LWD Sonic Logging in Large Diameter Surface Holes

Abstract Sonic logging in large diameter surface holes in offshore environments such as the Gulf of Mexico has traditionally been problematic from both operational and economic perspectives. Acquisition of high-quality wireline sonic log data in large diameter, shallow holes is often compromised by borehole enlargement or washout that may occur before wireline tools are run, and by the typically high signal attenuation encountered in shallow formations and large boreholes. Even if adequate quality wireline sonic data can be obtained, the rig time and cost required for the wireline run may be as great as that required for the actual drilling of the surface hole. However, sonic velocity data in these shallower intervals can be particularly important to geophysicists for use in seismic velocity modeling or time-depth correlation. Sonic log data can also be used for formation evaluation and for drilling applications such as pore pressure determination. To improve sonic data acquisition while eliminating rig time and costs associated with wireline logging, a 9 ½" diameter LWD sonic tool has been developed particularly for these logging environments. The tool is designed with two opposed transmitters capable of firing strong signals at multiple frequencies, and dual, seven-element receiver arrays for a high signal/noise ratio in the soft, highly-attenuating formations. The tool is mechanically compatible with larger BHA's used to drill large-diameter surface holes, while providing high-quality compressional slowness (Δtc) logs in boreholes ranging from 14 ¾" to 26" in diameter. Centralization is not required, and the tool is compatible with bi-centered bits; thus, it can also be used to provide porosity and pore pressure information in applications where bi-centered bits preclude the acquisition of high-quality LWD neutron porosity and density data. The sensor is combinable with 9½" directional, gamma ray, resistivity, and annular pressure sensors to facilitate both drilling and formation evaluation applications in real time in large boreholes. Along with the design and modeling of the 9-1/2" sonic tool, example data are presented from large surface holes (17 ½" to 26") in the Gulf of Mexico.