Borehole Acoustic Reflection Survey Experiments in Horizontal Wells for Accurate Well Positioning

Abstract

Abstract Reservoir management optimization requires a detailed description of thegeometry and the properties of the reservoir. For horizontal wells, knowingaccurately the position of the borehole in the reservoir is critical. However, seismic techniques do not have sufficient resolution to provide the position ofthe well with respect to the reservoir boundaries with desired accuracy. Sonicmeasurement on the other hand has the potential to provide such information. Weconducted experiments in horizontal wells in Oman using a commercialmulti-receiver sonic tool modified for imaging, with extended spacing betweenthe transmitter and the receivers. Although the tool characteristics are notoptimized for the imaging, we obtained interesting results when the reflectorshave sufficient strength, thus showing the potential of this technique. Thepre-processed and migrated sonic images provided useful information on thegeometry of the reservoir, which could be compared with seismic interpretationand field knowledge. Furthermore, we developed a specific processing techniquethat uses the four different waveforms acquired around the tool at eachreceiver station to separate the reflections coming from above the well and theones coming from below. In this way, we could obtain images that could beinterpreted more easily. In some instances, the position of the reservoirboundaries could be determined with an accuracy of 1 m up to a maximum distanceof 10 m. Introduction Reservoir management optimization requires a detailed description of thegeometry and the properties of the reservoir. Logging-while-drilling andWireline logging techniques only provide such reservoir information near theborehole as they probe on the order of centimeters to a meter deep into theformation. While surface seismic and borehole seismic survey techniques providean image of the geological structure and acoustic characteristics under thelarge survey area, the resolution is limited to more than tens of meters due tothe frequency band used and fundamental survey geometry. This resolution gapcreates various issues when mapping reservoir properties from logging data awayfrom the borehole. Sonic measurement on the other hand uses a frequency rangeof 3 to 10 kHz; it therefore has the potential to fill the resolution gapbetween logging data and seismic data. Refs. 1 through 5 report successfulcases of imaging bed boundaries and fractures away from the borehole usingsonic imaging techniques, which is called Borehole Acoustic Reflection Survey(BARS). These Refs. also report that the resolution of the sonic images isbetter than 1 m and the imaging distance is about 2 to 15 m away from theborehole depending on the survey cases. We conducted a series of experiments for a sonic imaging survey inhorizontal wells in Oman to evaluate the following potential applications:accurate positioning of the borehole with respect to reservoir boundaries,imaging of sub-seismic scale lithological boundaries and stringers away fromthe borehole,high resolution mapping of fractures and sub-seismic faults, andqualification of fracture continuities.