Single-Well Salt-Flank Seismic Imaging
Authors: Cameron Drake S. , Chen Sen T. 
Published: October 2008

Reservoirs located adjacent to near-vertical salt flanks can be difficult to image accurately using surface seismic techniques. Exxon has developed a single-well profiling tool to seismically illuminate the salt flank from a nearby well among other uses. High-resolution images obtained from this data will aid reservoir assessment and placement of up-dip wells.

Seismic imaging of a salt flank was analyzed using synthetic data generated from a computer model. The computer model, containing over three hundred layers, was based on a typical Gulf of Mexico geologic salt flank cross-section. The synthetic data were generated using a finite-difference solution of the two-dimensional acoustic wave equation. Prestack depth migration was used to obtain an image because (he velocity model contains rapid lateral variations. The sensitivity of the imaging process to errors in the migration velocity model was tested by migrating the data using various velocity models, As expected, the accuracy of the image varies with the accuracy of the velocity model.

The borehole seismic industry is excited about the development of single-well imaging technology to image salt flanks from a nearby well.three-dimensional modeling and imaging techniques will be developed by a consortium group on Single-Well Seismic Imaging of Salt Dome Flanks.

Introduction

Single-well seismic profiling occurs when the seismic source and receivers are both deployed in the same well as illustrated in Figure 1. A single-well profiling tool records both direct refracted arrivals and waves reflected from interfaces located near the well, The reflected data can be used to image salt flanks, faults, layer interfaces from horizontal wells, and below seismically-opaque layers such as thick interbedded evaporites in the Gulf of Suez, karst topography in Papua New Guinea, and volcanic sills in the northern North Sea. The direct refracted waves can be analyzed for seismic velocity and attenuation. Single-well imaging is a specialized tool which will add to an existing interpretation. It is not intended to be a stand-alone exploration tool. We will briefly describe the single-well profiling tool and seismic modeling and imaging of a salt flank using single-well profiling data.

A Prototype Single-Well Profiling Tool

A single-well seismic profiling tool deploys a seismic source and one or more receivers in the same well. Exxon's prototypesingle-well profiling tool (Chen, 1993) consists of a piezoelectric (PZT) source, multi-level three-component receivers, adjustable source/receiver spacing, and associated electronics. The tool is designed to operate in borehole temperatures up to 150 degrees Celsius at depths down to 20,000 ft in standard diameter boreholes. This prototype tool has the potential to obtain profile data from geologic features several hundred feet from a well depending on formation characteristics. It can also be used for low-frequency logging to measure in-situ seismic velocities and attenuation.

A more powerful vibrator source, which is under development by an industry consortium, is expected to extend the distance of investigation to 1500 ft and generate less tube-wave noise.