SeisCORK engineering design study

Abstract

The goal of SeisCORKs is to make simultaneous and co-located seismic, pressure, temperature, pore water chemistry and pore water biology measurements in the seafloor. We want to see the small events in the vicinity of the borehole for three reasons: 1) After an event fluid may flow in the formation in response to the changing stress regime. Down to what magnitude of event do the pressure transients in the well respond? 2) Fluid flow causes small earthquakes. One mechanism for example is by changing the temperature of the rocks which expand and contract, altering the stress regime. We want to look for this fluid flow. 3) Laboratory studies of rock deformation show that shear fracture is preceded by the coalescence of interacting tensile microcracks which are observed as “acoustic emissions”. By placing high frequency geophones next to faults it may be possible to observe these “acoustic” precursors to rock failure. Since in reservoirs on land small events appear in the frequency band 400-800Hz, no one has yet tried to observe them in oceanic crust. SeisCORKs also obviate the considerable logistical, administrative, and clearance difficulties associated with scheduling a shooting ship to run offset VSPs. We resolved to start with a “tubing conveyed” SeisCORK configuration consisting of four three-component sondes at 50m separation lowered on the outside of 4.5in casing (or drill pipe) inside 10-3/4in casing.