Companies
have long known that Venezuela’s Orinoco heavy oil belt contains
over 1.3 trillion barrels of oil in place, but getting it out of
the ground economically has proven a challenging task.
This has changed in recent years, thanks to technology innovations
utilized by Petrozuata (Conoco-PDVSA joint venture) in the areas
of horizontal drilling and geosteering, which have successfully
unlocked the vast reserves of this previously dormant giant.
Petrozuata has just recently completed the initial phase of its
field development in the Orinoco heavy oil belt, which has resulted
in one of the world's largest multi-lateral field developments.
The story begins in 1990, when Petroleos de Venezuela SA (PDVSA),
the state oil company, opened the Orinoco to international companies
through participation in strategic joint ventures as part of the
country's strategy to increase its production. Among those accepting
the challenge was Conoco, which in 1991 initiated a joint study
project in the Zuata region with Maraven, a PDVSA subsidiary.
After several years of technical, financial and political evaluations,
Conoco and PDVSA formed the first joint venture company to be given
operating rights in the Orinoco heavy oil belt. "Petrozuata" was
created in 1996.
Built from the ground up in less than four years, Petrozuata constructed
a heavy oil upgrader on the coast, built a 200-kilometer pipeline
system between the field and upgrader, and constructed production
facilities in the field to achieve a current production of 120,000
BOPD. To achieve this production level, 241 horizontal wells —
consisting of 435 laterals and 510 "fishbones" — have been drilled
and completed.
With over 21 billion barrels of oil in place, Petrozuata has a
35-year operating life and will require drilling up to 750 horizontal
wells at a rate of 15 per year to ultimately recover 1.6 billion
barrels of heavy oil.
"This has been a once-in-a-lifetime opportunity for the geoscientists
and reservoir, production and drilling engineers to be part of a
project of this magnitude," said Robert Kopper, chief geoscientist
for Petrozuata. "We have planned, navigated and drilled over 3.1
million feet of hole in three years in conjunction with an extensive
data acquisition program and reservoir characterization effort."
Geoscience
When the project was initially evaluated, Petrozuata relied on
the limited exploratory well control available in the area, which
consisted of 18 exploration wells drilled by PDVSA in the 1980s.
What Petrozuata geoscientists couldn't tell from these original
wells was that they were drilled in some of the field's thickest
sands, which biased the initial interpretation toward a more optimistic
model of widespread thick and laterally extensive sands. The plan
was to identify, using 3-D seismic, the thicker sand accumulations
within the field, and during the first two years of the project
preferentially develop sands having an average thickness of 56 feet.
Batch drilling started in 1997, and it didn't take long to discover
that the reservoirs were far more heterogeneous than anyone thought.
During the early production phase in 1998, decline rates were steeper
than anticipated due to thinner and less continuous sands.
As a result of these lower than expected flow-rates in single horizontal
wells, Petrozuata redesigned its extensive data acquisition program
in 1998.
This data acquisition program consisted of 146 vertical stratigraphic
wells and utilized a wide range of down-hole methods. These new
wells were all integrated into the 291-square-kilometer 3-D seismic
survey, resulting in a very accurate time-depth model. Following
this data acquisition/integration program, it was obvious the targeted
average sand thickness was less than expected — averaging about
42 feet. By integrating this extensive data set into the detailed
reservoir characterization efforts, the reservoir model was revised
to incorporate more complexity, compartmentalization and lateral
discontinuity of the different depositional facies, including fluvial,
distributary and tidal estuarine channel deposits.
The average sand thickness for the entire field was also revised
to approximately 33 feet.
Concurrent with this effort, Petrozuata began drilling more innovative,
complex multi-lateral wells to fit the revised depositional facies
complexities. In order to achieve this extensive reservoir characterization
interpretation and implement a four rig multi-lateral drilling campaign,
Petrozuata utilized geoscientists from partner companies (Conoco-PDVSA)
as well as direct local hire geoscientists.
There was also invaluable work done by Conoco and PDVSA’s
technology experts in such fields as sequence stratigraphy, biostratigraphy,
geostatistical modeling, geochemistry, petrophysics and sedimentology,
and seismic imaging.
Through this concerted team effort, Petrozuata was able to effectively
revise the reservoir characterization model and successfully complete
the initial phase of development of the Zuata Field.
Kopper said that because Petrozuata is drilling extended reach
horizontal wells of up to 1,900 meters in sands 20 and 40 feet thick,
an accurate depth model is critical. The integration of the vertical
well database with the seismic data provides the key link between
the advanced drilling technology developed in the project and the
production/reservoir planning, as it allows the precise placement
of boreholes within a stratigraphically complex reservoir. All of
the horizontal wells were planned and steered using depth-converted
3-D seismic data.
Drilling
With the requirement to maximize the flow rate potential in each
horizontal well, Petrozuata started utilizing complex multi-lateral
wells in 1999. Each of these horizontal multi-lateral wells is custom-designed
to the local geology based on extensive reservoir characterization
studies and then geosteered to access the reservoir in the most
efficient manner. The key to Petrozuata's success was finding people
with the right skill sets and giving them the latitude and authority
to do what was required to plan and drill high producing potential
horizontal multi-lateral wells.
In order to achieve success in this kind of project, where data-integrated
decisions had to be made rapidly, a new management style had to
be formed. It was recognized early on that geosteering horizontal
wells in heterogeneous target sands averaging 30 feet thick over
distances greater than 1,200 meters combined with average drilling
penetration rates of 300 to 1,500 feet per hour required a rapid
and accurate decision making process.
