraulic fractures used in reservoir model for GOR
study.
[00058] FIG. 11B Grid refinement near fractures and wells used in reservoir
model for GOR
study.
[00059] FIG. 12A. Production history matching BHP for old completion wells.
[00060] FIG. 12B. Production history matching cumulative oil for old
completion wells.
[00061] FIG. 12C. Production history matching GOR for old completion wells.
[00062] FIG. 13. Reservoir pressure and gas saturation evaluation at different
GOR stages of
old completion wells; where BHP > Pb, where BHP drops below Pb, Early Pressure
build-up, Later
Pressure build-up, and later flowing.
[00063] FIG. 14A. Production history matching of BHP for modern completion
wells.
[00064] FIG. 14B. Production history matching of cumulative oil for modern
completion wells.
[00065] FIG. 14C. Production history matching of GOR for modern completion
wells.
[00066] FIG. 15. Reservoir pressure and gas saturation evaluation at different
GOR stages of
modern completion wells; where Pwf > Pb, where Pf just dropped below Pb, and
where GOR
enters a plateau stage.
[00067] FIG. 16A. Old completion well forecasts of BHP.
[00068] FIG. 16B. Old completion well forecasts of GOR.
[00069] FIG. 17A. Modern completion well forecasts of BHP.
[00070] FIG. 17B. Modern completion well forecasts of GOR (short term).
[00071] FIG. 17C. Modern completion well forecasts of GOR (long term).
[00072] FIG. 17D. Modern completion well forecasts with less aggressive
drawdown BHP.
[00073] FIG. 17E. Modern completion well forecasts with less aggressive
drawdown GOR
(short term).
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1000741 FIG. 17F. Modern completion well forecasts of less aggressive drawdown
GOR (long
term).
1000751 FIG. 18A. Impacts of cluster spacing on BHP vs. time.
1000761 FIG. 18B. Impacts of cluster spacing on GOR vs. time.
1000771 FIG. 18C. Impacts of cluster spacing on cumulative (CUM) oil vs. time.
1000781 FIG. 18D showing tight cluster spacing (solid lines in FIG. 18A-C).
1000791 FIG. 18E showing wider cluster spacing (starred lines in FIG. 18A-C).
DETAILED DESCRIPTION OF THE DISCLOSURE
1000801 Herein, we present our findings on Bakken GOR trends based on high-
quality data
collected, integrated modeling and the analysis of hundreds of wells in the
area.
1000811 Unlike Permian and Eagle Ford, Bakken is a hybrid play of shale and
carbonate. The
reservoir matrix permeability is at the low single-digit micro-Darcy range. It
is about one order of
magnitude higher than other shale plays, however it is still much lower than
conventional reservoir
permeability. This has important implications on the Bakken reservoir
depletion process and GOR
evolution that will be discussed later.
1000821 Bakken reservoir fluids generally fall in black oil to volatile oil
fluid regimes with a
wide range of initial solution GOR R, from 500 to 2500 scf/stb in the study
area. The initial
reservoir pressure P, varies from 6500 to 7500 psi Many PVT samples were
collected and tested
across the field to characterize fluid properties. A field wide Equations of
State (EOS) model was
developed for various applications, including GOR modeling and facility
design.
1000831 Completion design is a key driver of Bakken well GOR behaviors. We
will use two
main completion design types (old completion and new completion) to illustrate
the typical GOR
stages and trends in the short and intermediate-terms in our proof of concept
demonstrations. Most
of the wells drilled before 2016 have old completion designs¨open hole sliding
sleeve, few stages
(30 or less) and low proppant volume (5 million lbs or less). Modern
completion designs normally
are associated with cemented plug-and-perf completions, more stages (30+) and
larger frack job
sizes (8+ million lbs proppant).
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GOR TRENDS FOR OLD COMPLETION DESIGN WELLS
[00084] The old completion wells were developed when reservoir pressure was
close to original
virgin pressure P1, i.e., there was no parent well depletion impact. Their
stimulated rock volumes
(SRV) were small with fewer long fractures, which can lead to faster drawdown
and lower well
productivity. These created unique GOR characteristics for this type of wells.
Examples are given
in FIG. 3 and FIG. 4, where oil rate, GOR and downhole gauge BHP pressure data
are overlayed.
