Innovative integration of the channel acquisition technique for 3D data acquisition in Well Ma-131 Block

Transcription

1 CHINA PETROLEUM EXPLORATION Volume 21, Issue 2, March 2016 Innovative integration of the channel acquisition technique for 3D data acquisition in Well Ma-131 Block Tu Wei, Huang Yongping, Peng Xiao, Liu Yafeng Xinjiang Geophysical Exploration Department, BGP Abstract: Areas around Mahu in the Junggar Basin contain abundant hydrocarbons, suggesting great potential for exploration. However, low SNRs and resolution of available seismic data for these areas negatively affect the discovery of high-quality reservoirs, verification of lithologic traps and detection of hydrocarbons. In order to address these deficiencies, integration of various sophisticated acquisition techniques was considered for 3D seismic data acquisition in Well Ma-131 Block. Employing the integrated approach of simultaneously conducting tests and practical operations, the technical problems associated with remote data transmission, increasing data sets, data acquisition efficiency, real-time quality control, and etc. were addressed. Ultimately, the integrated solution applied in Well Ma-131 Block, and in other areas, proven successful and yielded significantly improved seismic data qualities. Key words: channel acquisition, controllable seismic sources, real-time quality control, synchronous slip scanning, Junggar Basin, Well Ma-131 Block The proprietary channel G3i seismograph is designed for high-density, wide-azimuth acquisition, however there is no field data to support its actual channel capacity, field operation, and data quality or to provide troubleshooting guidance. It is necessary to conduct field tests to examine the performance of the G3i seismograph in high-density, wide-azimuth acquisition and relevant supporting techniques. 1. Overview of test area The Mahu area in the Junggar Basin has abundant hydrocarbon resources and huge exploration potential [1]. The Well Ma-131 Block with significant discoveries in recent years was the major block booked with proved reserves in the Xinjiang oilfield in Existing seismic data suffered from low signal to noise ratio, low resolution and poor imaging; thus, it was hard to accurately delineate structural features, especially minor faults, and to predict complicated oil reservoirs and gas-oil-water contacts. High-density, wide-azimuth seismic surveying is a practical solution to high-resolution seismic prospecting for reserves estimation and petroleum production. In 2013, a 3D seismic survey was deployed in the Well Ma-131 Block with a total area of 388 km 2. The Well Ma-131 Block lies in the Mahu sag at the northwest margin of the Junggar Basin (Fig.1). Surface conditions (including stone deserts, wind-eroded monadnocks in Devil City, oilfield job regions, alkaline lands and water areas) are complicated, but most of the area is covered with stone deserts and is of low relief, which is favorable for the utilization of vibroseis. The area of this 3D seismic survey was large enough for the test of high-efficiency acquisition via the channel seismograph and vibrator. Thus the Well Ma-131 Block was selected as the test area. 2. Technical challenges 2.1. Operation Distance separated slip sweep (DSSS) is a state-of-the-art technique developed based on slip scan for seismic acquisition. A number of vibrator fleets were to be moved and shot simultaneously, which theoretically would double the efficiency when compared to a conventional slip sweep [2 4]. DSSS receiver lines needed to be long enough and at least double the length of the receiver spread. The distance between the two fleets also needed to be large enough to avoid crosstalk. A 3D high-density, full-azimuth vibroseis survey conducted with DSSS using the channel G3i seismograph would meet the requirements for efficiently acquiring high-quality for Well Ma-131 Block. Geometry parameters are shown in Table 1. The geometry in Well Ma-131 Block has the following features. The first is a large coverage density over channels/km 2, more than 4 times as many as that for a con- Received date: 09 Oct. 2014; Revised date: 17 Nov Corresponding author @qq.com Foundation item: State Science and Technology Major Project High-density wide-azimuth seismic prospecting techniques (Grant No. 2011ZX ) and the PetroChina Xinjiang-Daqing Science and Technology Major Project Key exploration & development technologies for sustained production increase in Xinjiang and Tuha oilfields (Grant No. 2012E-34-11). Copyright 2016, Petroleum Industry Press, PetroChina. All rights reserved.

