IPTC Title: High Resolution, Simultaneous VSP and Land Seismic Acquisition. Scott Robinson, Qatar Petroleum

Transcription

1 Title: High Resolution, Simultaneous VSP and Land Seismic Acquisition Authors Scott Robinson, Qatar Petroleum primary author o P. O. Box 47, Doha, Qatar; (+974) ; s_robinson@qp.com.qa Salva R Seeni, Qatar Petroleum o P. O. Box 47, Doha, Qatar; (+974) ; seeni@qp.com.qa Michel Denis, CGGVeritas o 1, Rue Leon Migaux, 9134 Massy Cedex, France; (+33) ; michel.denis@cggveritas.com Saleh Barakat, VSFusion o P. O. Box 15425, Manama, Kingdom of Bahrain; (+973) ; saleh.barakat@vsfusion.com This is appropriate to be included in the following sessions: #32: New Advances in Geophysics or #39: Quantitative Geophysics: Data Acquisition to Reservoir Model Abstract The Dukhan field is a large oil field in Qatar established in 1941; it contains over 700 wells producing from 4 major reservoirs. There has been a complex history of production & development strategies used at Dukhan; starting with natural pressure depletion for more than 20 years, followed by power water injection since 1989 and gas cap cycling since Over its lifespan the Dukhan field has witnessed large changes in technology and its application. In order to maximize the long term economic recovery from the field, QP is committed to applying leading edge technologies as appropriate to aid in field development. New, state of the art, 3D seismic data combined with updated reservoir models will enable QP to continue to develop Dukhan field for many years in the future. This paper presents a case study relating to the 3D high resolution seismic acquisition currently in progress over the Dukhan Field in Qatar. Presented is how 3D VSP and surface seismic are being simultaneously acquired so as to use the VSP data to enhance the surface seismic project in: processing, interpretation and reservoir characterization. The combination of numerous 18 August, 2009 Page 1

2 VSP s and extremely high resolution seismic is unique to the industry, and offers an excellent example of how the next generation of seismic data are being acquired and used to give much higher resolution, quantitative interpretations. Design The tool used for the 3D VSP operations is a telemetric 100 levels tool with 3 components in each tool. Coupling is achieved through an articulated arm powered through the cable. The choice of the VSP tool helped to design an acquisition able to fulfill several key imaging tasks. The 100 levels tools with 15m between tools delivered a total logged depth of 1485m inside the borehole. Figure 1: Tool anchoring device Figure 2 : Tool electronic device The design should deliver the following information: - High resolution data from reservoir level (Arab D) up to the Mauddud formation, requiring a very long cable from reservoir at 2100m until 900m depth for Mauddud. - Fracture and anisotropy study at Arab D level requiring 180m of cable below the base of Arab D. - Have an optimized vertical seismic calibration over the whole depth to refine seismic surface processing. The inter bed multiples (or seismic events of similar effects) are one of the issues encountered by the processing of the previous pilot data in D modeling was preformed over dip and strike directions with a model including all the main geological interfaces in depth with their interval velocities. Sources were offset as far as 2.5km from the borehole and coverage maps were compared with different maximum offset for source points. Synthetic response for direct and reflected PP and PS waves were studied for different offsets in order to evaluate the differential between PP and PS waves for an optimum wavefield separation. Finally the effect of recording the full set of sources per swath in the East West dip 18 August, 2009 Page 2

3 direction was also studied, as it was cost effective, the swath acquisition progress being North South. In this case maximum offset in the dip direction were reaching 3.5km. In conclusion the source polygon to be acquired around the borehole was a rectangle area including the full swath length in the East West direction along 4km in the strike (North South) direction. The 100 level tool should have its deepest tool 180m below the Arab D reservoir level and consequently the shallowest one should be inside the Mauddud formation. In any case the minimum offset threshold was set at 1250m meaning that all the source points inside this radius should be acquired W E Simsima Lafan Mauddud Coverage Yamama Arab - D Khuff Figure 3: Modeling by ray tracing in the East West direction Acquisition As all the source points belonging to each 3D VSP were also seismic points of the surface seismic it was set from the start that the surface will never wait or be put on stand-by during the VSP operations. The challenges were quite important: - Rig up the 100 level VSP tool in time, demonstrate the perfect state of the equipment and record a rig source on the whole depth of the borehole before the surface seismic reach the source patch common with the VSP. The sweep used for the zero offset VSP had a much higher frequency bandwidth than the production one (200 Hz high instead of 120 Hz), in order to have a high frequency VSP log to help calibration with the surface seismic and wire-line logs. - Record several tens of thousand of source points with the tool inside the borehole without any major breakdown, and this for an average acquisition time of 10 days. The threshold for a major breakdown was set at two consecutive levels down out of August, 2009 Page 3