The main observations are.
[00085] 1) GOR started flat with GOR= Rs1, while well flowing bottom hole
pressure (P,,,f) was
above bubble point pressure (Ph-3000 psi), as shown in FIG. 3;
[00086] 2) GOR rose while Pflf dropped below Pb (arrow 1);
[00087] 3) GOR trended downward (arrow 1) when Af reached a temporary low
limit due to
rod pump production constraints in this case;
1000881 4) Post the operational shut-ins (e.g. downhole isolation for offset
fracking), GOR came
down to Rs, (arrow 3). It rose back up as production resumed and Ai dropped
further (arrow 4).
The mechanism and theory will be discussed in the modeling section.
[00089] The GOR rise-and-fall cycle repeated itself throughout the short to
intermediate-term of
the well production (FIG. 3). However, in each subsequent cycle, the Af built
up to a lower
pressure and GOR rose to a higher level though the rising pace was slow. This
GOR rise-and-fall
pattern is commonly observed among Bakken old-style completion wells.
[00090] A very strong correlation between producing GOR and BHP was observed
from this
data. GOR is much more sensitive to P1 changes in unconventional reservoirs
than in
conventional reservoirs thanks to UR low matrix permeability and limited
drained rock volume
(DRV).
[00091] It is also interesting to see that Afgradually increased while the
well was on production,
as indicated by arrow 2 in FIG. 4. This could be caused by rod pump curtailed
production and
relatively faster pressure recovery from areas surrounding fractures, given
the higher matrix
permeability in the Bakken as compared against other shale plays. It is also
consistent with early
discussions that GOR moved downward during the Afrising period. This
observation underscores
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the importance of collecting long-term downhole BHP data to understand GOR and
reservoir
depletion.
1000921 The flat GOR period was short, only 2 months in this example The
duration of the flat
GOR is dictated by the pressure difference between P, and Pb, and the pressure
drawdown, which
is a function of well operating practices and stimulated hydraulic fracture
system supporting the
well deliverability.
GOR TRENDS FOR MODERN COMPLETION DESIGN WELLS
1000931 Modern completion wells show different GOR trends from old completion
wells. An
example is given in FIG. 5, where oil rates, GOR and downhole gauge BHP
pressure data are
overlayed. The main observations are given below:
1000941 1) The flat GOR period lasted longer (5+ months). The modern
completion designs
created much larger SRV/DRV and resulted in higher well productivity, which
can keep p,,,f above
ph for a longer time.
1000951 2) GOR started rising as Piif dropped below Ph, but a modern
completion well GOR
rises slower than an old completion well in the early time as compared to
later (Arrow 1 v. 2). The
rising periods can last from several months to over a year in Bakken.
1000961 3) GOR reached a plateau as Pr further decreased in the intermediate
term. The plateau
had not been well established in this particular case.
1000971 It is also worth mentioning that Bakken GOR data do not show strong
critical gas
saturation (Sg,) behavior, i.e., GOR does not show a clear dip right before it
starts rising. Our
interpretation is because the drainage areas are very limited prior to gas
breakout, it does not take
a large amount of free gas to make the drained areas reach Sgc. This may
create a short GOR
dipping period (if there is one), which can be easily masked by the producing
GOR fluctuations.
1000981 Building on the GOR stages identified in the previous sections, we
also investigated
other GOR trend drivers based on hundreds of Bakken wells' production data
with the focus on
modern completion wells in the study area.
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FLUID PROPERTY SPATIAL VARIATION
1000991 The fluid property spatial variations can have significant impacts on
GOR. In this study
area, the south part has lower Rs, and Ph, and the oil is less volatile than
that in the north side. Four
similar PVT property areas are defined¨Areas 1, 2, 3 and 4 from south to
north. The first 25-
month average well GOR of each area is shown in FIG. 6A. The GOR rising
magnitude, defined
as normalized GOR by initial solution GOR (GOR/Rs,) is shown in FIG. 6B. As
the oil becomes
increasingly more volatile from south areas to north areas, the initial
solution GOR Rs, moves
higher and the producing GOR rises to a higher plateau at a faster pace.
