Permanent Downhole Seismic Sensors for Reservoir Morritoring: A Review
Authors: Bathellier E.J.M. , Czernichow J.A. 
Published: October 2008

Abstract
High resolution seismic measurements are now extending to the kilohertz frequency range in applications such as microseismic monitoring and time-lapse VSP. For this purpose, a new technology of downhole triaxial geophone sondes was developed by Created) Industries either for client or on its own funding. The sondes designed for permanent and wireline use can be installed at each step of the wellcompletion process: outside casing, inside casing, between casing and tubing and through tubing. Results of experimental tests show the receivers to have excellent three-component response at frequencies in the range of 30-800 Hz for 28 Hz geophones.

Introduction
Conventional seism ic exploration has been very productive in delineating reservoir-scale subsurface structures with spatial resolution on the order of 15-20 meters (upper frequency limit 100-150 Hz). However, new seismic applications as microseismic monitoring, time-lapse VSP and crosswell methods have the capability of resolving oil and gas structure details to dimensions of approximately one meter, corresponding to an upper frequency limit of about I kHz. Receivers must have a wideband frequency response, the highest signal-to-noise ratio, a response not modified by artifacts related to the sondes design, and provide more complete information by acquisition of both compressional and shear wave data. For this purpose was developed a new technology of triaxial geophone sondes capable of measuring artifact-free frequency response over the frequency range 1001000 Hz. A critical factor to this application is the physical coupling of the sensors to the formation and the accuracy ofthe geophones in responding to three-component ground particle Illation.

Sondes Design
The downhole triaxial geophone sondes were designed tellpermanent and wireline installations and a sonde model available for each step of the well completion process.

Outside Casing Sonde
A sonde having a nominal diameter of 1.375 inches wire designed to be bolted on casing welded wear pads and to be installed in the borehole annulus. Strong coupling to the formation is ensured by casing cementation. Fig. I illustratethis sonde. The mechanical assembly includes a triaxial array of 28 Hz geophones mounted in a solid stainless steel cylindrical block, sensor amplifiers and power supply. 7 wire logging cable supply power to the electronic circuits and transmit the geophones output signals to the surface.

Inside Casing Sonde
The sonde is a wireline logging tool to be run in casting and boreholes (from 4 in. (0) as shown in Fig. 2. Its body include a DC motor which opens and closes the spring blades. triaxial geophone block is fixed on a spring blade whit:11 delivers a high anchoring force. Blades are uncoupled fromthe rest of the sonde and electronic in a separate cartridge above the main frame. With 28 Hz geophones, the inside casing sonde has a flat frequency response up to 800 Hz.

On Tubing Sonde
This sonde is installed in between tubing and casing ;1IId has a tubular shape to let the tubing go through it as illustratein Fig. 3.