The prompt and accurate measurements of mud properties are well known drilling-response indicators.1,2 An automatic, remote sensing instrument is under development which will measure the following parameters when installed in riser joints down to 3,000 feet depth: mud/gas mass ratio, mud flow rate, and mud density. Measurements will be multiplexed along a single cable and interfaced to the computer-display system of the data recording system.
The purpose of this paper is to discuss the theory of operation, the engineering factors to be considered, and the experimental development fixture which is nearing completion. The test results will be a subject of a subsequent paper when available.
The evolving concept is directed to finding the engineering solutions to the many anticipated environmental and physical problems. These include high pressures, cuttings, and abrasives in the riser, interference from the drill stem, ambient noise conditions, gas bubble size distribution, gas solubility and water density cutting.
The measurement of the three properties will be discussed separately, beginning with the mud/gas mass ratio (?). We define ? as the ratio of the mass of mud to the mass of gas in a unit volume. Since mud is inherently much more dense than gas, even when gas is compressed at 3,000 feet depth, the normal operating range for ? is about 106 - 107. Values of 105 or lower would represent significant gas-cutting of mud.
The theory of operation is based upon the behavior of the speed of sound (c) in a mixture of fluids. For these purposes both mud and gas may be properly considered fluids since neither will support a shear stress. The speed of sound in mud itself as a fluid is determined largely by its porosity, the volume proportion of interstitial liquid to solid matter3. For porosity greater than about 0.5 the speed of sound is almost exactly that of the surrounding liquid. Normal drilling muds with density up to 20 lb/gal will thus behave as fluids acoustically.
Conceptually, as gas is added to the mud the speed of sound in the mixture will decrease from its nominal value of about 1,600 m/sec, since the speed of sound in any gas is much lower than this. When present, the solids in drill cuttings will have a slight opposite influence. The precise value of the speed of sound depends upon the type of gas, temperature and pressure in a known way; the relationship is4.
(Mathematical equation available in full paper)
A curve of c as a function of log10? for typical conditions is included as Figure 1.
The quantities ?1, ?2, c1, and c2 can either be directly measured or deduced from auxiliary measurements. The theory of operation is, therefore, to measure c and then to "compute" ?. Actually, rather than "compute," it is intended to use a statistical estimator for ? which not only handles the various uncertainties in the problem but also provides a predictor for future values.