4 - The surface acquisition is done with the technique, which is a simultaneous type of recording where four source points are acquired simultaneously. The synchronization between the surface set up and the borehole recording systems for the sources needed to be implemented during a very short time frame just after the full borehole tool set up is operational inside the hole. - For QC purpose, the VSP data will be correlated real time to evaluate the quality and perform some simple processing tasks such as rotation of the horizontal component s. The final data set need to have the same separation algorithm than the surface seismic applied in order to recover the individual source points. This will be done on site before shipment for VSP processing. The first 3D VSP was acquired late June 2009, the second one mid July 2009 and the third one late August For the first VSP, only 75 levels were operating but synchronization and separation were successfully implemented. On the second 3D VSP over source points were simultaneously recorded both in the surface and borehole recorder in about 10 days without any breakdown of the 100 levels inside the well. A deployment system was used for the second operations in order to secure the 100 connections during the rig-up phase. A fast evaluation of the data demonstrated an excellent quality data set inside a fully cased borehole and a excellent vector fidelity of the borehole tool. Figure 4: Borehole tool testing in deployment Figure 5: Borehole deployments racks 18 August, 2009 Page 4

5 PP down PP up PP up PS down PS up SS down Figure 6: Source point offset 1500m component Z, Y, Y, HR, HT, R & RT Interpretation of former VSP data A certain number of former zero offset VSP were evaluated and reprocessed on a reduced scale in order to understand some particular physical properties of seismic rays in the Dukhan field. For DKG28 VSP which is quite interesting as it reaches the Khuff formation there is a clear indication on the P direct arrival of a diffraction caused by an accident at Top Kharaib level followed at depth by a secondary arrival event refracted along the accident. This is correlated by surface seismic interpretation. 18 August, 2009 Page 5

6 trace 6 = 439, 07ft/kb T77 Arab-D trace 248 = ft/kb Signature = L 500 ms Refor 1.7 Gain 30 Diffracted event generated by an accident around Top-Kharaib level-3450ft (?), followed at depth by a secondary arrival event refracted along the accident, with apparent velocity slightly higher than direct arrival. Figure 7: Refracted event generated by an accident at Kharaib level on DKG28 zero offset VSP W- E- Accident possibly seen by the DKG28 VSP, from 650ms extending down to Arab-D Structure map at Mauddud Figure 8: Extension of accident seen on VSP, on former surface seismic on left and on Mauddud structural map 18 August, 2009 Page 6

7 Processing The processing sequence to process the 3D Cube in P mode is quite standard with the following steps: - 3C data QC, analysis and editing. - Geometry QC and update - 3C first arrival time picking - 3 Components rotations - Wavefield separation - Spherical divergence gain correction - Deconvolution - Shot statics - Velocity model building - Vector Kirchhkoff migration - Integration with surface seismic data One good QC of a proper geometry update and overall direct arrival quality is the first arrival pick time seen as an attribute. Results Technical Contribution The first point to make on technical contribution is that several very complete seismic VSP records were successfully acquired simultaneously with a major 3D surface seismic program. By using 100 receiver nodes there was no need to shift the VSP receivers to record the desired interval, therein eliminating the need to repeat 1000 s of shots specifically for VSP acquisition. Processing seismic data acquired over carbonate strata has unique problems associated with the relatively high rock velocities and the nature of carbonate and evaporate strata to generate inter bed multiples or reflections which contaminate the seismic record. Much of the processing sequence is based on algorithms that reduce or eliminate inter bed multiples; with the 18 August, 2009 Page 7

8 simultaneous acquisition of 3D VSP s we were much better able to identify where multiples contaminated the surface seismic data allowing parameters in the attenuation algorithms to be tuned to the most aggressive level without compromising the true signal. Finally this paper presents an exceptional case for combining a variety of data acquisition objectives to meet the demands of modern reservoir characterization project. Establishing high expectations pushed industry limits leading to a premium dataset that is clearly able to extract petrophysical properties at 2 to 4 times thinner increments than was possible from previous data. 18 August, 2009 Page 8