Abstract:
This paper analyzes the stress state of vertical wellbore formation in radial flows: phase, flow model and crustal stress. Also collapse volume is studied based on the Mohr-Coulomb criterion. Define a critical collapse condition: the stress state of northern adjacent rock on the wellbore satisfies Mohr-Coulomb criterion. While the collapse of a gas reservoir in Darcy flow, Darcy-Forchheimer flow or accelerating flow will continue to develop in the near-well formation after the stress of adjacent rock reaches certain conditions. The collapse of a gas reservoir in accelerating flow is more liable to develop in near-well formation and the adjacent rock in this kind of flow is the first to reach its critical collapse condition. However, under the dual effect of non-uniform stress field and accelerating flow, collapse may exist in the maximum horizontal stress direction when the stress of adjacent rock reaches its critical collapse condition. Besides, the critical collapse radius corresponding to maximum horizontal stress is likely to be the largest. And computational formula of collapse volume is given in this paper.
Introduction
When reservoir fluid flows into the wellbore, the pore pressure decreases all the way from the outer boundary to borehole wall [1]. Due to the interaction between fluid and fluid-saturated porous rock, changes in pore pressure will lead to changes in the formation wall stress and the effective stress, which may lead to the borehole instability [2]. Pore pressure gradient of near-wellbore formation corresponding to different flow models in gas reservoir development and testing is various, especially for overpressed gas reservoir. Accelerating effect should be considered in the flow model when flow velocity is large enough. While pore pressure gradient of near wellbore formation in accelerating model is nonlinear and may even be infinite. And the mass flow rate of the fluid has a maximum value [3-5], which will change the effective stress state of near-wellbore formation rock.
During the process of gas reservoir development and testing, adjacent rock is influenced by Darcy flow and non-Darcy flow, the phenomenon of formation sanding has been studied using plastic Coulomb yield criterion and tensile failure criteria, but accelerating effect and collapse volume calculation have not been considered yet [6-9]. Research of compressible fluid showed that there exist blocking flow and infinite pore pressure gradient in the flow model with accelerating effect, however, borehole collapse is not involved [10-13]. Jin Yan et al. [14] studied radial stress of adjacent rock of accelerating flow; they concluded that when the pore pressure gradient is very large, the effective radial stress may be tensile stress. Still circumferential stress analysis and collapse volume calculation were not taken into account.