Vertical Seismic Profile in Horizontal Wells

Abstract

Summary. An offset vertical-seismic-profile (VSP) survey was performed on Lacq 91, a horizontal well drilled in a reservoir layer about 50 m thick. This paper describes the field acquisition and the specific data-processing sequence that led to a usable seismic section. The paper also demonstrates the usefulness of horizontal well seismic surveys for reservoir studies. Introduction Horizontal wells have proved to be an efficient way to produce hydrocarbon reservoirs, leading to substantial gains in productivity and recovery. Many horizontal productivity and recovery. Many horizontal wells have been drilled throughout the world and many more are expected. It is therefore time to develop techniques to optimize production through the few hundred meters of production through the few hundred meters of drainhole offered by a horizontal well. From this point of view, one of the more important needs is to evaluate the reservoir in the horizontal well area. Moreover, because the oil industry currently is concerned with reservoir characterization, we should acknowledge the potential horizontal wells have for providing better reservoir appraisal. Among possible reservoir characterization techniques, seismic surveys used in horizontal wells seem to be very promising in giving an image of the reservoir and the surrounding layers over a large area. But the implementation of VSP's in horizontal wells encounters two main difficulties: the ability to run the seismic tool into the horizontal well without damaging the quality of the recordings and the ability to process the seismic data in the case where reflectors are almost parallel with the borehole axis. In June 1988, a borehole seismic survey consisting of a set of three offset VSP's was performed in a horizontal well to prove the performed in a horizontal well to prove the feasibility of and the usefulness in horizontal well seismic. With these objectives, an experiment was performed in Lacq 91, a horizontal well drilled in a 50-m-thick layer of a reservoir in France. At the time of the experiment, Lacq 91 was not producing. Data acquisition used three vibroseis trucks and a three-component downhole tool, including a hydrophone and an inclinometer. Offset VSP's were recorded on 69 different downhole tool positions spaced irregularly at vertical depths (between 352 and 665 m) associated with horizontal departures that varied between 84 and 575 m from the wellhead. Drillpipes pushed the downhole tool in the horizontal section of the well. The processing sequence included data editing, tool-rotation correction, wave separation, deconvolution, and migration. In horizontal wells, ascending and descending events with close apparent velocities cannot be separated by conventional apparent-velocity filters. In this experiment, the ascending waves were very weak compared with the descending waves, so they were separated by spectral matrix filtering. After wave separation, the deconvolved migrated data represented a VSP/common-depth-point (CDP) stacked section with a lateral investigation of about 500 m. Results obtained with geophone recordings are compared with those obtained from the hydrophone recordings, which exhibits a strong level of tube waves. The comparison correlates with information from cemented and perforated zones. To the best of our knowledge, the Lacq 91 experiment was the first VSP operation done in a horizontal well. This paper further describes the field acquisition and the specific data-processing sequence used in the Lacq 91 experiment. It also demonstrates that use of a borehole seismic profile in a horizontal well confirms information available from geological and reservoir engineering studies. VSP Data Acquisition In Horizontal Wells Data acquisition in Lacq 91 involved running a borehole geophone through a wireline into the most vertical part of the well and through a specific downhole system (HORSEIS) in the most horizontal part. Fig. 1a shows the HORSEIS downhole system, which is used to run a seismic sonde into a highly deviated or horizontal borehole to make VSP'S. It can be used on- or offshore with conventional VSP equipment (i.e., vibrators or air guns, a seismic sonde, and a data-acquisition system). Equipment Description. A seismic sonde is linked by a logging wireline several meters long to the sonde support of the HORSEIS system (Fig. 1b). The borehole sonde preferably should include a seismic cartridge containing three geophones. In this experiment, three ungimbaled geophones were mounted in a trirectangular pattern. JPT P. 1486