Pore Throat Controlling Liquid Yield in Shale — Mismatch Between Dry Produced Gas at Surface and Wet Gas or Condensate in the Reservoir

Abstract

In the search of liquid rich hydrocarbons in shale, abnormally dry gas has been occasionally encountered and produced in unexpected locations among otherwise wet gas or even condensate wells. In many of these wells, the isojar data, when available has shown compositions much wetter than the isotubes. Our preliminary series of studies integrating geochemistry and core analysis indicate a relationship between very small pore throats and larger hydrocarbon molecules being retained within the reservoir. Series of phase envelopes have been generated for each couplet isotube-isojar as a complement to a carbon isotope analysis and to Pixler plots (slightly modified for shale reservoirs). Our core based integrated work clearly indicates the link between pore throat and retention of larger hydrocarbon molecules. The larger the difference between the isojar and isotube phase envelopes being linked to a larger molecule retention problem. Such a combined sample analysis, after calibration with cores, can be successfully applied to the horizontal legs of any well, delivering a cheap but reliable way of looking at the shale reservoir quality in the absence of cores. As the lithological change is more gradual than in a vertical well, the difference between the phase envelopes of the isojar and isotube is more reliable, making the technique perfectly suited for horizontal wells; the depth match between isotube and isojar is much better in the horizontal part of the well. The same approach can also be used when comparing gas chromatography and blended cuttings gas samples. This comparison involving blended cutting gas is not new and was extensively used in the past in exploration wells as a semi-quantitative indicator of permeability. Applying the approach to shale is just a simple and natural step; it is relatively cheap, especially if blending is done at a later date in the lab and not at the well site. Pore throat apertures are directly linked to rock fabric and to mineralogical composition, the latter two can be addressed by XRF analysis of drill cuttings that gives the elemental composition of the rocks penetrated. To study old wells with no or limited gas composition data, integration between XRF and Phase Envelopes would thus allow extrapolation to areas and wells that may need a closer look.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016