Strategies Give Reservoirs Character

Key to Reserve Growth

Got an aging oil or gas field that's on its proverbial last leg?

Not to worry - the tonic du jour is integrated reservoir characterization.

It's a proven relatively low cost, low risk approach to building reserves, and it's used on a regular basis to breathe new life into mature, often marginal fields.

"Field re-exploration strategies like resource-targeted infill drilling and field or play extension wells, along with strategic recompletions and waterflood optimization strategies based on sophisticated reservoir characterization programs are the preferred approach to revitalize old fields," said Roger Tyler, a partner in Advanced Reservoir Characterization and Exploration Services (ARC).

"These applications have captured almost two-thirds of the 40 billion barrels of oil that have been added to the U.S. reserve base over the past 20 years."

Tyler noted an added benefit of applying reservoir characterization technologies to mature fields: New wells typically are more cost effective.

In Texas, for example, the volume of reserves added per new well drilled has increased by 40 percent in the first half of this decade as a direct result of these methods.

Tyler co-authored a paper with colleagues from ARC and the Texas Bureau of Economic Geology (BEG) addressing integrated reservoir characterization of mature fields and the application of 3-D visualization techniques to reserve growth strategies. The paper was presented at October's Gulf Coast Association of Geological Societies annual meeting in Houston.

Image Caption

New views: Three-D seismic data and virtual reality imaging, according to many geoscientists, could revolutionize our approach to reservoir characterization. Graphics courtesy of Roger Tyler.

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Got an aging oil or gas field that's on its proverbial last leg?

Not to worry - the tonic du jour is integrated reservoir characterization.

It's a proven relatively low cost, low risk approach to building reserves, and it's used on a regular basis to breathe new life into mature, often marginal fields.

"Field re-exploration strategies like resource-targeted infill drilling and field or play extension wells, along with strategic recompletions and waterflood optimization strategies based on sophisticated reservoir characterization programs are the preferred approach to revitalize old fields," said Roger Tyler, a partner in Advanced Reservoir Characterization and Exploration Services (ARC).

"These applications have captured almost two-thirds of the 40 billion barrels of oil that have been added to the U.S. reserve base over the past 20 years."

Tyler noted an added benefit of applying reservoir characterization technologies to mature fields: New wells typically are more cost effective.

In Texas, for example, the volume of reserves added per new well drilled has increased by 40 percent in the first half of this decade as a direct result of these methods.

Tyler co-authored a paper with colleagues from ARC and the Texas Bureau of Economic Geology (BEG) addressing integrated reservoir characterization of mature fields and the application of 3-D visualization techniques to reserve growth strategies. The paper was presented at October's Gulf Coast Association of Geological Societies annual meeting in Houston.

Revolutionary Approach

It's estimated that more than 100 billion barrels of movable oil will remain in place in heterogeneous reservoirs in the United States after recovery of existing reserves, he said. This sizeable resource is in reservoir compartments that are untapped or either vertically or laterally bypassed by injected fluids, making them ideal targets for integrated reservoir characterization programs.

Tyler predicted that reservoirs where the greatest reserve growth will be realized are those with the most accurate reservoir descriptions - where there is a high degree of internal heterogeneity and substantial volumes of untapped or bypassed hydrocarbons.

This reserve growth, according to the geoscientists, can best be achieved by taking the reservoir characterization process a giant step forward, enhancing it via the rapidly evolving virtual reality technology.

"Immersive virtual reality promises to revolutionize the way we traditionally do reservoir characterization," Tyler said.

"Reservoir characterization programs rely on thorough integration of geological, seismic, petrophysical and production engineering data into a quantified description of reservoir architecture and rock property structure by individual professionals possessing specialized knowledge, but in distinctly different disciplines," he explained.

"It's this, the very nature of how we do reservoir characterization, that can benefit from immersive virtual visualization technologies."

By synthesizing shared representational objects of the evolving reservoir model, virtual reality removes the barriers indigenous to mentally imaging multiple complex data sets, Tyler added.

The interactive manipulation of the integrated models and application of cutting-edge visualization techniques, such as temporal mapping of reservoir dynamics, significantly enhances the process of reservoir characterization, he said - and identifying reserve growth opportunities.

Case Studies

Tyler and his colleagues have successfully applied virtual reality analysis to stratigraphically and structurally complex fields in Lake Maracaibo, Venezuela, and along the Texas Gulf Coast. The effort took place at the BEG's Virtual Imaging Visualization Environment (VIVE).

In the Venezuelan field, the main Eocene producing reservoir is divided by a fault system, with numerous minor faults influencing fluid flow in the reservoir.

Production performance differs markedly on either side of the main fault. Production on the west side is driven by an active aquifer, which provides pressure support, and gas cap expansion provides the drive mechanism to the east. Gas injection was first implemented in 1962 to supply producing energy to the reservoir on the east side.

"Our analysis was placed in the virtual environment and focused on time-sequence motion mapping of the aquifer encroachment, gas cap expansion and production response to the injected gas," Tyler said. "The fluid dynamics were illustrated in outputs from the virtual imaging and provided a profound understanding of the compartmentalization in this reservoir.

"More important, the virtual imaging facilitated the identification of bypassed compartments of the reservoir that formed the basis for strategic infill drilling and recompletion recommendations to target the estimated 100 million barrels of remaining mobile oil."

Recent advanced reservoir characterization efforts by Tyler and his colleagues along the Texas Gulf Coast included research and development programs focused on multiyear oil and gas field analysis in Texas, with an emphasis on Texas State Lands.

To identify the residency of the remaining oil and gas resource base, the group's approach has been to define and deploy on a field-to-field basis advanced recovery strategies to ensure maximal recovery efficiency.

Tyler cited, for example, the Umbrella Point Field in Galveston Bay, Chambers County, which is producing oil and gas from Gulf Coast Frio barrier island strandplain sandstones.

Through mid-1997, the field's mature Middle Frio reservoirs had produced a total 17 million barrels of oil and 103 billion cubic feet of gas from 36 wells developed in 15 zones between 8,000 and 10,000 feet. Estimates indicated the daily production rate in the field was 189 barrels of oil and 2.8 million cubic feet (MMcf) of gas from 11 completions in 10 wells.

As part of the State of Texas' Advanced Resource Recovery Program, 3-D seismic data and virtual reality imaging were used to help define bypassed gas compartments there, and a new gas well was recommended and subsequently drilled into the lower Frio in January 1998.

According to Tyler, the well tested 11.5 MMcf per day and 220 barrels of liquid daily. Cumulative gas production for the well was more than 812 MMcf as of May 1998, increasing total field production more then 2,600 percent since January 1998.

Such results, however, are only the tip of the iceberg when you look at the numbers applicable to Texas State Lands.

The research team concluded 1.6 billion barrels of mobile oil will remain on these lands at reservoir abandonment, which nearly equals the cumulative production to date. Team members said this will be foregone without the continued application of advanced geological and engineering technology.

This prediction holds true for natural gas reservoirs as well, where an estimated 10 trillion cubic feet, or the equivalent of cumulative production thus far, will remain unrecovered without advanced recovery technology.

"Rather than being unattainable, we believe a large volume of this remaining oil and gas is recoverable through the strategic, or targeted, deployment of advanced reservoir characterization technologies that should incorporate 3-D seismic and virtual reality imaging," Tyler said.

"In fact," he added, "incorporating virtual reality imaging to reservoir characterization should be an integral part of all advanced reservoir characterization programs along the Gulf Coast."

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