Overlying salt obscures surface seismic images

The Rotliegend Dethlingen sandstone reservoir was discovered in Germany in 1993 using 2D reflection seismic. Follow-up 3D seismic shot in 1994 was the basis for drilling 15 wells that have produced 10 million m³ of gas.

After Well 9 became watered out, the operator intended Well 15 to produce its remaining updip reserves. Production from Well 15 was less than anticipated, however, considering that it intersected the same zone as two of the best-producing wells in the field. The operator wanted to sidetrack the well, but first needed reliable seismic information to determine the cause of the low production. The previously obtained seismic sections were uninterpretable for the complex geology because they were compromised by the presence of salt structures in the overburden.

Dual-set imager acquires borehole seismic data

To evaluate the viability of sidetracking Well 15, a 3D VSP survey was conducted using the VSI versatile seismic imager, which integrates Q-Technology point-receiver seismic hardware and software with three-axis geophone accelerometers acoustically isolated from the tool body to maintain data fidelity.

Data was acquired with a dual setting of the VSI imager—one tool above and one in the salt. Because presurvey modeling optimized the number of shotpoints to 2,100, three-component (3D) traces totaled 126,000. The dual VSI imagers' arrays provided data redundance for proper wavefield separation and dense target coverage, and the 3D data redundancy enabled analysis of 3D ray bending.

For data processing, Q-BorSeis borehole seismic processing delivered high-quality answers with a significantly faster fivefold improvement in turnaround time. Further interpretation was conducted with the Petrel E&P software platform, which effectively integrated the surface and borehole data for quality control and interpretation of the 3D VSP.

Efficient survey clearly shows subsalt structure

The entire survey, including prejob surveying, permitting, shotpoint drilling, data acquisition, and cleanup, took just under 1 month. The VSI imagers contributed to the efficiency with

• rapid mechanical deployment
• very little time between stations
• short shot-cycle time during remote-source surveys
• real-time data processing during acquisition

The migrated 3D VSP data clearly imaged the subsalt structure, revealing additional faults that were previously invisible on the surface seismic. The operator was able to update the velocity-depth model, which allowed remigration of the surface seismic. With the information from the 3D VSP and advanced processing, the operator was able to plan the Well 15 sidetrack for better reservoir access.