THEORY FOR THE EFFECTS OF FREE GAS IN SUBSEA FORMATIONS ON TIDAL POREPRESSURE VARIATIONS AND SEA-FLOOR DISPLACEMENTS

Loading of the seafloor by regional-scale pressure variations, such as those imposed by ocean tides, is supported by both the rock matrix an d interstitial fluid. The nature of the partitioning of the support be tween the two depends primarily on the compressibility of the fluid an d the compressibility and fluid-transport properties of the rock matri x. In this paper, we examine theoretically the influence of free gas o n pore fluid compressibility, on the nature of time-dependent load par titioning, and on the consequent vertical rock deformation and seafloo r displacement. An example is the gas trapped below deep-sea gas hydra te. We have derived an expression for the steady state compressibility of pore fluid considering the influence of gas solubility in water. T he effect of gas solubility is seen to be important at low, such as ti dal, loading frequencies and thus must be included when observations o f tidally induced pore fluid pressure variations or seafloor displacem ents are used to constrain gas content. For very low gas concentration s n(g) (much less than 0.1%), the steady state fluid compressibility c an be enhanced by gas solution/dissolution over the loading cycle by s everal factors at high ambient pressure and more than an order of magn itude at low ambient pressures (< 5 MPa). At n(g) > 2%, the fluid comp ressibility increases sensitively with n(g) and greatly affects the ti dal 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 co nstrain the quantity if n(g) > 2%. This method is complementary to usi ng acoustic velocity to constrain the quantity of free gas, which work s well in the n(g) = 0.2-2% range. We have also given an expression fo r the vertical deformation of subsea formations and hence of the seafl oor displacement under tidal loading. The presence of free gas enhance s tidally induced seafloor displacement, but the maximum effect is lim ited 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.