Quantifying Seals and Saturations: Capillary Pressure, Pseudo-capillary Pressure and Quantitative Show Assessment

Abstract

Capillary pressure exerts a fundamental control on seal capacity and reservoir behavior. Pore throat radius is a key factor in seal capacity, with the smallest pore throats requiring the most buoyancy pressure to displace water in the pore systems with hydrocarbons. Interfacial tension and wettability play additional roles. When converted to height above free water plots, capillary pressure data can give a good approximation of the free water level if the water saturation is known.

Capillary pressure data can be expensive and time consuming to require. However, by studying flow units in rocks using Winland analysis, porosity and permeability can be used to estimate a pore throat radius. Once this is done, pseudo-capillary pressure plots can be make which give a good approximation of the height above fee water from the permeability and porosity data alone. These data can then be used to estimate seal capacities and reservoir performance.

Seals and traps along migration routes can be modeled using software packages or with simple grid manipulation. Quantitative show analysis can help identify seals, force modification of paleogeographic maps to fit hydrocarbon shows, or help determine seal capacities on faults. Once this is done, migration can be modeled using seals along the migration pathways, providing a much more robust prospect inventory than can be achieved by looking for four-way closures alone.

In addition, Appendix B shows how to build an Excel spreadsheet to visualize potential pore throats sizes. Appendix C illustrates how to build Excel spreadsheets to analyze mercury-injection capillary pressure data and Appendix D similar solutions for pseudo-capillary pressure using porosity and permeability as inputs. Appendix E provides some tips on how to create a seal grid for trap analysis using ARCGIS shapefiles and feature classes.

The major intent of this chapter is to get the reader to the point where a simple knowledge of porosity and permeability, from any source, can lead to a quantitative estimate of seal capacity or reservoir performance. To get to this point, you will learn how to construct pseudo-capillary pressure curves from calculations using regression equations which estimate pore throat size from porosity and permeability data.

Being able to assess seal capacity or position in a trap from Sw in terms of height above free water is key to understanding traps and hydrocarbon shows.