Reliable Measurement of Saturation-Dependent Relative Permeability in Tight Rock Samples
- Andres Gonzalez (The University of Texas at Austin) | Saurabh Tandon (The University of Texas at Austin) | Zoya Heidari (The University of Texas at Austin) | Pavel Gramin (BP) | German Merletti (BP)
- Document ID
- Society of Petrophysicists and Well-Log Analysts
- SPWLA 60th Annual Logging Symposium, 15-19 June, The Woodlands, Texas, USA
- Publication Date
- Document Type
- Conference Paper
- 2019. held jointly by the Society of Petrophysicists and Well Log Analysts (SPWLA) and the submitting authors
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- 136 since 2007
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Reliable laboratory assessment of water and gas saturation-dependent relative permeability in low permeability rock samples can be challenging. In tight samples, the use of steady-state techniques such as stationary liquid method can also be very time consuming. On the other hand, unsteady-state methods often fail to address viscous and capillary effects which are commonly present in low-porosity and low-permeability core samples. The objectives of this paper are (a) to compare unsteady-state and steady-state methods (stationary liquid) using two desaturation techniques, porous-plate (high pressure membranes) and centrifuge desaturation, for relative permeability measurements, (b) to improve relative permeability estimates by monitoring fluid distribution along the core length using Nuclear Magnetic Resonance (NMR) measurements, and (c) to establish the range of rock properties for which different methods provide reliable results.
First, we selected core samples from multiple tight-gas reservoirs with a wide range in absolute brine permeabilities. The selected core samples have total porosity and Klinkenberg corrected gas permeability values ranging from 5% to 12% and 0.009 md to 2.120 md, respectively. Next, we measured absolute gas and brine permeabilities of each sample. Subsequently, we desaturated the cores to different saturation levels using centrifuge and porous-plate desaturation techniques. At each saturation level, we measured effective permeability to gas of each sample using the pulse decay technique. Then, we measured T2 (transverse relaxation) distribution to quantify fluid saturation at each desaturation stage. Additionally, we measured saturation profile along the core length at each stage in order to monitor and control fluid distribution in the core samples. Finally, we used the unsteady-state technique for measuring relative permeability to gas curves for comparison purposes.
The NMR saturation profile measurements revealed the presence of saturation gradients along the core length after centrifuge desaturation in all samples. Rotating the samples by 180 degrees and further desaturating them mitigated the influence of capillary end effects on measured relative permeabilities.
This paper provides a workflow to estimate relative permeabilities to gas in tight samples with a wide range of porosity and absolute permeability values. Results showed reliable relative permeability to gas measurements with the steady-state method for all the samples, and reliable results for the unsteady-state methods in core samples with klinkenberg corrected gas absolute permeabilities as low as 0.669 md. In core samples with klinkenberg corrected gas absolute permeabilities below 0.669 md unsteady-state measurements were strongly affected by viscous and capillary end effects. Furthermore, the introduced workflow provides robust and accurate gas relative permeability measurements for the range of rock quality included in this paper.
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