Abstract

This paper reports a complete description of a device, a numerical method and of a new experimental procedure that allows direct measurements of low and very low permeability on cuttings (small fragments of rock, down to 1–2 mm in diameter).

The results show the determination of shale permeability even in the order of few nanodarcies. This technique is included in a complete package of standard' measurements on cuttings (sonic, rock strength, porosity) and in particular it extends the range of permeability measurements, technique already developed for reservoir cuttings and presented elsewhere. Thanks to this technique it is now possible to obtain, while drilling, a complete permeability log' of rock formation even for very low values.

Traditionally, permeability can be determined on cores by a steady state flow rate measurement. To measure a permeability from 0.1 mD to 1 nD (=10-9 darcies) the pulse decay technique can be used. This technique is a transient method and it is implemented under unsteady-state conditions.

In a rock saturated with fluid, diffusion of a pressure gradient is much faster than the steady state flow of fluid itself. The transient method is essentially a measurement of the pressure gradient diffusion and it is applicable even for very low permeable materials. The technique applied to cuttings is described in this paper.

Some limits are intrinsic in this technology: the measurement is feasible only if cuttings have at least few mm in diameters; no discrimination is possible from a cutting's measurement in order of directionality (horizontal or vertical permeability); only matrix permeability is revealed from this measurement as no fracture permeability is detectable from the sole measurement on cuttings.

A new experimental apparatus is developed to run the tests, even at the rig site. Field results are presented and discussed along with the numerical methods to calculate permeability values from unsteady-state tests in the transient method.

From a rock mechanics perspective one application of the permeability log while drilling in shale formations is the calibration of wellbore stability models. Permeability is one of the key parameters which governs pore fluids re-equilibrium while and after drilling and definitely affects the time related instabilities.

Using this technique in the laboratory, along with other measurements on cuttings representative of field situations, improves shale-fluid interactions evaluation, aiming at optimising mud selection.

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