Theory for the effects of free gas in subsea formations on tidal pore pressure variations and seafloor displacements

Abstract

Loading of the seafloor by regional-scale pressure variations, such as those imposed by ocean tides, is supported by both the rock matrix and interstitial fluid. The nature of the partitioning of the support between the two depends primarily on the compressibility of the fluid and the compressibility and fluid-transport properties of the rock matrix. In this paper, we examine theoretically the influence of free gas on pore fluid compressibility, on the nature of time-dependent load partitioning, and on the consequent vertical rock deformation and seafloor displacement. An example is the gas trapped below deep-sea gas hydrate. We have derived an expression for the steady state compressibility of pore fluid considering the influence of gas solubility in water. The effect of gas solubility is seen to be important at low, such as tidal, loading frequencies and thus must be included when observations of tidally induced pore fluid pressure variations or seafloor displacements are used to constrain gas content. For very low gas concentrations ng (much less than 0.1%), the steady state fluid compressibility can be enhanced by gas solution/dissolution over the loading cycle by several factors at high ambient pressure and more than an order of magnitude at low ambient pressures (< 5 MPa). At ng > 2%, the fluid compressibility increases sensitively with ng and greatly affects the tidal response of the pore fluid pressure regardless of the solubility. Thus, with careful experimental design, tidally induced pore pressure variations may be used to detect very small amounts of free gas and constrain the quantity if ng > 2%. This method is complementary to using acoustic velocity to constrain the quantity of free gas, which works well in the ng = 0.2-2% range. We have also given an expression for the vertical deformation of subsea formations and hence of the seafloor displacement under tidal loading. The presence of free gas enhances tidally induced seafloor displacement, but the maximum effect is limited by the compressibility of the matrix frame. Given relatively low frame compressibility, tidally induced seafloor displacement is small, of the order of 1 mm, which is presently difficult to detect at tidal frequencies.