Summary

Surface gas mud logging has long been overlooked as a critical real-time tool for accurate formation evaluation, despite the potential cost and operational benefits which could be derived. This is due to a perceived inability to yield reliable, quantitative data that truly reflects real gas-in-mud values and subsequently the reservoir fluids. However, recent developments in gas extraction technology have taken place that allows more quantitative gas-in-mud data to be generated.

The most recent advance in gas extraction at wellsite utilises diffusion of hydrocarbon and non-hydrocarbon gases across a semi-permeable membrane as its extraction technique. The less soluble a gas is within a given mud, the higher partial pressure that is exerted on the membrane leading to a higher rate of diffusion (Henry’s law). Previous investigations have shown that extraction of gases from the drilling fluid across the membrane varies in relation to the properties of the drilling fluid, with fluid type and temperature being the most significant factors affecting gas solubility. In modelling the relationship between fluid type, temperature and the specific gas components it has been possible to obtain gas-in-mud values suitable for formation evaluation purposes in a wide variety of situations. However, during the drilling of a well the mud system is subject to many other changes that will influence the take up of gas within the system and the retention time therein. In order to provide improved quantification of the gas values present within the mud, further investigation into the relative importance of other mud rheology factors was deemed necessary. A test rig was constructed to investigate the effect of these fatctors on gas extraction across a semipermeable membrane. In addition to a range of mud types, the mud rheology parameters investigated included temperature, flow, pH, oil/water ratio, viscosity and salinity.

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