WELL OPERATING STRATEGY/DRAWDOWN
1001001 As mentioned earlier, one of the biggest drivers in well GOR is the
drawdown of the
well and thus the lower P1. Many of the earlier wells were lifted with rod
pumps after the flowing
period. Often the installation of a rod pump early in the life of the well
will constrain the amount
of the fluid it will produce. A fluid column will then form in the well,
keeping some back pressure
on the formation and higher Pwf These wells would then stay above Ph longer
and with less
aggressive drawdown would not see the GOR rise as quickly.
1001011 Comparatively, the industry has switched to more electrical
submersible pumps (ESPs)
and gas lift design early on in the well life. Both are capable of handling
more fluid, and in turn
drawing the bottom hole pressure down lower. This more aggressive drawdown
will cause more
rock volume to drop below Pb and faster. The GOR is then seen to rise more
early on and reach
higher peak GORs than rod pump wells with similar completion designs.
1001021 Error! Reference source not found. compares 2 well's GORs on the same
pad with the
same formation, completion design, and timing. Both wells flowed until around
October 2018
with identical GORs. After that time, one well was put on gas lift and the
other on rod pump. One
can see the GOR differences post AL installation with the gas lift well GOR
rising much faster
and higher than the other. This proves the value of reducing drawdown rates.
OFFSET PARENT WELL DEPLETION
1001031 As an unconventional field is developed into a mature asset, the
parent (existing wells)
and child (offset new wells) situation becomes more and more common, and it
can have important
impacts on new development well GOR. Inevitably, child wells will start
production at a lower
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pressure than the original P, due to the parent well depletion. This leaves
smaller room for the
child well's P,,f to drop before it reaches Pb. FIG. 8A compares the GOR
trends between parent
wells and child wells. The flat GOR period of child well is much shorter than
that of parent wells.
In addition, child well GOR rises to a higher plateau. FIG. 8B plots the GOR
rising magnitude
against cumulative oil volume instead of production months. We can draw the
similar
conclusion¨the child well cumulative oil production is lower before the start
of rising GOR.
FRACTURE HITS
[00104] Fracture hits (cross-well communication created by hydraulic
fracturing) on parent
wells are created by offset well fracturing. A parent-child well schematic is
given in FIG. 9A.
Since the stress and pressure near parent well areas are lower due to
depletion, child hydraulic
fractures tend to asymmetrically grow towards parent wells and generate strong
fracture hits, as
indicated by the tubing pressure jump post fracture hits in FIG. 9B. In the
Bakken, the parent well
production normally benefits from fracture hits. See the oil rate uplift post
fracture hits in FIG.
9B. Also, it is worth noting that the rising GOR prior to fracture hits is
suppressed to a much lower
level for a long per iod time. Possible reasons include re-pressurization of
the parent well prevents
more areas dropping below Pb., or fracture hits create new fractures near
parent wells that contact
new higher-pressure rock volumes.
WELL OIL AND GAS PRODUCTION RATES
[00105] Bakken well oil production rates normally begin with a plateau in the
early time, given
the higher reservoir pressure and facility constraints. It is also common that
a well's oil rate plateau
end coincides with the beginning of the GOR rise. The oil rate can show faster
decline with rising
GOR because the high gas mobility enables gas to move preferentially to oil
from the reservoir to
the wellbore. See FIG. 5 and FIG. 10A-B. Statistically, Bakken data show the
GOR rising
happens when oil rates drop from 700 stb/day to 300 stb/day (FIG. 10A).
However, the gas
production rates are relatively stable in the short- and intermediate-terms,
i.e., the declining oil rate
and rising GOR make the gas rate relatively flat. See FIG. 10B.
GOR MODELING AND LONG-TERM FORECAST
[00106] A GOR modeling workflow was developed to capture key GOR drivers,
match available
data and predict short- and long-term GOR trends.
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[00107] The model includes the following key components:
[00108] 1) A reservoir model with Bakken geological layers and reservoir
properties (FIG.
11A).
[00109] 2) Hydraulic fracture representations for old and modern completion
designs. As
discussed, old completion jobs created fewer fractures with a larger fracture
area per fracture,
whereas modern completion jobs created more complex fractures. The fracture
properties were
based on the insights gained from relevant data collected and model
calibrations.
