Until now the use of permanent systems has required the drilling of additional monitor wells. In terms of instrumentation, permanent downhole seismic sensors represent the cornerstone for the implementation of full-field continuous passive seismic monitoring. The use of permanent downhole seismic sensors for use during 4D studies offers the prospect of accurate well ties, wavelet characterisation, and VSP on demand. A series of tool deployments within active wells has demonstrated that standard tool designs result in a noise level that is too high for viable microseismic monitoring. Common noise levels in such an environment vary from around 1 µ/s RMS to over 100 µ/s RMS depending on flow rate and completion design. Given that most recorded microseismic signal amplitudes are below 0.5 µ/s RMS, it is unsurprising that conventional downhole tools are unsuited for microseismic monitoring. The development of the PS3 (Permanent Seismic Sensing System) tool and the ?-lok mechanism solves this problem, providing a solution for the viable monitoring of microseismic activity from active wells. This is achieved by properly decoupling the sensor array from the flow noise in the tubing. Unlike conventional "decoupling" methods, the ?-lok completely detaches itself from the tubing. This feature results in a noise floor that is limited only by system noise and vibration in the formation itself. Although the combined value proposition for permanent passive seismic monitoring and 4D seismic remains undecided, the downhole instrumentation required to investigate this proposition is now real and present.


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