November-December 2000
Volume 1 No. 6
ABSTRACT
Stress direction has been determined to about 4 km depth under the Mahanadi Offshore basin using four-arm dipmeter caliper logs for six wells spanning an area of about 18,000 square kilometers between the Mahanadi coast and the Eocene “hinge zone.” The hinge zone is a regional feature delimiting the seaward extent of the Mahanadi pull-apart basin. The hinge zone supposedly marks the continent-ocean transition against the Bengal Fan. Breakout orientation deduced from data of the six wells shows a prevalent modal class represented in the histogram plots between azimuth versus resultant of cumulative zone length of breakout interval as well as between azimuth and the frequency of occurrence of breakouts. The minimum horizontal compressive stress direction varies between N40°–60°E for the Eocene- Pliocene sediments lying at depths between 500–2300m, but the direction changes to N40°–80°W for deeper litho-horizons (below 3000m) while approaching the Eocene hinge zone and further oceanward. This interpretation is corroborated by stress analysis results determined from a single well oceanward of the hinge zone by previous researchers. Breakouts are clearly distinguished by the symmetric elongations in caliper difference plots. These features are consistent with known history of formation of the Mahanadi pull-apart basin and general northward movement of continental India.
ABSTRACT
A computer code based on the finite difference method is developed to model multi-coil induction tools in complex 3-D formations such as layered reservoirs with various invasion profiles. The resulting linear system is solved by the Spectral Lanczos Decomposition Method in the frequency domain. The main characteristics of the code are presented, including the grid generation and the algorithm used to facilitate convergence in the Lanczos decomposition process. The code can simulate induction logging in highly dipping beds and in horizontal wells with various invasion patterns such as step, ramp, slope, or annulus profile. Also, non-circular boreholes and invasion profiles can be modeled. Several numerical examples are presented to illustrate the effects of those profiles on induction logs. The code has been tested against 1-D and 2-D codes in various formations with satisfactory results. It has also been used to produce synthetic ILD and ILM logs in dipping beds with invasion zones. When compared with array induction logs, it is seen that the ILM responses are similar to 30-inch AIT logs and the ILD logs are similar to 60- and 90-inch AIT logs. The code requires a minimum of 24 Mbytes of RAM and approximately 60 seconds of CPU time to compute responses of several receivers at one transmitter position on a 300 MHz personal computer.
Field Measurements
of Resistivity Dispersion Using Two Frequency MWD Propagation Resistivity Tools
ABSTRACT
Laboratory data indicating that resistivity is a strong function of frequency at 2 megahertz (MHz) has recently been published. The most common frequency for measurement while drilling (MWD) resistivity tools is 2 MHz. If oilfield formations were dispersive at 2 MHz, then resistivity dispersion should be a major factor in the interpretation of MWD resistivity data. Some published laboratory data even suggest that water saturations measured at 2 MHz are grossly inaccurate due to dispersion effects. However, the analysis of hundreds of MWD logs with data at two frequencies (400 kHz and 2 MHz) demonstrates that very little resistivity dispersion occurs in oilfield formations at these frequencies.
Analysis of practical porous oilfield formations using theoretical models shows that resistivity dispersion should not become apparent unless the frequency is much higher than 2 MHz. Dispersion effects occur at all frequencies. However, the dispersion is much smaller at low frequencies. The question is at what frequency do these effects become a significant factor in data interpretation. Resistivity dispersion is seen in wireline field data when high frequency dielectric tools are used in the same formation as low frequency induction tools. However, when 2 MHz tools and 20 kHz induction tools are used in the same borehole the results are usually comparable. When the results are different, some effect other than resistivity dispersion is usually the cause. The same theoretical models indicate that dielectric dispersion should be significant at 2 MHz, which is in agreement with the two-frequency MWD field data.
Hundreds of two-frequency logs through thousands of formations have been analyzed and several typical examples are shown in this paper. The two different measurements are made almost simultaneously using the same antennas. Separation between the resistivity data at the two frequencies is common. However, other effects such as anisotropy, invasion, dielectric effects, and differences in vertical resolution are found to explain these differences more accurately than resistivity dispersion. All of these other effects cause separation of attenuation and phase resistivity data, separation of data from different spacings, or both. Therefore, these effects can be distinguished from resistivity dispersion when all available data are compared with the theoretical formation responses.
Only one clear case of resistivity dispersion has been found in any of the two-frequency data sets. This log, like all other field logs that are claimed to be dispersive at 2 MHz, is in a shale. There is no known field log from any source which exhibits significant dispersion at 2 MHz in a reservoir rock.