A new wellbore seismic technique uses the vibrations produced by a drill bit while drilling as a downhole seismic energy source. The technique is described as “inverse” VSP because the source and receiver positions of conventional VSP are reversed. No downhole instrumentation is required to obtain the data and the data recording does not interfere with the drilling process. These characteristics offer a method by which borehole seismic data can be acquired, processed, and interpreted while drilling. Interchanging the conventional VSP source and receiver positions improves the efficiency of recording multioffset surveys for imaging a 3-D data volume in the borehole vicinity. The continuous signals generated by the drill bit are recorded by a pilot sensor attached to the top of the drillstring and by receivers located at selected positions around the borehole. The pilot signal is crosscorrelated with the receiver signals to compute traveltimes of the arrivals and to attenuate incoherent noise. Deconvolution and time shifts of the pilot signal compensate for the effects of propagation from the drill bit to the top of the drillstring. By repeating this process for an interval of the well, a VSP‐equivalent data set is generated. Results from a test well demonstrate that the processed drill‐bit data are comparable to conventional VSP data.

No abstract

No abstract

Crosswell seismic is an emerging technology that provides highly detailed images of the subsurface at the reservoir scale. The technology has the potential not only to delineate complex structures, but also to monitor the effectiveness of hydrocarbon-recovery and CO 2 -sequestration strategies. The technique employs tomographic surveying, whereby a transmitter and receiver are deployed in separate wells. With this setup, interwell velocity profiling and structure can be obtained from direct-wave and reflection processing, respectively.With increased hydrocarbon recovery and CO 2 sequestration becoming more prominent in the oil and gas industry, the ability to monitor the efficiency of these strategies is paramount. In this paper, two case studies of crosswell seismic surveying are discussed, with the focus on high-resolution imaging and monitoring during CO 2 injection for improved oil recovery. A brief description of the measurement theory and its capabilities is provided, followed by a description of the processing workflow and, finally, a discussion of the acquired results.The two cases discussed demonstrate that crosswell seismic was able to successfully monitor the injection through velocity profiling in time lapse and provide answers as to why and how the flow has occurred through interpretation of the crosswell reflection seismic section. The results clearly show that crosswell seismic could significantly reduce the uncertainty and risk associated with injection processes for improved recovery and can extend the technique to sequestration monitoring.

A gas discovery in the Shengping area of the Daqing Oilfield in China was made recently in a large-scale volcanic depositional environment. Because gas in the heterogeneities of formations broken by tectonic activity and localized volcanic eruptions is not common, researchers sought a more detailed reservoir characterization before developing the field. Crosswell seismic data were used to augment existing 3D surface seismic, log, and core data. This provided data at five times the resolution of the surface seismic data to bridge the gap in resolution between surface seismic and well data. Crosswell seismic data were acquired in two wells, 832 m apart, and processed to provide images of reflectivity, velocity, and formation properties from sections produced by amplitude-versus-angle ͑AVA͒ inversion. The state of the art in crosswell seismic is summarized briefly, reviewing progress in data acquisition and data processing over several decades of crosswell technology development. A detailed description of the data acquisition and data processing applied to the data from the Shengping area is also given. An integrated interpretation of the crosswell images with the surface seismic and log data was used to produce a more detailed geologic model. The enhanced geologic model is being used to plan strategic development of the reservoir and to evaluate possible infill well locations. REVIEW OF THE STATE OF THE ART IN CROSSWELL SEISMIC Crosswell seismic technology has been under development since the early 1980s with many research groups inside major E & P companies involved in early experiments ͑Lines et al., 1993͒. Initially re