Case Study: High-order repeatability - The first Q-on-Q 4D survey

Overview

The first use of Q-Marine for both the baseline and monitor surveys of a 4D (time-lapse) seismic project was over the Norne field in the Norwegian Sea.

Very high repeatability of source and receiver positions, the fast turnaround of 4D difference cubes, and deterministic processing sequences were all key features of the project. A 4D amplitude difference volume was produced on the vessel within 10 days of the end of the survey and a 4D acoustic impedance difference cube was generated in two more days. Based on these results, Statoil was able to adjust their horizontal drilling plans within 30 days of the monitor survey completion. Estimated savings from moving a preplanned well path to a more productive track were $29 million.

Introduction

The 4D technique assumes that the only differences between surveys are related to oil and/or gas production. Normally, the 4D signal originates from the response to changes in pore fluid type, although other production-related events in and above the reservoir, such as localized compaction and fracturing, or changes in temperature and pressure, may also be detectable. If non-production-related changes have occurred, the 4D signal can become masked by, or confused with, 4D noise. Historically, differences in the acquisition of baseline and monitor surveys have been the main source of 4D noise. Q-Technology significantly reduces these differences. With better 4D signal-to-noise ratios, the time between surveys can be reduced and reservoirs with relatively small 4D effects become prospects for the technique.

The principal aims of the 4D program over Norne are to map accurately the fluid movements in the reservoir resulting from production, flow paths and barriers, and to provide accurate fluid and rock property information for a better understanding of the reservoir's past and current behavior. In addition, the information enables forward modeling of fluid movements so production plans can be optimized and the life of the field extended.

Field description

The Norne field, discovered by Statoil (and partners Petoro, Norsk Hydro, Enterprise/Shell, and ENI) in 1991, has been producing oil since 1997 and gas since 2001. The reservoir is around 3 x 9 km in extent, at a depth of approximately 2.5 km below the sea surface; it is comprised of lower and middle Jurassic sandstones in a NE/SW trending horst block. The gas cap is approximately 75 m thick with an oil column of 110 m. Sub-horizontal shale and calcite permeability barriers and faulting have a major impact on gas and water injection and on reservoir production.

Seismic data acquisition

During June 2003, WesternGeco acquired a Q-Marine monitor seismic survey over Norne, repeating a Q-Marine survey acquired in August 2001. A second Q monitor survey will be acquired in the summer of 2004. The Geco Topaz will acquire the 2004 monitor as well as the 2001 and 2003 surveys. A single source array and six 3200-m steerable single-sensor streamers with 50-m lateral separations were used. The ability to steer the streamers permitted the receiver array to be towed within 40 m of the FPSO, allowing imaging of the center of the reservoir.

Very low feather was achieved, with 78% of the 2003 data acquired with feathering of less than ??? degree. This compares with a value of 14% for non-steered streamer data acquired in the same direction over Norne. Figure 1 shows how well the feathering was repeated. Around 90% of the 2003 data were acquired with feathering within ?1 degree of the 2001 data.

Low feather angles, straight streamers, constant streamer separation, a minimal undershoot footprint, and consistent CMP fold all have benefits for processing. Additionally, to achieve high 4D signal quality, it is necessary to steer accurately to repeat positions of the sources and receivers. This is especially true on shallow and/or high frequency targets and deeper lower frequency targets contaminated by poorly repeatable sea-surface multiples and diffractions, or distorted by a rapidly varying overburden. Although variable cross-currents in 2003 of up to 45 cm/s caused the vessel to deviate from the preplot map, over half the survey's shots were within less than 3 m crossline from the preplot and 95% of the shots were within 9 m.

Seismic data processing

The 2001 baseline survey was reprocessed before the monitor acquisition started. The 2003 data were then processed on the Geco Topaz through an identical sequence, including deterministic corrections for measured acquisition system variations. The two migrated volumes were differenced 10 days after completion of the survey. The single-sensor information, calibrated source and receivers and the high positional accuracy played an important role in enabling such rapid 4D differencing, and without which, neither the fast response time nor the ability to detect subtle 4D signal would be possible.

Onshore processing of the 85-km2 datasets employed computer-intensive and tailor-made techniques to further enhance the 4D results. Free-surface demultiple had failed on the previous Norne 4D because, although the 2001 monitor was Q-Marine acquisition, the baseline survey was conventionally acquired. The new monitor survey was acquired with free-surface demultiple in mind. The two-boat undershoot lines were extended so that when joined with the single-boat deadhead lines shot in the opposite direction, there were sufficient data in the common receiver-station domain for the free-surface demultiple to prevent a low-fold edge effect at the join between the two. The resulting excess CMP fold was regularized by coincident trace selection, based primarily on similarity of source and receiver azimuth. Figure 2 shows the benefit of applying such regularization techniques.

A section of the baseline and 4D difference are shown in Figures 3. The excellent 4D signal can be seen by the clear imaging of the production and injection effects.

Results and conclusions

Within 30 days of survey completion, Statoil had formulated a revised drilling plan, 20 m above and to the side of the original. This very successful horizontal well has now been completed. Had the original well path been drilled, at a cost of $29 million, the oil-water contact would soon have risen into it. The Q-on-Q 4D results revealed that a thin, heavily calcified, horizontal permeability barrier was not tight, and the waterfront actually moved upwards rather than sideways.

Streamer steering for zero feather and constant cable separation provided safe and clear imaging beneath the FSPO and achieved a very high level of repeatability. This enabled the 4D repeat survey, revealing subtle time-lapse effects, to be successfully acquired in a relatively short time-lapse period. The rapid delivery of high-quality results made an immediate and valuable impact on Norne field reservoir management.

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