1001101 3) Tartan gridding near the well and fractures to better characterize
the local pressure
gradient and gas saturation changes (FIG. 11B).
[00111] 4) PVT EOS model that was tuned to fluid sample lab data.
[00112] 5) Relative permeability curves based on lab tests and production
history matching
model calibration.
[00113] 6) High quality production data, including oil/gas/water rates and
downhole measured
BHP data.
[00114] The model was controlled by BHP to match oil production rates and GOR.
The
production history matching process was conducted in a probabilistic manner to
obtain multiple
equal-probable models for uncertainty quantification.
[00115] The old completion well matched results are given in FIG. 12A-C, where
the early flat
GOR period and intermediate term rise-and-fall patterns were successfully
matched using the
proposed modeling approach. The reservoir gas saturation (Sg) maps at various
production periods
are shown in FIG. 13 to illustrate the gas saturation evolution. At the
beginning (first stage¨see
left-most gas saturation element), there was no free gas in the reservoir
while Pvvi > Pb. The second
stage shows the Sg map when GOR reaches the peak of the first GOR rising
cycle. Small amounts
of gas came out of solution and was concentrated near fracture areas due to
low reservoir matrix
permeability and small drained rock volumes by that time. The third stage is
the Sg map at the end
of the first build-up. Some free gas from the second stage was pushed back
into reservoir by
recovered pressure near fractures. The fourth stage is the Sg map at a later
GOR peak. The high
Sg regions were expanded further from fractures, as compared with those in the
second stage, due
to the increased cumulative production volumes, and the partially recovered
BHP pressure in this
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period was not able to make free gas solute back into reservoir. See the final
stage (gas saturation
element on the far right).
1001161 The data matched results for the modern completion well are given in
FIG. 14A-C. The
flat-rise-plateau GOR stages during the early and intermediate production
periods were well
matched.
1001171 The Sg and pressure evolution maps are shown in FIG. 15. Note that
modern completion
wells' higher fracture density resulted in earlier inter-fracture production
interference and larger
reservoir drainage volumes. We believe this is the main reason that the old
completion wells' rise-
and-fall GOR pattern and long-term slow rising trend were not observed in the
modern completion
wells.
1001181 The history matched models were used to predict long-term GOR. FIG.
16A-B shows
the GOR prediction of old completion wells. The GOR increases at a slow pace
in the long term.
1001191 For modern completion wells, two GOR scenarios were predicted based on
the pressure
drawdown or well operating strategy. A more aggressive drawdown (FIG. 17A) led
to quicker
GOR rise and a shorter GOR plateau period (FIG. 17B) before it changed to the
downward trend
in the long term (FIG. 17C). Whereas a less aggressive drawdown (Error!
Reference source not
found.D) slowed down GOR rise and created a prolonged GOR plateau (FIG. 17E-
F). Again, this
shows the importance of collecting long-term BHP data for GOR prediction.
1001201 Given the insights gained on Bakken UR well GOR trends and key
drivers, some
measures can be taken to mitigate rising GOR. Four potential methods related
to completion
design, development sequence and well operation strategy are discussed below.
1001211 1) Widen fracture cluster spacing: If the rest of completion design
parameters are kept
the same, widening fracture cluster spacing tends to create larger fracture
area per fracture and
delay production interferences between fractures, which will enhance well
productivity and lower
GOR as illustrated in FIG. 17A-F.
1001221 2) Recharge high GOR parent wells with offset child well fracture
hits: As
mentioned early, fracture hits can suppress parent well GOR rising and create
parent well
production uplift. We can take advantage of this unique situation in Bakken
and strategically select
the timing of fracturing the child well as parent wells enter high GOR stages.
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[00123] 3) Use less aggressive drawdown: An aggressive drawdown can create
sharp pressure
sinks near fractures or wells, given the low matrix permeability in
unconventional reservoirs. It
drops the pressure in these limited drainage areas below Pb faster and
triggers the GOR rising
events. On the other hand, a less aggressive drawdown will extend the flat GOR
period and
maximize the production prior to the accelerated decline caused by rising GOR.
1001241 4) Extended well shut-in: This might be a temporary solution, but long
shut-ins can
lower GOR when the production is resumed thanks to the pressure build-up.
