Reanalysis of in situ permeability measurements in the Barbados de ´collement

摘要

A cased and sealed borehole in the Northern Barbados accretionary complex was the site of the first attempts tonmeasure permeability in situ along a plate boundary de ´ collement. Three separate efforts at Hole 949C yieldednpermeability estimates for the de ´ collement spanning four orders of magnitude. An analysis of problems encoun-ntered during installation of the casing and seals provides insights into how the borehole conditions may have lednto the wide range of results. During the installation, sediments from the surrounding formation repeatedlynintruded into the borehole and casing. Stress analysis shows that the weak sediments were deforming plasticallynand the radial and tangential stresses around the borehole were significantly lower than lithostatic. This perturbednstress state may explain why the test pressure records showed indications of hydrofracture at pressures belownlithostatic, and permeabilities rose rapidly as the estimated effective stress dropped below 0.8 MPa. Even afternthe borehole was sealed, the plastic deformation of the formation and relatively large gap of the wire wrappednscreen allowed sediment to flow into the casing. Force equilibrium calculations predict sediment would have fillednthe borehole to 10 cm above the top of the screen by the time slug tests were conducted 1.5 years after thenborehole was sealed. Reanalysis of the slug test results with these conditions yields several orders of magnitudenhigher permeability estimates than the original analysis which assumed an open casing. Overall the results basednon only the tests with no sign of hydrofracture yield a permeability range of 10)14n–10)15nm2nand a rate ofnincrease in permeability with decreasing effective stress consistent with laboratory tests on samples from thende ´ collement zone.