P Edward Knight 1, James Raffle 2, Sian Davies 2, Henna Selby 2, Emma Evans 2, Mark Johnson 1. Abstract

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1 P De-risking Drill Decisions. A case study on the benefit of re-processing conventionally acquired seismic data with the latest broadband processing technology Edward Knight 1, James Raffle 2, Sian Davies 2, Henna Selby 2, Emma Evans 2, Mark Johnson 1 1 EnQuest, Aberdeen, UK, 2 ION Geophysical Corporation, Chertsey, UK Abstract Acquiring and processing a new vintage of seismic data can often fall outside the time frame of ongoing field development, In this case, careful and detailed reprocessing of vintage seismic data can be a practical and timely way of de-risking any imminent drilling decisions. Here we consider one such case study over the Thistle field, in the Northern Sector of the North Sea, demonstrating how contemporary de-ghosting of conventional marine streamer data, combined with refined demultiple techniques and iterative non-parametric tomographic presdm model building facilitate more reliable well-track planning. Introduction The Thistle Alpha platform was installed in 1976 and commenced production from the Thistle Field in 1978 and the adjacent Deveron field in 1984 for BNOC/Britoil/BP. After the demerging of Lundin s UK assets in 2010, EnQuest became operator and embarked on a late life extension project including major platform upgrades and refurbishments along with the reactivation of the drilling rig and a multi-year drilling campaign. Platform production rates increased from approximately 4,000 bopd to 11,000-12,000 bopd in late With a field wide recovery factor approaching 60%, and water cut rates in excess of 90% for many of the existing producing wells, the accurate placement of additional wells is key to extending the future life of the field. In the current low oil price environment, maximising the value of existing seismic data is crucial for optimising infill well locations. Consequently, during , Enquest re-processed a acquired survey over the Thistle and Deveron fields in preparation for a renewed drilling campaign in The seismic has been reprocessed using the latest deghosting and model building technology and also making use of recent enhancements in demultiple techniques. An iterative inverse filter approach to deghosting technology (Zhou et al., 2012) followed by a hybrid gridded tomography approach to velocity model building resulted in significant improvements over the crest of the field. This case study shows how the use of these data resulted in beneficial changes to interpretation and were used to de-risk the planned well trajectories of four successful wells drilled in Input Data Quality The data covered an area of 200sq.km centred over a known structural high. Data quality was generally good which is to be expected given the relatively recent acquisition (2007). Conventional denoise techniques to target impulsive, linear and guided wave energy were employed to remove noise in preparation for deghosting. In addition, water depths of m resulted in a significant amount of short period multiple. In order to attenuate this, a proprietary shallow-water demultiple technique based on wave-equation datuming was used. Long period multiples were removed with a delayed-start 3D-SRME and residual multiple was then attenuated using a high resolution Radon filter. Figure 1 shows the results of the demultiple sequence.

2 1Se c 1Km Figure 1- Stack section before (left) and after (right) demultiple Deghosting Following extensive testing, an optimal result was found by deghosting the data after the application of demultiple. An iterative inverse operator was extracted from the data and then applied to suppress side-lobes and increase bandwidth from 10-40Hz (2 octaves) to 6-44Hz (2.9 octaves) in the target interval. Figure 2 shows the data before and after deghosting. 1Sec 1Km Figure 2- Stack section before (left) and after (right) deghosting: enhanced continuity of the low frequencies is especially noticeable

3 Model Building Six iterations of hybrid gridded non-parametric tomography (Jones, et al., 2007; Jones 2010) were utilised in the model building update. Calibration against several well markers was made throughout the processing to help guide anisotropy parameters and a TTI anisotropic migration algorithm was employed. Figure 3 shows a comparison of the interval velocity models for the initial and final stages of the velocity model building both overlaid on the final seismic volume. It is clear that the final velocity model conforms to the structure (as expected in this environment), leading to a clearer image. Figure 4 shows the vintage prestm versus final comparison presdm images for an inline showing deeper imaging is significantly improved. De-risking Drilling Decisions The example shown in Figure 5 shows how the reprocessed data resulted in a change to the well trajectory for one of the wells drilled in The concept of the well was to drill a narrow, rotated terrace block near the crest of the field to target the remaining oil in a shadow zone against the main Thistle bounding fault. It was critical to ensure that the well was positioned as close as possible to the fault to minimise the oil left up dip and also to keep away from the influx of water from down dip. However by being too aggressive, there was also the risk of missing the reservoir entirely or drilling through the fault and exiting reservoir too early. A comparison of the vintage prestm and the reprocessed presdm showed a lateral movement of 50m of the fault plane. This meant that there was a very real risk that the original well path, planned on the prestm, would miss the reservoir. As a consequence, the well path was revised to mitigate the risk of overshooting the terrace and requiring a costly side-track. The results show that the drilled well successfully encountered a full hydrocarbon bearing reservoir section and well positioning against the reprocessed seismic was considered key to the success of the well km/s Figure 3 Initial velocity from smooth stacking velocities (left) and final results after 6 iterations of hybrid gridded tomography (right)

4 Figure 4 Vintage prestm (top left) with yellow arrow indicating remnant multiple obscuring tilted faults, and current final presdm converted to time (bottom). Power Spectrum (right) shows increase in bandwidth from 10-40Hz in the vintage prestm (blue) to 6-44Hz in the current presdm (red) at target level resulting from deghosting prestm 2007 BCU ghost presdm 2015 Figure 5 (Left) Vintage prestm; (centre) ION final presdm and the change to the planned well trajectory and (right) final well results from the revised well trajectory. In addition, several other areas showed improvement following the re-processing and this directly resulted in a change to the structural interpretation. Improved resolution of faulting helped to explain how two previously drilled wells had encountered a significantly different thickness of Kimmeridge Clay Formation. The presdm revealed a growth fault where no obvious fault was visible on the vintage data. Also, removal of the BCU ghost through broadband processing helped resolve the top reservoir reflection and subcrop subtleties resulting in a marked improvement in the depth prognosis validated by the post well results.

5 Conclusions It is well known that reprocessing vintage seismic data can often result in uplift in the data quality. Improvements in processing technology and attention to detail in the pre-processing and model building can greatly enhance existing data. Here, one such successful case study has been shown, which resulted in a crucial repositioning of well locations due to improved geological understanding, ultimately de-risking the drilling programme. Acknowledgements We thank EnQuest for permission and encouragement to present this work, Peter Brown, Clare Goodall and Juergen Fruehn for their contribution to the success of the project and Ian Jones for help in preparing this manuscript, and to ION Geophysical for permission to present the processing technology. References Jones, I.F., Sugrue, M.J., Hardy, P.B., [2007], Hybrid Gridded Tomography. First Break, 25, no.4, Jones, I.F., [2010], An introduction to velocity model building, EAGE, ISBN , 296 pages. Zhou, Z. Z., Cvetkovic, M., Xu,B., and Fontana, P., [2012], Analysis of a broadband processing technology applicable to conventional streamer data : First Break, 30, no. 10,