Twin Wavefield (Pump-Probe*) Exploration G. Quiroga-Goode* -We are always looking for better ways to define not only the geological macrostructure but also petrophysical properties such as porosity, permeability, fluid type, etc. Actually, it’d be better if we could tell first if there’s hydrocarbons and then focus our resources to define the macrostrucure later to place the geometry in its right place. -It is clear however that if we based our models as if the multiphase porous reservoir rocks behaved as single-phase solids, meeting those goals is going to be a very tough job. -Here I’m presenting the basis of a novel approach for direct hydrocarbon detection which uses a couple of wavetrains, as opposed to single wavefields used in conventional seismic exploration. -It is still in the early stages of development and there are lots of things yet to be done. The theoretical and numerical results that I’ll be showing here clearly reflect this. Nevertheless, there is one thing I’m sure of: the evidence appears to indicate that the method could work. -The preliminary numerical modeling I’ve done so far provides inconclusive evidence. -As you will see further ahead, it turns out that this method is very similar to Seismic Time-Lapse. -I’d love to get feedback from you. Instituto de Investigacion en Ingenieria Universidad Autonoma de Tamaulipas Tampico, Mexico *formerly IMP, Mexico, *from Ursenbach & Stewart, 2004

The basic idea In viscoelasticity, the material properties change with strain Material properties are attached to fluid content Can we use two different waves (strain sizes) to determine fluid-related properties?

t t c2 t TRANSIENT MECHANICAL BEHAVIOUR elastic viscoelastic f theory relaxation creep t viscoelastic strain stress stress t Pilot Wave f c2 r MR MU Velocity Dispersion Tracking Wave strain MR r MU Tracking Wave + MU + MU t

2D P-SV Biot poroelastic theory (1956, 1962) x s = R 22 [ xx + xy ] - R 12 + B 2 f - ], . . . . (1) y yy ], . . . . (2) v x f = -R 12 [ s xx + xy ] + R 11 + B 1 - ], . . . . (3) y yy (4) t [ho (v f - v s ) ] s = ( Q + R ) Ñ · v + R / h o . . . . . . . . . . . . (8) t xx = ( P + Q ) - 2 N y + Q / (5) yy x Ñ · [ho (v f - v s ) ] (6) xy = N ( + ) , . . . . . . . . . . . . . . . . . . . . . . . . . . . . (7) R ij = r ij / ( 11 22 - 12 ), i, j = 1,2 B 1 h o m f ( R + R ) 2 22 ______ _____ k

] TEST MODEL: geology distance (m) depth (m) ho = 0.05 K = 10 mD homogeneous porous background ho = 0.05 K = 10 mD ] pseudo-fractured porous media ho = 0.60 K = 10 D depth (m) Whole model is saturated with water except within the ellipse, which is permeated with gas.

TEST MODEL: geometry . . reflection data transmission data 512 X 512 square cells (Dh = 0.025 m) . distance (m) . transmission data reflection data 1 85 receivers depth (m)

Center of mass velocity ho rfo vf + (1 - ho) rso vs r Pore fluid pressure Fluid content s ho y y pf = - x = -Ñ· [ho (vf - vs ) ] vy= 4.0 5.0 6.0 7.0 8.0 0.0 1.0 2.0 3.0 distance (m) depth (m) + + fluid increase - - fluid decrease

CREWES Collaboration Viscoelastic modeling to have a priori knowledge of Q attenuation Analytical solution considering twin waves in a homogeneous and in a heterogeneous viscoelastic medium Investigate exactly how to implement Analyze converted waves