A MECHANICAL MODELING OF THE PRIMARY MIGRATION

In order to adress the question of oil-induced microfracturing, we pro pose under specific assumptions (plane circular kerogen flake surround ed by an homogeneous microfractured porous medium) an analytical metho d for the determination of the oil pressure increase. It is based on a mechanical modelling of the kerogen-oil-rock interaction at the ''mic roscopic'' scale of a kerogen particle. It is shown that the oil press ure tends towards an asymptotic value when the chemical transformation of kerogen is completed. The effect of the macroscopic stress variati on during oil formation process proves to be negligible. However, this effect must be taken into account for describing the evolution of oil pressure at earlier stages of oil formation process. The increase in burial depth induces an increase of oil pressure as well as a variatio n of the macroscopic stress which both determine the microscopic stres s field. The possibility of microfracturing depends on the position of the microscopic stress state with respect to the fracture criterion. If the duration of the oil formation process is short enough, so that the macroscopic stress change associated with the corresponding (small ) burial depth increase can be neglected, it is found that microfractu ring is likely for the usual values of rock tensile strength. However, in the general case, neglecting the macroscopic stress change can sig nificantly overestimate the possibility of fracture initiation due to oil-pressure increase. Considering now the ''macroscopic'' scale of th e source bed, the evolution equation of the oil pressure are derived w ithin the framework of Blot's poroelasticity theory. The oil pressure rate proves to be the sum of a diffusion term which accounts for oil m igration within the source bed, and of two source terms respectively a ssociated with the volume expansion tendency of the kerogen --> oil tr ansformation and the overburden pressure increase.