MODELING OF THE TEMPORAL EVOLUTION OF EFFECTIVE STRESS, PORE PRESSURE, COMPACTION, FILTRATION, AND GROWING OF GAS HYDRATES IN THE CASE OF THE SEQUENTIAL ACCUMULATION OF SEDIMENTARY LAYERS WITH DIFFERENT RHEOLOGICAL PROPERTIES

The processes of the evolutions of the effective stress, compaction, and fluid and gas filtrations in the permeable sedimentary rocks during their sequential accumulation are described in the frame of a poro-visco-elastic (Maxwell-type) constitutive law. But such a process is disturbed by the precipitation and accumulations of a species in its P-T stability zones. This processes lead to decreasing of porosity and permeability. Mathematical model of coupled processes of sediment compaction, pore fluid and gas migration and pore feeling by precipitation is developed.

Mathematical formulation of physical problem of low-viscosity fluid- gas flow in a deformable poro-visco-elastic matrix with moving boundary consists of the system of nonlinear partial differential equations with appropriate boundary conditions. Because the permeability nonlinearly depends upon porosity, and the effective pressure gradient is interrelated with deformation of sediment matrix under the loading, the dynamic of fluid is controlled by the behavior of deformable matrix.

Calculations with parameters that are within the framework of the available geophysical data show that the accumulation of sedimentary layers with a permeability or viscosity differing from that of the main basin fill leads with time to the formation, deep within the basin, of layers that have a higher porosity and a different overhydrostatic pore pressure as compared with the surrounding layers. The lower-viscosity layer of sediment is compacted more rapidly, thereby creating an obstacle for the pore fluid and gas motion toward the surface. Subsequently, as the overlying sedimentary layers accumulate, the compaction process leads to the formation of two zones above and below the lower-viscosity layer, in each of which the porosity decrease with increasing depth becomes more pronounced with time. The pore pressure in the lower zone increases more rapidly than in the upper one due to a rapid porosity decrease in the lower-viscosity layer. On model examples shown as a sequential accumulation of porous sediments with different viscosity quantitatively affects on the evolution and contemporary values of effective stress, pore pressure, fluid and gas filtration and accumulation of gas hydrates in the pores.