Multi-frequency LWD Acoustic Tool for Shear Slowness Measurement.

Abstract

This paper presents some case histories involving multi-frequency quadrupole acoustic measurement to obtain the formation shear slowness under varying conditions. Quadrupole modes in high frequencies for fast formations acquired in parallel with low frequencies for slow formations are processed such that the final result is that the combination of both responses yield the shear slowness across the range encountered. These conditions range from large shallow boreholes with unconsolidated formations to consolidated formations where the shear velocity is faster or close to mud compressional velocity. As is well known in the industry, the monopole mode allows the measurement of shear slowness in fast formations. However, for slow formations this refracted shear does not exist which requires other means to be employed. The application of the dipole measurement which is used in wireline encounters problems in the Logging While Drilling (LWD) environment. These problems are due to the presence of the collar which greatly influences the flexural mode in the formation along with the additional signal caused by the collar flexural wave 1. Cases when the shear slowness lies in the range of the fluid slowness, a special treatment needs to be considered. The fact that one frequency (high or low) is unable to accurately respond above and below this threshold emphasizes the advantage and need for having multi-frequency LWD acoustic data to accomodate the demands of the environment. Also the challenging case where the shear slowness is in the same range as the mud compressional slowness will be discussed. All these results point to the importance of the use of a multi-frequency LWD acoustic tool for shear measurements. Some quality control considerations such as the rate of penetration (ROP) need to be taken into account in order to have a successful job. Introduction Accurate shear slowness data have become a fundamental measurement used by the oil industry in many areas: the shear slowness is used to identify the rock strength, shear failure, direction of stresses, and other geomechanical parameters involved in wellbore integrity; is also valuable in conjuction with the compressional slowness when pore pressure prediction models are built to avoid over pressure zones or borehole collapses. In seismic, the shear slowness allows estimating values of poisson 's ratios that are used to generate AVO (amplit ude versus offset) models for hydrocarbon identification or in ties with seismic when the compressional slowness response is not reliable, e.g. when gas zones are present on the seismic line. Because of these results, measuring the shear slowness only in fast formations is insufficient to create a complete borehole or reservoir analysis, especially when slow formations are present. Tang et al. 1 demonstrated that the LWD quadrupole tool is the best candidate for the shear measurement because the tool quadr upole wave is absent when operating in the low-frequency range and the quadrupole wave in a slow formation travels at formation shear velocity at low frequencies. In fast formations, the formation shear velocity can be measured from the second mode of the quadrupole wave. LWD acoustic data have been acquired in two wells from the North Sea and the Gulf of Mexico: The first example shows the acoustic processing over the North Sea data where the shear from the slow formation and the shear from the fast formation have been integrated to recreate a complete shear slowness response. The second example from the Gulf of Mexico, contrasts the shear slowness generated in fast formations from the high frequency quadrupole mode and the refracted shear processed from the high frequency monopole mode, showing the dispersive character of the refracted shear wave (result supported by modeling); also an example of the Acoustic Log Hydrocarbon Indicator (ALHI)7 and the hydrocarbon correction are presented.