Acoustic Logging While Drilling

The acoustic velocities of sub-surface formations are critical in a number of analyses in the oil industry, ranging from borehole stability calculations that aid in drilling wells to formation evaluation and pore fluid identification. It has been possible for several decades to make reliable velocity measurements after drilling operations are complete. While post-drill analysis is valuable, there is a substantial benefit to being able to perform these calculations while drilling operations are ongoing. Real-time seismic ties, pore pressure estimation, and formation evaluation enable better planning and execution of expensive drilling and evaluation operations. Since the mid-1990's it has been possible to make compressional velocity measurements while drilling (MWD). The drilling environment presents many unique challenges to these measurements which impact tool design and data interpretation. The most recent MWD tools are adding the ability to estimate shear velocities. While shear logging is difficult in slow formations under any conditions, in the MWD environment there are additional complications beyond those encountered by wireline or compressional-only MWD tools. Shear velocity is an input into a variety of subsurface calculations. With current technology, we are forced to estimate this quantity when performing real-time evaluations. This necessarily makes the results questionable in cases where lithology and fluids are uncertain - precisely those situations we wish to evaluate. As MWD shear velocity estimates become more reliable and timely, these calculations will become more useful to oilfield operations. The potential of such improvements will be illustrated with a few examples.