Orinoco Requires Heavy Lifting

Lateral Maze Improves Plumbing

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.

Image Caption

Zuata Field 3-D
A full field 3-D view of the subsurface well bones in Venezuela's Zuata Field showing the intricate placement of 241 horizontal wells. This display also illustrates how fishbones can effectively connect many parts of the reservoir which may have been left undrained if fishbones had not been used.
Graphics courtesy of Petrozuata.

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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.

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