Borehole seismic: what does it bring to the understanding of the reservoir?

Abstract

When thinking about what we need for a more comprehensive approach to reservoir description and management, the range of information is large and various. The objective is always to improve our ‘vision’ of the geological setting, including a precise rock-type characterization and a good understanding of hydrocarbon flow paths. Each of these elements of the reservoir is determined with a varying level of uncertainty. Today, there is general agreement that integration of competences in business units is the optimal way to take the full benefit of technology breakthroughs in each individual discipline. Borehole seismic is identified as being critical to increasing the quality and the reliability of processing and interpretation of surface seismic data. If until recently, borehole seismic applications were focused on seismic calibration using vertical seismic profiles (VSP) and sometimes on imaging when surface seismic defaulted, today the link to well data is growing stronger. Since not all disciplines involved in reservoir management can work at the same scale, this raises the problem of scale and model compatibility between the respective disciplines. Here again borehole seismic can play a key role. This is becoming possible because advances have been made in a variety of techniques and methods leading to applications such as multi-walkaway and 3D-VSP, 4C and 4D-VSP, single-well and crosswell acquisitions, amplitude variation with offset (AVO) calibration, and scale change. Moreover, improvements in technology make the acquisition and processing cost-effective. Multilevel, three- or four-component receiver strings and permanent seismic sensors are examples of a continued development of operationally efficient hardware. New borehole seismic techniques such as 4D-VSP or the improvement in more mature ones such as crosswell seismic, offer new images of the reservoir. Although these images may be spatially limited, they are usually of a higher resolution than surface seismic and can offer significant benefit for reservoir management and monitoring. This paper illustrates the benefits of borehole seismic with three case studies, illustrating (1) how we can include uncertainties in the VSP imaging process and its impact on structural interpretation of a North Sea reservoir, (2) why AVO calibration using a walkaway technique is mandatory for a better prediction and interpretation of seismic attributes in terms of lithology and fluid content and (3) 4D walkaway and passive seismic surveys using fibre-optic seismic permanent sensors for monitoring an onshore gas storage.