However, this
lowered GOR tends to be short-lived, since the flush production post shut-ins
can cause more
aggressive drawdown and GOR will go back to the original rising trend within
few months.
[00125] In summary, we have demonstrated comprehensive analysis and modeling
studies on
Bakken UR well GOR providing the benefit of forecasting, improved design,
completion trends,
and correlations between GOR and BHP. Forecasting GOR and gas rates are
crucial to
development plans, offtake strategy, facility design and flaring reduction.
The Bakken field has
seen a large increase in GOR and gas rates over the last several years driven
by completion design,
depletion, operational strategy, and fluid PVT properties. Old sliding sleeve
completion well GOR
shows cyclical rise-and-fall patterns with a gradually increasing long-term
trend, whereas modern
completion wells show rising GOR trend after their BHPs drop below Pb and then
reach an
intermediate-term plateau.
[00126] Strong correlations between GOR and BHP pressure were observed.
Downhole gauge
BHP pressure is critical data for GOR behavior analysis and long-term
forecast. No strong critical
gas saturation behavior was observed from Bakken wells. GOR can be used a
diagnostic tool to
monitor reservoir depletion and drainage. A focused GOR modeling approach can
be crucial in
the forecasting. Several measures can be taken to mitigate the high GOR, while
ultimately
improving well economics.
HARDWARE & SOFTWARE
[00127] The present disclosure also relates to a computing apparatus for
performing the
operations described herein. This apparatus may be specially constructed for
the required purposes
of modeling, or it may comprise a general-purpose computer selectively
activated or reconfigured
by a spreadsheet program and reservoir simulation computer program stored in
the computer. Such
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computer programs may be stored in a computer readable storage medium,
preferably non-
transitory, such as, but is not limited to, any type of disk including floppy
disks, optical disks, CD-
ROMs, and magnetic-optical disks, read-only memories (ROMs), random access
memories
(RAMs), EPROMs, EEPROMs, magnetic or optical cards, or any type of media
suitable for storing
electronic instructions, each coupled to a computer system bus.
1001281 In one embodiment, the computer system or apparatus may include
graphical user
interface (GUI) components such as a graphics display and a keyboard, which
can include a
pointing device (e.g., a mouse, trackball, or the like, not shown) to enable
interactive operation.
The GUI components may be used both to display data and processed data and to
allow the user
to select among options for implementing aspects of the method or for adding
information about
reservoir inputs or parameters to the computer programs. The computer system
may store the
results of the system and methods described above on disk storage, for later
use and further
interpretation and analysis. Additionally, the computer system may include on
or more processors
for running said spreadsheet and simulation programs.
1001291 Hardware for implementing the inventive methods may preferably include
massively
parallel and distributed Linux clusters, which utilize both CPU and GPU
architectures.
Alternatively, the hardware may use a LINUX OS, XML universal interface run
with
supercomputing facilities provided by Linux Networx, including the next-
generation Clusterworx
Advanced cluster management system.
1001301 Another system is the Microsoft Windows 7 Enterprise or Ultimate
Edition (64-bit, SP1)
with Dual quad-core or hex-core processor, 64 GB RAM with Fast rotational
speed hard disk
(10,000-15,000 rpm) or solid state drive (300 GB) with NVIDIA Quadro K5000
graphics card
and multiple high resolution monitors. Of course, such systems may be updated
with time, as
computer hardware continues to improve at great rates.
1001311 Slower systems could also be used because the processing is less
computation intensive
than for example, 3D seismic processing.
1001321 Reservoir simulation programs can be any known in the art, possibly
modified for use
herein, or any novel purpose-built system. Existing commercial packages
include MEERA,
ECLIPSE, RESERVOIR GRAIL, 6X, VOXLER, SURFER, the CMG suite, LANDMARK
NEXUS, and the like. Open source packages include BOAST ¨ Black Oil Applied
Simulation
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Tool, MRST¨the MATLAB Reservoir Simulation Toolbox and OPM ¨ The Open Porous
Media
(OPM).
[00133] The following references are each incorporated by reference in its
entirety for all
purposes:
[00134] Carlson, C. G.; Anderson, S. B.; Sedimentary and tectonic history of
North Dakota part
of Williston Basin. AAPG Bulletin; 49 (11): 1833-1846.
doi.org/10.1306/A663386C-16C0-11D7-
8645000102C 1865D.
