There are a myriad of remote sensing technologies available that provide reservoir management teams with the ability to gather information to assist them with reservoir characterization. From cores at one end of the spectrum which sample minute physical areas but allow for ultra-high definition analysis, to surface seismic on the other end of the spectrum which samples extremely large physical areas but only allows for low definition analysis.
Borehole seismic is a class of technology that bridges the gap between surface seismic technologies and well logging technologies. Borehole seismic has evolved into a powerful technology for 3D imagery and reservoir characterization because it can provide much higher definition images of the reservoir than surface seismic and image the reservoir area surrounding a wellbore far beyond the reach of logging techniques.
Inserting seismic sensors deep into oil and gas wells allows the recording of much higher frequencies as compared to placing sensors at the earth’s surface. The reason for this is simple: seismic waves have to propagate only once through the weathered layer in a confined zone near the source. In contrast, during surface seismic surveys, waves must travel through the weathered layer twice. Each traversal of the weathered layer attenuates high frequencies much more than the low frequencies, thus reducing the image resolution. The frequency content of borehole seismic is typically more than twice that of surface seismic, which provides an increase in subsurface resolution.
Borehole seismic surveys, commonly known as Vertical Seismic Profiling (VSP), have been an industry standard technique for several decades. In the past, however, these data have been used primarily for check-shot type velocity surveys and for reflection mapping at the well location in a one-dimensional fashion. This 1D measurement can be extended to 2D by using one or more walk-away lines of surface source points. The 2D method works well enough for imaging simple layered stratigraphy, but in a complex reservoir a full 3D data acquisition and imaging solution needs to be pursued. The typical 3D borehole seismic image volume is cone shaped with the top of the cone coincident with the top receiver in the borehole array. The size of the base of the cone is determined by the depth of the image volume and the offset of the surface seismic sources.