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Visualizing Fluid Flow in Structurally Deformed Carbonates Using 4-D GPR and Dynamic Modeling


Structurally deformed Cretaceous rudist grainstones strata in the Madonna della Mazza quarry (Majella mountain, southern Italy) served as a natural laboratory for a novel 1-10 m scale experiment of time-lapse 3D Ground Penetrating Radar (4D GPR) to assess the role of structural heterogeneities in fluid flow in carbonates. For the infiltration experiment 2952 liters of water were infiltrated in 30 hrs from the surface into the host matrix (poro= 35%, perm= 630 mD) in a location where open faults and clusters of deformation bands intersect. Overall, 16 3D GPR surveys were acquired, 2 before and 14 after the infiltration (survey area= 20x20 m). Water decreases the speed of electromagnetic waves increasing the traveltime of subsurface reflections. The application of a warp algorithm allows automatic extraction of timeshifts between pairs of surveys. Values of local water content changes are then calculated using the Topp petrophysical transfer function. Deformation bands play a key role in influencing the fluid migration: these cataclastic features are thin sheets of reduced porosity in the fault zone and have sealing properties for cross-fault fluid migration. Data show a pronounced asymmetry of the infiltrated water mass towards the undisturbed matrix, experiencing higher water content changes (3.5-4%) if compared to the deformation bands area with lower values (1.5-2%). This sharp difference in magnitude of water content changes shows the effect of deformation bands as fluid barriers especially in fully-saturated conditions (i.e. early stages of infiltration). In addition, a noticeable lateral distribution up-dip along a fault plane indicates its role as a preferential flow path. In preparation for dynamic simulation, a static 3D model of the surveyed area was constructed using information from GPR volumes and measured petrophysical parameters from plugs. For the dynamic simulation, due to computational constraints, the static model had to be simplified in terms of precision (less accurate geometries) and resolution (fewer cells). This dynamic modeling fails to reproduce the fluid flow observed with 4D GPR. In particular, the role of structural heterogeneities on the flow behavior is lost. This comparison indicates that standard dynamic modeling does not capture the influence of small-scale structural features on fluid flow.