Using Cross-Dipole Sonic Logs and
Rock-Physics
Principles for Enhanced Detection of Commercial Gas Sands from
Seismic Data in the Offshore Mediterranean, Egypt
By
Theodore Klimentos1, Bill T Bryant2
(1) Schlumberger, Cairo, Egypt (2) bp Egypt, Maadi Cairo,
Recent exploration in the offshore Mediterranean, Egypt, has resulted in
many gas discoveries, at various depths reaching down to 4.5 Km, approximately.
One of the main challenges in this environment is the ability to accurately
detect commercial hydrocarbon plays from seismic information. Variations in
density and velocity result in impedance changes which, under favorable
conditions, can be detected from seismic amplitutes. However, the probability of
success of this technique heavily depends on several reservoir parameters, i.e.,
depth, lithology, rock and fluid properties, pore pressure, etc. This paper is a
case-study of using cross-dipole sonic logs and Rock-Physics
principles for a
better understanding of the gas-effect on the elastic-wave propagation in porous
sands, and thus enhancing the probability of commercial gas detection from
seismic data.
In a recent exploration activity, offshore Mediterranean Egypt, a well was
drilled aiming to encounter light hydrocarbons at a depth of approximately 2000
m. The strong seismic amplitude response observed at this depth indicated the
presence of a gas bearing sand. However, later on, all acquired log and test
data indicated that the sand is effectively water-bearing with probably only
small amounts of residual gas being present. To improve the knowledge of the
reservoir quality, several high-tech logging tools were acquired, including NMR,
Cross-Dipole shear-sonic, high resolution induction and lateralog resistivity
logs, in conjunction with formation pressure and mobility measurements.
Following the log data acquisition, a detailed evaluation of petrophysical and
geophysical properties, was carried out. This integrated evaluation proved the
following: presence of minor amounts of residual gas and no evidence of
shear-wave anisotropy over the sand. Moreover, the rock-physics
evaluation
indicated that the Kuster-Toksoz elastic-wave propagation model matched the
measured P- and S-wave velocities and thus may explain the strong seismic
amplitute anomaly observed.