INTRODUCING SONIC IMAGING WITH A SLIM-DIPOLE MEASUREMENT IN HIGH-ANGLE WELLS

Abstract

Sonic imaging has been traditionally provided using wireline acoustic logging tools in vertical or deviated pilot wells, and, in horizontal wells, using tractors and drillpipe conveyance, to provide information such as lithological changes and natural fractures extending tens of meters from the borehole. Providing a sonic imaging capability on other conveyance methods, particularly closer to drill time, has been a long-held ambition to reduce rig time and provide more timely information during the well construction process. Particularly where conventional wireline operations are not advisable or not possible, the slim-dipole logging technology offers greater flexibility and reduces operational risk. The small-diameter logging tool can be conveyed through the drillstring using a mud pump flow to deliver the tool out through a specialized 2 ½-in. portal in the bit to the open borehole. Sonic logging conveyed through the drillstring was primarily intended for estimating formation slownesses along extended reach wells providing essential information for designing completions and optimizing perforation performance along the reservoir interval. Providing a sonic imaging capability for this conveyance platform involved a tool firmware modification to extend the waveform listening times for both the monopole and dipole sources and optimizing on-board memory for data storage. New workflows were developed to reduce the interference of the direct borehole modes that typically obscure the underlying reflected arrival events, which account for the differences in the signal response, as compared to a traditional larger diameter wireline tool. Using finite difference modeling, we quantify the effects of the tool’s smaller diameter on the azimuth precision of the sonic imaging measurements and confirm the capabilities for providing useful sonic imaging results. Automated sonic imaging and migration workflows were used to convert these reflected arrival events into a 3D formation model and corresponding log of true dip and azimuth useful for formation evaluation and completions design in a timely manner. We present data from the study area introducing far-field imaging on slim dipole technology. Monopole and dipole sonic imaging waveform measurements were acquired along a highly deviated well through a complex fractured carbonate formation. Horizontally polarized shear (SH) reflections from major horizons through the formation were identified in the filtered dipole waveforms to provide structural insight for layers not crossing the wellbore. In addition, the automated workflows identified mode-converted refracted arrivals and P reflections from fractures along the lateral, and thus complemented the borehole image analysis.