Rig-site and Laboratory use of CWT Acoustic Velocity Measurements on Cuttings
Authors: Nes O.-M. , Horsrud P. , Sonstebo E.F. , Holt R.M. , Ese A.M. , Okland D. , Kjorholt H. 
Published: December 2007

A continuous wave technique, CWT, for measurement of acoustic phase velocities on cuttings is presented. The equipment is particularly well suited for testing of small samples like cuttings, and measurements even on sub-mm thick shale cuttings have been performed. This yields potential access to a new source of data on the drilled formation that can also be attained in quasi real-time at the rig-site. The prototype equipment developed is portable, fast and easy to use. Tests have been performed both at the rig-site, as well as in the laboratory. Potential applications are e.g. within estimation of mechanical properties of shales, effects of various fluids/drilling muds, estimation of seismic parameters, and estimation of pore pressure.

Introduction

Acoustic velocities may represent a valuable source of information on formation properties due to the interaction between the propagating wave and the medium it travels through. Traditionally, seismic data are attained prior to drilling. Sonic log data are mainly acquired after, but to some extent during, drilling. Laboratory measurements, usually at ultrasonic frequencies, can be made subsequent to drilling in intervals where successful coring has taken place.

Cuttings produced during drilling represent another, potential quasi real-time source of information that can be attained at the rig-site, as well as a source of information in intervals lacking proper logs and cores. The utilization of cuttings for acoustic measurements has been limited so far, partly due to the difficulties in performing measurements on very small samples. We are only aware of one related paper on this topic, which describes a pulse transmission technique. Furthermore, the ability to perform acoustic measurements on small samples generally yields access to more material, allowing a larger variety of experimental parameters to be tested on a limited amount of material.

Here we present the development and examples of use of a Continuous Wave Technique, CWT, implemented to quickly and inexpensively attain accurate acoustic phase velocities (P- and S-wave) on small samples, in particular cuttings, at ultrasonic frequencies. The CWT methodology has been patented. CWT is particularly well suited for measurements on fine-grained materials like shale, where samples of sub-mm thickness may be handled. CWT has been tested both at the rig-site and in the laboratory, including comparison between velocities as derived from both CWT and sonic logs. Potential applications of CWT include:Estimation of mechanical properties in shales from measured velocities. Such correlations have been published and may allow for a tuning of the mud weight to try to prevent borehole instabilities in shale during the drilling operation. Furthermore, the data may be utilized to improve the selection of drilling parameters.Effects of exposure to various fluids/drilling muds. Knowing that e.g. the mechanical properties of shales tend to be sensitive to exposed fluids, quantification of such effects is important when deciding mud composition during drilling. Since the samples are small, fluid effects will occur relatively fast, reducing the time required to observe fluid effects in materials of low permeability.Estimation of seismic parameters which are needed as input to seismic interpretation. For instance, velocities can be provided in intervals not logged. Furthermore, when proper samples can be found, velocity anisotropies may be determined through measurements on samples with various orientations.Estimation of pore pressure.

In addition to presenting velocity measurements themselves, an application specifically related to shale-fluid interaction will be illustrated in this Paper.

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