This paper presents a supposition that significant fluid heterogeneity (GOR | OGR | composition)1 may exist within the drainage volume of a tight unconventional well. The main question to be answered is whether "well-box"2 fluid heterogeneity can have a significant impact on how produced GOR changes with time and, consequently, how it impacts the rate-time performance and ultimate recovery of oil and gas.
We recommend the use of detailed reservoir simulation for a well-box drainage volume, combined with geochemical and petrophysical modeling of in-situ fluid and rock-property distributions to provide a basis for such a study. Simulation results should be compared with actual GOR performance for a large sampling of wells in major basins containing documented, basin-wide variations from leaner gases to lower-GOR oils (e.g. Eagle Ford, Barnett, Montney, and Woodford). Both correlated and uncorrelated spatial variations of key variables - particularly composition (solution GOR | OGR) and permeability - should be studied. A detailed EOS fluid model and comprehensive compositional library of relevant reservoir fluids is also required.
Correlation of geochemical "maturity index" (based on well logs) to GOR and composition has been developed for the Barnett shale. Correlation of mud gas composition to GOR and composition has been developed in the Eagle Ford. In both of these instances, a quantity (log data and/or mud gas) can be measured along the wellbore trajectory to provide a range of fluid and petrophysical properties that can be used to populate an entire well-box volume - e.g. with random distribution, or correlated and mapped distributions.
The main purpose of our paper is to prompt a discussion about the potential impact of in-situ spatial fluid heterogeneity and permeability distribution on producing GOR performance and ultimate EURs of oil, condensate, and gas. Given the little (if any) existing studies of this subject, we hope that this paper will forge a multidisciplinary alliance of geochemists, geologists, petrophysicists, and reservoir modelers together with solicitation of industrial partners who can share production performance data illustrating GOR-time behavior that has not been readily understood with conventional assumptions of uniform fluid within the well-box drainage volume.
