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.