Spatial-temporal evolution of reservoir effective stress during marine hydrate depressurization production

Abstract

In tests of marine hydrate depressurization production in many countries, accidents such as collapse and failure of boreholes have occurred, which caused by the evolution of the effective stress field have motivated further attention. However, in existing effective stress models, the pressure of the fluid in the wellbore is taken as the boundary condition, which did not account for the effects of the wellbore on the effective stress distribution. This paper is aimed at proposing a methodology to research the spatio-temporal evolution of the effective stress during the hydrate depressurization production, in order to provide a theoretical benchmark against which to analyze the borehole stability. We derived a mathematical model of the effective stress by taking into account the interaction between the wellbore and the reservoir on the basis of the theory of elasticity, and coupled with deposited infiltration using fluid-structure interaction theory, a coupled multiple field model considering the kinetic effects of hydrate decomposition was then established. And a solver was developed independently. A case study shows that the wellbore has significant effect on the effective stress within the radius of 0.5 m of the reservoir. At the borehole wall, the radial effective stress is 23.58 MPa, whereas the circumferential effective stress is 7.31 MPa, such nonuniformity of the effective stress lead to the risk of borehole failure. As the production progresses, the effective stress increases rapidly at first and then slowly. The effective stress is slightly affected by the size of the wellbore, it is found that when the outside diameter of the wellbore is increased from 100 mm to 500 mm, the change is less than 3.04%.