A Pragmatic Guide to the Identification of Net Pay in Shale-Gas Deposits
Worthington, Paul F.; Majid, Azlan A.
Gaffney, Cline & Associates, Singapore, Singapore.
The identification of Net Pay is an evolving subject for unconventional reservoirs and yet it is emerging as one of the design criteria for completions in shale-gas formations. This emergence has arisen despite the fact that there is no generally accepted definition of Net Pay for these complex host rocks and there is no widely recognized methodology for its evaluation. This paper redresses these shortcomings by setting out the criteria for Net Pay identification in shale-gas formations and by outlining how they should be applied. The first step is the adoption of Net Pay parameters: these are the references for Net Pay cut-offs and they must describe all the key factors that influence Net Pay. They are: total organic carbon referred to thermal maturity; fracturability to include sand-volume fraction and thence brittleness; natural fracture density; effective interparticle permeability to gas in the presence of (sub-)irreducible water; and total porosity. The second stage requires an assessment of the extent to which parametric cut-offs should be specific to reservoir zones. This is not trivial because diverse methods of reservoir zonation and data partitioning can give rise to very different reservoir characters and associated parametric relationships. The third step is the quantification of Net Pay cut-offs in a manner that is driven by dynamic data and by taking due account of analogs. The latter should be used with extreme caution because of the complex nature of shale-gas host rocks. The cut-offs have a higher degree of uncertainty than those used in conventional reservoirs. The last stage is the application of cut-offs in a way that honors scale: in most cases, scale is determined by the spatial resolution of the principal well logs that are used for formation evaluation. It is important to reconcile the scale of application with the ordering effects of sedimentary deposition, e.g. sediment cyclicity. These considerations lead initially to proposed sets of Net Pay cut-offs for all the pertinent parameters. The cut-offs can be used until reservoir-specific information is sufficient to allow the generation of data-driven cut-offs for a given shale gas play or zone. In this way, the identification of Net Pay and thence candidate intervals for completion becomes more assured. This is in accord with the aim of guiding development decision-making to increase the chances of success in the face of pronounced reservoir complexity.
AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012