High Resolution, Simultaneous VSP and Land Seismic Acquisition
Authors: Barakat Saleh , Denis Michel , Seeni Salva R. , Amortila Philip , Robinson Scott 
Published: May 2016

This reference is for an abstract only. A full paper was not submitted for this conference.

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 1998. 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 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.

Application

This is a unique opportunity to combine and extremely high resolution land 3D seismic program with the simultaneous acquisition of 100 node VSP's. By combining these two acquisitions we are able to: Acquire a simultaneous 3D VSP where receivers cover 1500 feet of wellbore in a single deployment combined with 1000's of vibrator points from the surface seismic therein ensuring consistent source parameters for the VSP's and surface seismic. Have an optimized vertical seismic calibration data set to refine processing parameters of the 3D land project. In particular the VSP data set allows accurate identification of inter bed multiples (or seismic events of similar effect) in the surface seismic along with very high resolution wavelet calibration and identification of thin reservoir units. VSP data are also used for derivation of VTI anisotropy parameters, Epsilon and Delta, and their variation with azimuth. These VSP measured parameters serve as critical input for processing of the surface seismic data. Collect a complete set of wireline logs, core and FMI data providing a very quantifiable reservoir characterization data suite. With calibration of the VSP data to the surface seismic data a world class opportunity is created to extract high resolution petrophysical data from 3D seismic. Identify fracture zones; the VSP is high resolution both vertically and horizontally, and is acquired with a fully populated azimuthal coverage extending from 2 to 5 km from the wellhead. As the VSP is recorded across target intervals these data can be analyzed for shear wave splitting, leading to identification of fault and fracture zones. Using this as a calibration point, the fracture analysis can then be applied to the broader 3D surface seismic.

Results

Thoughtful design allowed a series of wells to be prepared ahead of the 3D surface seismic survey in preparation of simultaneous data acquisition both on the surface and in the wellbore. By casing the wells it was possible to run the VSP rigless with no risk of the wells flowing. This was a critical procedural step that best coordinated the availability of VSP wells with the surface seismic acquisition schedule. The 3D VSP acquisitions provided excellent data that greatly enhanced the analysis and interpretation of the 3D surface seismic data. By collecting numerous VSP's, inter-bed multiples were mapped across the surface survey area allowing this noise to accurately be attenuated from the 3D surface seismic during processing. With the suite of VSP's, a very accurate vertical velocity profile was defined that further enhanced processing velocity analysis by clearly identifying zone with velocity inversions. Perhaps the most useful result was the inclusion of numerous VSP wells to provide reservoir characterization data. These wells were of excellent quality and allowed a very quantifiable analysis of the seismic data for the purpose of extracting petrophysical properties from the high resolution surface 3D seismic. In particular the 10 to 110 Hz bandwidth allowed beds as thin as 15' to be recognized with seismic inversion. The extreme detail provided with the inversion data will form the basis for future reservoir modeling efforts. Though this field is not considered to be highly fractured and faulted, the maturity of the development is such that smaller scale structural features are beginning to play a more significant role in depleting the reservoirs. Identification of faults adjacent to wells with VSP data allowed a much higher confidence interpretation of faults in the larger surface 3D seismic. It has not been determined whether fracture detection was successful, this may be due to little or no fractures being present (few were identified in the FMI analysis) or to the inability of the seismic data to capture the relevant detail required for fracture detection.

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 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.