2 2 CHINA PETROLEUM EXPLORATION Fig. 1 Table 1 Structural location of Well Ma-131 Block Main parameters of the observation system Spread 36L(2 6)S420R Longitudinal layout Fold 36 (xline) 35 (inline) = 1260 Bin size Coverage density / (104-channel km-2) Group interval/m Shotpoint spacing/m 25 Receiver line spacing 150 Perpendicular offset 150 Aspect ratio 1 ventional high-density survey. The second is the full-azimuth system. The third is a more regularized layout, small receiver line spacing, and perpendicular offset (of 150 m); conventionally the spacing is larger than 200 m. The fourth is symmetric sampling with group interval and shotpoint spacing (of 25 m) and receiver line spacing and perpendicular offset (of 150 m) Vol. 21, No. 2, 2016 Technical problems to be solved The following four issues were to be addressed for the vibroseis survey with the channel and DSSS acquisition in Well Ma-131 Block Data communication The distance for data communication was large. The maximum shotpoint span and distance of communication were 29.4 km and 16 km, respectively and the survey area was 230 km2, more than twice as many as those for a conventional survey. Complicated surface conditions, e.g. semi-mountainous regions and wind-eroded monadnocks in Devil City and gullies, would have great impacts on data communications between the seismic apparatus and sources. The size of real-time data transmission is large. The size of real-time transmission of source-related data for a seismic survey with 20 vibrators and DSSS acquisition at least doubles that of a conventional survey Operating efficiency The acquisition, with a large number of spreads and groups, as well as DSSS for a doubled number of vibrator fleets, needed better channel capacity, apparatus performance and stability. Methodical management was also needed to ensure normal operation Bandwidth Wide-band excitation was a prerequisite to wide-band data for more detailed structure interpretation and reservoir prediction Real-time monitoring The size of single-shot data was 180 M, and per-day data size exceeded 1.5 T, more than three times as much as those of a conventional survey. Real-time on-site supervision was needed to ensure high-efficiency acquisition and data quality Solutions Data communication Four measures were taken to enhance the signals for remote transmission. The first was to prepare the instrument truck with the communication station, which is equipped with a proprietary tower, mounted lifting pedestal of 30 m, and high-gain antenna. The second was to equip the station for the apparatus with a high-gain antenna of 6 m.

3 Tu Wei et al., Innovative integration of the channel acquisition technique for 3D data acquisition in Well Ma-131 Block 3 The third was to equip the source with a high-gain antenna of 3 m. The fourth was to erect signal relay stations in those areas with hills, wind-eroded monadnocks, gullies and scarps Operating efficiency Three measures were taken to improve operating efficiency Source-driven technique [5] The utilization of the source-driven technique was dependent on the installation of DSD Network, GPS, and etc. When the plate struck the ground, the source would send COG coordinates and a Ready signal to the seismograph which would then automatically match COG coordinates to the closest shot point in the shooting table. If the matching error was accepted, a firing order would be launched automatically for data acquisition [6]. The source-driven mode could be achieved with the G3i seismograph. This time-saving technique could guarantee accurate shot point positioning and operating efficiency because no manual intervention was needed Visualized navigator A digitized seismic crew technique and visualized navigation equipment was used in Well Ma-131 Block after technical field tests were conducted according to operators feedback and production requirements. Visualized navigation equipment loaded with high-definition satellite images positioned and monitored shot points in real-time, which reduced the time of subshot and improve operating efficiency Stakeless acquisition Stakeless acquisition in the Junggar Basin was first employed in Well Ma-131 Block. The job region had relatively low relief. RTK technique was adopted by the digitized crew to ensure the accuracy of vibrator point measurement and reduce the workload while improving operating efficiency. There were stakeless shots in Well Ma-131 Block, accounting for 28% of total shots Bandwidth The precision of seismic data was dependent on wavelet amplitude and side lobe width. The resolution was enhanced thanks to the increase of relative bandwidth (octave). The scanning frequency of a conventional vibrator is 6-84 Hz. The expansion of one octave may be achieved by increasing 84 Hz to 168 Hz at a high frequency end or decreasing 6 Hz to 3 Hz at a low frequency end. Currently, octave expansion at the low frequency end would be more feasible in practice. The signals were grouped into two bands, i.e. the low frequency band (below 6 Hz) and the normal band (above 6 Hz). The purpose was to extend scanning time at low frequencies to further lower the low end, but severe signal distortion occured at the joint between non-linear signals and linear signals, or between low frequencies (below 6 Hz) and normal frequencies (above 6 Hz) [7]. Thus, a long slope (increased from 0.25 s to 0.9 s) was designed to reduce signal distortion [8 9]. The scanning frequency actually used in this survey was 3-90 Hz, which was increased by one octave compared to a conventional survey Real-time monitoring The efficiency of acquisition was greatly improved by DSSS vibroseis. Daily efficiency was increased to above 7000 shots on average, with single-shot data of 180 M and total raw data over 1.5 T per day. In order to ensure single-shot quality [10 11], source performance, receiver spreads and excitation, and data quality was supervised and evaluated in real-time. The following measures were taken Source performance monitoring Performance monitoring by one operator was shared by several operators using visualized navigators in real-time. Supervisors monitored average phase, peak phase, average distortion, peak distortion, average force, peak force, and force signal acquisition rate Process monitoring During data acquisition, information such as return of source quality control data and source signals, source point position overrun, missing shots, data recording quality, ambient noises, spread state, contingencies, new spread testing, etc. would be closely watched via the G3i monitoring system Single-shot real-time monitoring system The high-efficiency acquisition of mass data from Well Ma-131 Block meant it was almost impossible to examine and evaluate data quality manually. Thus the KL-RtQC (i.e. RealQc) system developed by BGP was used for real-time alerts of acquisition parameter and spread definition errors, missing spreads, codes or acquisition data, source out-ofcontrol, non-acquisition, abnormal initial readings, severe ambient interference, etc. Thus, problems occurring during acquisition could be remedied immediately. Efficient quality control was quantified and computerized through the application of KL-RtQC to avoid the low efficiency and inconvenience of manually examining singleshot records and omissions through instrument monitoring. 4. Application 4.1. Highest daily efficiency in China The 3D seismic survey in Well Ma-131 Block was the first case of DSSS acquisition in China. Average daily efficiency reached 7269 shots and the maximum daily effi-

