Toru Ikegami

Sonic dispersion curves identify TIV anisotropy in vertical wells

Summary Sonic dispersion curves have been shown to be useful at identifying many formation behaviors, such as the type of azimuthal anisotropy when there is shear wave splitting. In this case, stress-induced anisotropy has a signature of crossing dipole dispersion curves while TIV anisotropy, when the borehole is not along the TI axis, has noncrossing or “parallel” dispersion curves. With a borehole along the TI axis, however, no shear wave splitting occurs. In this case, comparing both the dipole and Stoneley dispersion curves to dispersion curves from an equivalent Homogeneous-Isotropic (HI) formation highlights key features of the TIV formations (e.g., shales). In this paper, we highlight five points using field data from Norway, Nigeria, and the US: a) Stoneley and dipole dispersions have systematically lower slownesses (i.e., higher speeds) in TIV formations than in an equivalent HI formation, except for the dipole at zero frequency; b) the Stoneley wave slowness dispersion can even be lower (i.e., higher speeds) than the dipole dispersion at zero frequency when C66 >> C44, c) in strongly TIV formations, the shear log characteristically shows an anomalously high slowness; d) the three parameter ANNIE model is a good approximation for calculating dispersion curves for boreholes along the TI axis; and e) fitting the ANNIE model to the field data suggests that real formation can have a ratio of C66/C44 as high as 2.

Optical Sensors for the Exploration of Oil and Gas

There is a large variety of sensing devices for the exploration of oil and gas. These sensors must operate in exceedingly challenging environments, such as high temperatures and high pressures. The majority of sensors are based on electronics because of the maturity of the technology, but optical sensors have been used successfully for specific measurements where no replacement technology exists. We introduce distributed optical sensing and downhole optical spectroscopy, and their unique measurements and value for the exploration of oil and gas are explained.

Borehole flexural waves in formations with radially varying properties

Elastic wave propagation in a fluid-filled borehole is affected by near-wellbore alteration of formation properties. Near-wellbore alteration can be caused by several sources, such as overbalance drilling, borehole stress concentrations, shale swelling, near- wellbore mechanical damage and supercharging of permeable formations. Optimal completions of a well for production require both identification and estimation of the radial extent of alteration in reservoir intervals. Measured borehole flexural dispersions in the presence of radial gradients in formation properties can be inverted to estimate the radial extent of mechanical alteration. However, the presence of a tool structure that carries the acoustic transmitters and hydrophone receivers also introduces certain amount of bias on the measured borehole flexural dispersions. This paper describes the Backus-Gilbert inversion of synthetic borehole flexural data for radial variation in formation shear slowness (slowness is inverse of velocity). The inversion algorithm accounts for the tool bias on the measured data by introducing an equivalent structure of a heavy-fluid column placed concentrically with the borehole axis. This simple structure enables computation of the eigensolution for a reference homogeneous and isotropic formation that are used for calculating the data kernel in the perturbation integral equation. The solution of this integral equation yields the radial variation in the formation shear modulus in terms of fractional differences in the measured and reference dispersion at various wavenumbers. Results are presented for both radially increasing and decreasing shear slownesses away from the borehole.