A DCAMM seminar will be presented by
Principal Reservoir Engineer Ole Jørgensen
Mærsk Oil and Gas A/S
Copenhagen, Denmark
Abstract:
Stress-induced elastic anisotropy results in earth materials when moduli are stress dependent and the state of stress is non-hydrostatic. A known example is that of a borehole, where stress concentration effects cause a complex spatial variation of elastic anisotropy. Here the appropriate material model is that of an elastic orthorhombic medium with material axes locally aligned with the principal axes of stress.
The dispersion characteristics of waves propagating along a borehole in a stress-sensitive formation are studied for the purpose of
inversion. To include sensitivity to stress in the analysis, a finite-element modeling approach which includes the stress-velocity
relationship, the perturbed stress and velocity field near the borehole, and solutions to the full coupled anisotropic and 3D wave equation
is derived.
Of particular interest is the phase velocity of the quadrupole wave, which at low frequencies provides an estimate of formation shear-
wave velocity. Results indicate that the quadrupole mode dispersion is relatively insensitive to the hoop stress effects around the borehole giving greater confidence to shear-wave estimates made from these modes.
The proposed ring-element formulation, which includes Fourier expansions of the field variables in the direction of the circumference,
can be adapted to include a finite-length borehole. On this note the feasibility of “looking a head of the drill-bit” applications will be discussed.
Danish pastry, coffee and tea will be served 15 minutes before the seminar starts.
All interested persons are invited.