4 4 CHINA PETROLEUM EXPLORATION ciency was shots, which are currently the highest in China Improved data quality Seismic data quality has been improved greatly with better image, higher resolution and better amplitude preservation due to the acquisition of wide-azimuth, wide band and Fig. 2 Fig. 3 Vol. 21, No. 2, 2016 high density in Well Ma-131 Block. Fig.2 shows the legacy data of merged processing and new poststack migration data in Well Ma-131 Block. As per the interpretation of the Triassic Baikouquan Formation (T1b), a target stratum in this area, the resolution of new data has been greatly improved. Fig.3 shows the spectra of legacy data and new data. The Existing and updated sections of Well Ma-131 Block Frequency spectrum of existing and updated sections of Well Ma-131 Block

5 Tu Wei et al., Innovative integration of the channel acquisition technique for 3D data acquisition in Well Ma-131 Block frequency band of new data is expanded by 18 Hz and the dominant frequency is increased by 7 Hz. Fig.4 shows the horizon slices of curvature along T1b for legacy data and new data. The new slice exhibits a better fracture image with more minor faults and styles. 5. Fig.5 shows sand interpretation on legacy data and new data. The new profile exhibits a better image of sands (plotted in black dotted lines) in the Triassic Baikouquan Formation (between two pink lines). Fig. 4 Existing and updated formation curvatures in Well Ma-131 Block Fig. 5 Sandstones on existing and updated sections of Well Ma-131 Block Conclusions The success of Well Ma-131 Block benefitted from the following three points. (1) The vibroseis survey with DSSS and channel capacity was accomplished with proprietary software and hardware systems and proprietary intellectual property rights. (2) The technique of DSSS contributed greatly to seismic prospecting with wide-azimuth, wide band, and high density and to the improvement of data quality in Well Ma Block. (3) Innovative integration of ideas, methodologies, software and equipment supported the application of mass data acquisition. References [1] Kuang Lichun, Tang Yong, Lei Dewen, et al. Exploration of fan-controlled large-area lithologic oil reservoirs of Triassic Baikouquan Formation in slope zone of Mahu depression in Junggar Basin [J]. China Petroleum Exploration, 2014, 19(6): [2] Ni Yudong, Wang Jingfu, Ma Tao, et al. Advances in vibroseis acquisi-

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