Sunday, June 7, 2015

Shale Plays Need Seals Too

In an earlier post, I argued that there may be significant lateral migration within shale reservoirs that can lead to higher maturity fluids produced from lower maturity areas, and even occasionally dry gas production in the oil window. In this post, I would like to propose that shale reservoirs also need seals to work. 

Sedimentary rocks have a wide range of pore sizes. In a conventional reservoir, HC saturation builds up due to higher capillary pressure caused by the buoyancy of the column (Schowalter, 1979). Saturation is highest at the crest of the reservoir. 

In a shale reservoir, there may not be an effective column. The increase in saturation and capillary pressure is caused by generation of hydrocarbons. However, it will also require the presence of tight rock facies (above, below and laterally) to prevent migration out of the shale due to the increased capillary pressure. From MICP studies on shales, we see that shales have a wide range of displacement pressures (Pd), from 200 psi to >10,000 psi. The typical tight facies may have a Pd of ~6,000 psi mercury-air (~320 psi oil-water). After saturating the adsorptive kerogen, the generated HC fluid begins to fill the zones with larger pores (the reservoirs with low Pd) first. As saturation in the reservoirs builds up due to continued generation, capillary pressure increases, as hydrocarbons invade progressively smaller pores. Saturation may reach >50% when the capillary pressure exceeds the Pd of the seal and migration out of the shale begins. Obviously, without the sealing facies, a homogeneous rock cannot retain high saturation.

HC wet or partially HC wet pores may initially build up saturation without increasing capillary pressure.
From the above reasoning, higher Pd for the seals inter-bedded with the more porous zones at various scales ( millimeters, inches, to feet ) leads to higher saturation in reservoir intervals. Some shale plays may not work due to the lack of seals rather than the lack of porosity. Most studies on shale plays to date have focused on the porosity of the reservoirs, which ranges between 5 and 15% typically. If you agree with the above argument, perhaps it is also important to look at the seals (with less than 5% porosity) inter-bedded with, above and below the reservoir zones. 

Zhiyong He, ZetaWare, Inc.

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