The North Sea has been the proving grounds for one of the most significant technologies to come along in decades — time-lapse or 4-D seismic. The technique involves acquiring 3-D data over a field before production begins and then acquiring repeat or monitor surveys after production begins to detect changes in reservoir properties such as pressure, saturation, or geomechanical effects. A successful 4-D survey relies on the detectability of the 4-D signal and the repeatability of the survey. Due to a confluence of opportunities in the North Sea — availability of vessels, a willingness by operators to try new technologies, incentives by the Norwegian government to increase recovery, and R&D spending — most of the early work was done in that region.

This photo shows an aerial view of the Surmont field in Alberta, Canada. This SAGD project is benefitting from repeat 4-D surveys. (Photo courtesy of ConocoPhillips Canada Ltd.)

But things change, and not only in reservoirs. The technology is becoming quite mature, and research undertaken over the years has improved the ability of interpreters to understand production-related changes in their reservoirs, even when the signals aren’t strong. In a technical presentation during the recent Society of Exploration Geophysicists annual meeting, several papers revealed successful 4-D surveys thousands of miles from northern Europe. Below are two examples.

Offshore

Fields offshore West Africa have been the beneficiaries of many 4-D studies, and with good reason — the expense of intervening in these deepwater wells encourages engineers to know as much as possible about what’s going on in the reservoir through less invasive means. Additionally, many of these fields are developed in partnership with national oil companies, which often require produced gas to be injected back into the field for future development. Knowing where that gas is going is crucial to future planning for the field.

Several ExxonMobil and Sonangol authors reported on their success in the Dikanza field, located about 120 miles (193 km) offshore Angola. The main objectives of the study were to monitor the water sweep from the downdip water injection wells, to monitor movement of the oil/water contact, to monitor gas exsolution and the formation of a secondary gas cap, and to identify opportunities to improve the depletion strategy and ultimate hydrocarbon recovery from the field.

The baseline survey was acquired during ExxonMobil’s Kizomba A development program in 2002. The monitor survey was acquired in 2008 as part of a larger 4-D program.

Key to the success of this project was the rock and fluid physics modeling done prior to the monitor survey. This modeling suggested that 4-D responses would be detectable for both water replacing oil and gas replacing oil. In the rock physics modeling, gas exsolution was expected to cause a 10% change in acoustic impedance, secondary gas cap formation a 22% change, and water sweep a 12% change.

Additionally, the geologic model and the simulation model were combined through petrophysical relationships to generate 3-D synthetic seismic volumes for both surveys. These were analyzed to predict changes in both reflectivity and travel time differences. The real 4-D data were later used to recalibrate the rock physics models.

Before this second survey was acquired, the baseline survey was reprocessed to maximize data quality for comparison to the newer dataset. The new data were treated to the same flow to produce an early 4-D interpretation volume. The two surveys were then processed simultaneously to optimize the data quality and then cross-equalized to improve the spectral match of the data and to remove global and production-related time shifts from the monitor data. Spectral shaping and differencing completed the process.

The result was a 4-D dataset of excellent quality and consistent with forward-modeled data, the structural and stratigraphic interpretation, and the production data. The team observed dramatic 4-D responses associated with gas exsolution, water injection, aquifer water sweep, and pore pressure increase associated with high-pressure water injection. Plans are underway to integrate the 4-D response with a new geologic model.

“In summary, the 4-D data at Dikanza has been a success in many aspects: optimization, planning, execution, and integration with other geologic datasets,” the authors note. “It will have a significant impact on reservoir simulation modeling and key business decisions impacting Dikanza reservoir management.”

Onshore

In quite a different application, 4-D monitoring was applied to steam-assisted gravity drainage (SAGD) operations at the Surmont field in Alberta. Presenters from ConocoPhillips Canada Ltd. discussed the project, under joint development between ConocoPhillips and Total, and the decision to use 4-D monitoring in the field.

The benefit comes from the fact that the acoustic properties of heavy oil sands exhibit a strong response to temperature changes through a velocity decrease in the zones that have been thermally altered by SAGD. Time-lapse studies can be used to monitor the thermal evolution of the steam over time. Several highly repeatable surveys have been acquired at Surmont in six-month intervals since production began in 2007.

The SAGD project began in 1997 as a pilot, and pilot 4-D surveys were calibrated to 13 observation wells continually measuring pressure and temperature in the reservoir. When the 4-D amplitude difference was calibrated with downhole temperature data, it was observed that the top 4-D anomalies were consistently positioned ahead of the steam chamber interface.

“Although not a direct indicator of the steam, it became clear that the 4-D seismic added value by providing a 3-D characterization of the thermally altered zones in the reservoir,” the authors note. “Observations from the pilot 4-D program were used to justify continued 4-D monitoring over the 20 commercial Phase 1 well pairs, which started SAGD production in 2007.”

The first phase of the program consists of three survey areas, each of which cover individual SAGD drainage patters about .6 sq miles (1.6 sq km) in area. Acquisition geometries and designs of these surveys are slightly different to help determine the most cost-effective and fit-for-purpose design. Single-component analog buried geophones and a dynamite source are used for the surveys.

An optimized processing flow uses an amplitude-versus-offset-compliant Kirchhoff prestack time migration with minimal poststack processing. The resulting migrated gathers demonstrate very good repeatability.

The 4-D results at Surmont are being used quantitatively to estimate current recovery factors along each well pair for history matching and reservoir monitoring. They also are being used for production optimization, leading to more efficient management of steam and improving recovery.

ConocoPhillps recently announced plans to expand the Surmont heavy oil project in Canada. The Phase 2 Project, slated to begin initial construction in 2010, is expected to increase the company’s Surmont production from 10,000 to 50,000 net b/d of oil, with plateau production expected in 2017.