Evaluating New Designs of Land Hydrophones and Geophones for Permanent Reservoir Monitoring

R. Burnstad¹, A. Bakulin¹, R. Smith¹, and Jervis¹

¹Saudi Aramco

Abstract

This study investigated the performance of buried hydrophone and geophone sensors for onshore reservoir monitoring. Careful sensor design is essential to enable the weak 4D signal caused by CO2 injection to be mapped in a region characterized by a stiff carbonate reservoir and a complex near surface. Burying the sensors greatly reduces 4D noise but introduces receiver ghosting, which adversely impacts our ability to image changes in the reservoir. Dual sensor summation (summed geophone and hydrophone) has been utilized to overcome this problem. However, previous tests showed hydrophone repeatability to be significantly worse than for the geophones. Therefore, one primary goal of the study was to develop a system which resulted in comparable repeatability performance for both sensors. A field study was conducted in order to test various sensor types and deployment configurations. Parameters of interest included sensor location (above or below the water table), sensor coupling (sand or cement) and sensor manufacturer. In order to test the sensor performance quickly and efficiently, a new methodology was developed which allows an estimate of signal-to-noise ratio and short term repeatability to be made based on pre-stack data without the necessity to acquire large amounts of data with many sensors and produce a stack. The investigation found that comparable repeatability performance is obtained when both sensor types are deployed below the water table. However, the optimum coupling medium is dependent on both the sensor type and design. It was also observed that repeatability is offset dependent, with reduced repeatability at near offsets due to the influence of the surface noise. Therefore, careful survey design is necessary to ensure satisfactory coverage over the critical offset ranges.