Numerical Simulation of Water- and Oil-Base Mud- Filtrate Invasion in Vertical Wells and Corresponding Impact on Monopole and Dipole Sonic Waveforms

Date

2009-08

Authors

Rahmani, Amir Reza

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Abstract

The effect of mud-filtrate invasion on borehole sonic measurements plays an important role in the interpretation of acoustic logs. In this work, I quantify the effects of both water- and oil-base mud-filtrate invasion on monopole and dipole sonic waveforms. Pastefforts have been made to study these effects, but the actual spatial distribution of fluid saturation due to mud-filtrate invasion has not been considered. Studying these effects will improve the interpretation of sonic logs.First, I si mulate both water- and oil-base mud-filtrate invasion using a commercial multi-phase fluid-flow simulator. I assume that the formation is initially saturated with water and gas, and exhibits a capillary transition zone. Simulation of mud-filtrate invasion yields two-dimensional spatial distributions of fluid saturation. Fluid-saturation distributions are then transformed into spatial distributions of acoustic properties, namely rock density as well as compressional- and shear-wave velocities. Subsequently, I simulate the corresponding monopole and dipole sonic waveforms assuming axially symmetric variations of acoustic properties. The studies consider four rock types with different petrophysical properties, including porosity, permeability, capillary pressure and relative permeability. Measurements are simulated in a vertical well at two different measurement locations: at the first measurement location, the receiver array is located at the top of the capillary transition zone; at the second measurement location, half of the receivers are inside the water zone and half are outside the water zone at the bottom of the capillary transition zone. I assess acoustic propagation modes and their velocities withfrequency dispersion analysis. Simulations show that mud-filtrate invasion causes a decrease in the high-frequency asymptote of flexural-mode slowness in frequency dispersion results. Furthermore, the deeper invasion associated with water-base mud can cause an increase in the low-frequency asymptote of flexural-mode slowness in frequency dispersion results. Such anincrease depends on the specific petrophysical properties of the rock under consideration. I also show that variations of rock petrophysical properties impact frequency dispersion results; different frequency components are affected in different manners. For instance, sonic measurements acquired in a tight-gas sand are almost insensitive to invasion for both water- and oil-base mud even though mud filtrate penetrates deeply into the formation.

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