Single-sensor acquisition and processing techniques successfully imaged and characterized a mature onshore field for which previous attempts to derive reservoir properties from seismic were largely unsuccessful. The first point-source/point-receiver (single-sensor) onshore seismic survey in the Middle East was conducted in early 2004 as a pilot study under a Joint Technology Agreement between the Kuwait Oil Company and WesternGeco.

The study demonstrated that single-sensor seismic systems such as WesternGeco's Q-Land technology and methods can efficiently and properly sample the 3-D wavefield to effectively remove noise and apply perturbation corrections before group forming. This results in increased bandwidth and the preservation of high frequency and signal fidelity in prestack data. The significant data quality improvements are being used to optimize field management and bode well for further use of the single-sensor approach in similar onshore settings.

Survey setting

The Minagish field, one of Kuwait's main producers since its discovery in 1959, was selected for a pilot study to address multiple exploration and development objectives. One of these was to improve the resolution of the seismic time-lapse response across the producing oolitic reservoir. Fluid movements in the reservoir are complex; water influx in higher permeability layers overrides oil.

Acquiring seismic in an operating oil field requires detailed planning, hazard identification and mitigation to ensure workforce and equipment safety. Operational safety has been especially challenging in the Minagish field because of recent military activity in the area. The desert location is strewn with unexploded ordinances (Figure 1). Two oil gathering stations, numerous flares, a water injection plant, producing wells, a quarry, pipelines, roads and overhead power lines also lie within the survey area. In addition to being potential surveying hazards, these sites generate significant non-repeatable noise on surveys.

Early surveying challenges

A 48-fold 3-D survey was acquired over the Minagish field in 1996. The seismic crew returned 2 years later to acquire a trial 4-D (monitor) survey over a 15-mile (24-km) area centered on the main injector well. Baseline seismic data for the 4-D study was extracted from the 1996 volume, and both surveys were reprocessed using the best processing techniques then available.

The inability to acquire a clear time-lapse signal in 1998 was caused by resolution limitations within the reservoir itself, sub-optimal noise attenuation during acquisition and low signal-to-noise ratios. Bottom line, the data acquired in 1996 and 1998 with 165-ft (50-m) receiver arrays could not be used to properly image 4-D effects. These datasets were adequate for structural interpretation but not for reservoir characterization because of aliasing effects in the higher frequencies. It was evident that future studies would require different acquisition technology and techniques to reduce non-repeatable signal and coherent noise.

A single-sensor solution

The 2004 Joint Technology Agreement pilot study was established specifically to determine if WesternGeco's Q-Technology service could deliver the seismic data desired for improved management of Minagish field production. The study began by forming a multidisciplinary team of client and service company experts that followed a structured and integrated workflow. This team determined the data required to evaluate multiple targets within Minagish field and designed the Q-Land survey accordingly.

The four new technologies used under the Joint Technology Agreement included:

• The Q-Land single-sensor acquisition and processing system capable of recording 20,000 live channels (at 2 millisecond sample rate) and applying proprietary noise suppression techniques within the digital group forming (DGF) process.

• Several 80,000-lb peak force Desert Explorer vibrators to provide an ultralow distortion seismic source.

• The multilevel, single-sensor Q-Borehole Versatile Seismic Imager tool to acquire zero-offset and walkaway vertical seismic profiles (VSPs).

• Well-driven seismic technology utilizing information from VSPs and well logs to calibrate, at every stage, pre- and post-stack processing sequences to the borehole.

Project design and execution

Establishing an integrated overall design was the most important phase of the project. It was critical that the acquisition layout, source and processing parameters were optimally defined to achieve the objectives. With single-sensor acquisition and processing, source- parameter testing is emphasized as the quality of the seismic source has a greater impact on the returned seismic signal, which now can be more faithfully preserved.

The Q-Land pilot study consisted of three separate yet complementary phases, some of which included multiple objectives. In Phase 1, the team evaluated the signal and noise characteristics of the survey area to determine an optimum geometry for phases 2 and 3.

In Phase 2, the seismic source effort required for the new project was determined, and the ability of the seismic system to detect fractures was investigated. Involvement of the Kuwait Oil Company interpretation staff at this early stage allowed the acquisition parameters to be calibrated against borehole data.

Phase 3 was the acquisition stage. A new 9-sq-mile (24-sq-km) 3-D pilot baseline survey was conducted using the source and receiver parameters established in Phases 1 and 2. Since the 1998 survey, an oil-gathering station with three large flares and a water injection plant had been built within the survey area, adding significantly to environmental noise.

Survey results

The new baseline survey acquired using single-sensor acquisition and processing techniques clearly delivered better signal-to-noise ratios and higher vertical and lateral resolution than the previous conventional datasets. It was relatively easy to determine that the survey had been successful based on the predetermined metrics, especially because the main metric of increasing the bandwidth at the Minagish target was achieved with an increase from 44 Hz to 70 Hz. Comparison stacks of the previously acquired conventional data and Q-Land results are shown in Figure 2.

In addition, an infield migrated cube was processed by the crew and delivered only 6 days after acquisition ended. The cube comprised 16 terabytes of 384 million uncorrelated single-sensor seismic traces, which equates to a single-sensor trace density of more than 16 million traces per sq km - a densely sampled wavefield by any definition.

The total integration of data processing and acquisition into a single integrated workflow enabled the delivery of the migrated field volume in a short time frame. Data were transferred from the recorder to the infield processing center by means of removable hard disks. A proprietary recording format was used to ease data input to the processing sequence. Simultaneous processing and acquisition were achieved.

Specific noise attenuation processes were performed on the uncorrelated data before having perturbation corrections and spatial resample filters applied during the DGF process. A "normal" processing sequence, without well-driven seismic, was then applied to the group-formed data. The quick availability of a migrated field cube allowed the Kuwait Oil Company interpreters to immediately investigate the data volume.

Parallel to this processing, the data were sent to WesternGeco's regional data processing center in Cairo. A well-driven seismic processing sequence was applied with input from Kuwait Oil Company geoscientists.

Possibilities for surveying

The Minagish pilot project demonstrated the use of high-density, single-sensor seismic data to assist with field development planning. Single-sensor methods are equally applicable to exploration projects, in which lower-density datasets are suitable to give a structural overview of the subsurface. Having raw data-input records with increased signal-to-noise ratios permits the recording and processing of lower-fold exploration data with better data quality than a conventional survey. Later in the field life, these data can be integrated with the results from subsequent single-sensor surveys, providing high-density volumes for reservoir characterization and time-lapse reservoir imaging (4-D).

The industry has only begun to explore the possibilities for single-sensor acquisition and processing.

The Joint Technology Agreement made possible by the Kuwait Oil Company, and its willingness to thoroughly collaborate in the Minagish pilot study, helped advance the application of single-sensor technology, which could well prove to be a step change equal to that of the mid-1980s when the industry embraced 3-D applications.

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