Small two- to three-person multi-disciplinary teams were formed,
and all geosteering, drilling and operational decisions were pushed
down to key individuals on those teams. Proper skill sets were paramount
to a cost effective, successful operation, and as a result Petrozuata
sought out geoscientists who possessed a wide range of technical
expertise, particularly strong in both geology and geophysical interpretation.
These geoscientists were responsible for geosteering the well and
were empowered to make all geosteering decisions, ensuring that
geologic objectives were achieved in a cost-efficient .
These innovative custom-designed multi-lateral horizontal wells
were implemented to maximize production rates and ultimate recovery
per well at a decreased cost per barrel. It was determined early
on that the incremental cost in drilling multi-laterals was heavily
outweighed by the value of the increased rates and reserves per
well accessed through these designs.
Petrozuata also used multi-lateral wells to develop portions of
the reservoir from a single surface location, thus reducing the
number of surface drilling/production pads needed to access the
reserves — and at the same time minimizing the surface footprint
of field operations.
The various forms of multi-lateral wells drilled by Petrozuata
included stacked dual laterals, stacked triple laterals, "gullwing"
dual laterals, "crow's foot" triple and "pitchfork" dual laterals
and a "fishbone" multi-lateral concept designed to effectively drain
thin, stacked, heterogeneous and poorly connected sands. Several
different well designs are used to fit the particular geologic and
geographic features at each well pad.
During the well planning stage, each multi-lateral is custom designed
to effectively connect the oil-in-place and develop the reservoir
in an efficient and economical manner. During drilling, each lateral
is then geo-steered using 3-D seismic, geologic interpretation and
mapping to maximize sand content and optimize lateral placement.
The first design change from single laterals was to implement a
stacked dual lateral well. These are drilled much like a single
lateral well, but a window is cut in the casing above the first
lateral and a second lateral is drilled.
p
An application of a second design innovation was the "fishbone"
well. The first fishbone wells were drilled due east or due west
of the surface pad as single lateral wells with five to nine fishbones
or "ribs" drilled off the lateral's "spine." The ribs arc away from
the spine and generally extend approximately 300 meters lateral
to the spine. At the same time, these ribs cut vertically 5-30 meters
through the section. The ribs are designed to penetrate laminated
flow barriers within sands and to better contact locally disconnected
sand lenses.
Petrozuata's third multi-lateral design is the gullwing well. This
concept was designed to develop the 600 meter by 1,600 meter drainage
rectangles due north and south of existing surface pads without
having to construct new well pads centered over those drainage polygons.
This design alone should save 50 to 70 surface pads over the life
of the field, officials said.
A gullwing follows the same basic casing design as east-west multi-lateral
wells, except the build section is drilled in the north or south
direction. The first horizontal lateral is then turned to the east
or west and drilled to completion; at which time a window is cut
in the casing and a second lateral is drilled in the opposite direction.
p After gaining experience drilling and completing the stacked
dual, fishbone and gullwing type multi-lateral wells, Petrozuata
started drilling triple lateral wells.
There are several variations of the triple lateral well. Each of
the stacked lateral, fishbone and gullwing type wells can have a
third lateral easily added. Currently there are stacked triple laterals
in two dimensions and in three dimensions. One triple lateral design
that serves two distinct purposes is the crow's foot triple lateral
well.
The fishbone well design is especially versatile and has evolved
to meet several needs in the Petrozuata development. While the design
was initially developed to drill across numerous reservoir baffles
within one sequence, over time the fishbone technology has been
used to deplete adjacent overlying sequences and also to evaluate
overlying stratigraphy for subsequent target definition for planned
horizontal drilling. Also, when thin isolated sands exist in overlying
sequences, long reach fishbones can be used to produce these thin
sands at very low cost. Some of these thin sands would never economically
justify a separate lateral, however, through extended reach fishbones
that drill as much as 200 feet vertically, overlying thin sands
can be easily accessed and produced.
Another use of these extended reach fishbones is to evaluate the
reservoir thickness of overlying sequences. While drilling horizontal
laterals, often non-reservoir silts are encountered along the horizontal
lateral between vertical well control points. Some of these layers
are thin and insignificant, while others can be quite extensive.
Knowing beforehand where these non-reservoir units exist prior
to drilling a lateral would greatly enhance the potential value
of the subsequent horizontal well, because the lateral could be
properly placed in the stratigraphic interval containing the highest
quality reservoir sands.
Often, when a subsequent lateral was planned above the lower lateral,
extended reach fishbones would be drilled to evaluate the overlying
stratigraphy and verify what portion of the sequence should be targeted.
These fishbones provided critical information that helped ensure
that overlying laterals were drilled in the most effective location
and did so cost effectively. The fishbone design was taken another
step when Petrozuata began drilling inverted exploratory sidetracks.
These designer wellbores are certainly more expensive than a standard
single lateral horizontal well, but Kopper said Petrozuata has found
that the production increases gained with these various designs
makes the investment worthwhile. Petrozuata estimates the relative
cost of drilling and completing a nine-rib fishbone well is 1.2
times the cost of a single lateral well. A stacked dual lateral
is 1.6 times higher and a gullwing dual lateral is 1.7 times higher.
The more complex crow's foot triple lateral is 2.5 times more than
a single lateral well.
One of the greatest advantages of multi-lateral wells is that the
reservoir can be depleted to a greater extent than single lateral
wells would allow, and at a much reduced cost per barrel.
For example, a single lateral well that has an economic limit rate
of 50 barrels of oil a day, when combined in a dual lateral configuration
with an identical lateral in another sand target, might effectively
produce down to 25 barrels of oil daily before reaching a 50 barrel
a day economic limit for the dual lateral well. A triple lateral
well should allow even greater recovery.