Coupled Pressure-Transient Behavior and Geomechanical Deformation in the Near-Borehole Region of Unconsolidated Clastic Rock Formations

摘要

We describe the development and successful testing of anaxial-symmetric simulation algorithm that couplesgeomechanical deformation with pressure-transient analysis ina stressed vertical borehole. The objective is to quantify thesensitivity of two-phase flow properties of unconsolidatedclastic sediments to presence of wellbore stress. We considerthe situation in which the mechanically stressed rockformations are probed with a dual-packer formation tester toassess petrophysical properties. The simulation takes intoaccount the mechanical deformation of the rock due to in-situstresses as well as by wellbore stress imposed by overbalancemud circulation and by the dual-packer itself. Mechanicaldeformation of the compressible rock causes variations of theporosity and permeability in the near-wellbore region that areevidenced by the pressure transients measured with the dualpackerpressure probes.The processes of multi-phase flow in the near-wellboreregion due to mud-filtrate invasion and fluid pumpout aresimulated with an IMPES finite-difference formulation thatassumes axial-symmetric petrophysical properties and fluiddistributions around the axis of a vertical borehole. On theother hand, mechanical deformation is simulated with a finiteelementcode that assumes 2D axial-symmetric lineardeformation. The latter simulator assumes a pre-stressed initialcondition due to mud overbalance and computes theincremental deformation due to fluid pumpout with theformation tester. Coupling of mechanical deformation withporosity and permeability assumes a staggered-in-time looselycoupled volumetric model that is enforced by the timemarchingIMPES formulation. Sensitivity studies for the caseof unconsolidated clastic formations indicate that relativevariations as high as 15% in porosity and as high as 13% inpermeability can ensue in the near-wellbore region due tooverbalance mud pressures of 700 psi and wellbore pressuresof 6500 psi applied by the inflatable packers. Such spatialvariations of petrophysical properties can significantly bias thecorresponding pressure-transient measurements performedwith the dual-packer formation tester.