[00135] Cipolla, C.; Litvak, M.; Prasad, R. S.; McClure, M. "Case history of
drainage mapping
and effective fracture length in the Bakken." Paper presented at the SPE
Hydraulic Fracturing
Technology Conference and Exhibition, The Woodlands, Texas, USA, February
2020.
doi.org/10.2118/199716-MS
[00136] Carlsen, M. L.; Whitson, C. H.; Alavian, A.; Martinsen, S. 0.;
Mydland, S.; Singh, K.;
Younus, B., Yusra, I. "Fluid Sampling in Tight Unconventionals." Paper
presented at the SPE
Annual Technical Conference and Exhibition, Calgary, Alberta, Canada,
September 2019.
doi.org/10.2118/196056-MS
[00137] Gaswirth, S. B.; Marra, K. R.; Cook, T. A.; Charpentier, R. R.;
Gautier, D. L.; Higley,
D. K.; Klett, T. R.; Lewan, M. D.; Lillis, P. G.; Schenk, C. J.; Tennyson, M.
E.; Whidden, K. J.
"Assessment of undiscovered oil resources in the Bakken and Three Forks
Formations, Williston
Basin Province, Montana, North Dakota, and South Dakota, 2013" USGS National
Assessment
of Oil and Gas Sheet 2013-2013, p.4.
[00138] Jones, R. S. "Producing-Gas/Oil-Ratio behavior of multifractured
horizontal wells in
tight oil reservoirs." SPE Res Eval & Eng 20 (2017): 589-601.
doi.org/10.2118/184397-PA
[00139] Lei, G.; Cheng, N. "Liquid-rich shale versus conventional depletion
performance."
Paper presented at the SPE/EAGE European Unconventional Resources Conference
and
Exhibition, Vienna, Austria, February 2014. doi.org/10.2118/167788-MS
[00140] Liu, Y.; Bordoloi, S.; McMahan, N.; Zhang, J.; Rajappa, B.; Long, H.;
Michael, E.
"Bakken infill pilot analysis and modeling: Characterizing unconventional
reservoir potentials."
Paper presented at the SPE/AAPG/SEG Unconventional Resources Technology
Conference,
Virtual, July 2020. doi.org/10.15530/urtec-2020-2177
21
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1001411 Luo, S.; Lutkenhaus, J.; Nasrabadi, H. "A framework for incorporating
Nanopores in
compositional simulation to model the unusually high GOR observed in shale
reservoirs." Paper
presented at the SPE Reservoir Simulation Conference, Galveston, Texas, USA,
April 2019.
doi.org/10.2118/193884-MS
1001421 Pradhan, Y. "Observed gas-oil ratio trends in liquids rich shale
reservoirs." Paper
presented at the SPE/AAPG/SEG Unconventional Resources Technology Conference,
Virtual,
July 2020. doi.org/10.15530/urtec-2020-3229
1001431 Raterman, K.; Liu, Y.; Warren, L. "Analysis of a drained rock volume:
An Eagle Ford
example." Paper presented at the SPE/AAPG/SEG Unconventional Resources
Technology
Conference, Denver, Colorado, USA, July 2019. doi.org/10.15530/urtec-2019-263
1001441 Raterman, K.; Liu, Y.; Roy, B.; Friehauf, K.; Thompson, B.; Janssen,
A. "Analysis of a
multi-well Eagle Ford pilot." Paper presented at the SPE/AAPG/SEG
Unconventional Resources
Technology Conference, Virtual, July 2020. doi.org/10.15530/urtec-2020-2570
1001451 Whitson, C. H.; Sunjerga, S. "PVT in liquid-rich shale reservoirs."
Paper presented at
the SPE Annual Technical Conference and Exhibition, San Antonio, Texas, USA,
October 2012.
doi.org/10.2118/155499-MS
1001461 ASTM D1298 "Standard test method for density, relative density or API
gravity of
crude petroleum and liquid petroleum products by hydrometer method."
1001471 ASTM D4052 "Standard test method for density, relative density and API
gravity of
liquids by digital density meter."
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