GAS MIGRATION THROUGH SELF-SEALED OR INTACT CLAYSTONE

摘要

This contribution investigates gas migration through fully water-saturated CallovoOxfordian(COx)claystone. Our originality is to evidence the nature of progressive passage through initially macrocracked and water-saturated(i.e. self-sealed)claystone,or through intact(undisturbed)matter. The sample is subjected to hydrostatic pressure,identical to the mean in situ principal stress. We use a mix of gases detected on the sample downstream side by a mass spectrometer accurate to 2-5 ppm(i.e. 2-5 10~(-6)particles). We show that gas breakthrough occurs first,by discontinuous capillary passage,i.e. snap off,followed by continuous breakthrough(i.e. permeation),whether the upstream pressure value is increased or kept constant. Snap off is characterized by small amounts of gas detected at the outlet at random frequency and amplitude,until all gases in the mix are detected continuously and simultaneously. At continuous breakthrough,gas permeability is on the order of 10~(-21)m~2(1nD). For selfsealed claystone,gas breakthrough pressure(GBP)varies hugely in the low range 1.45-3 MPa. For undisturbed claystone,GBP ranges between 4.38 MPa(20 mm thickness)to 5.43 MPa(30 mm thickness),in good accordance with former research. It is concluded that self-sealed COx claystone has significantly weaker gas breakthrough properties than undisturbed matter.