--> How Typical Langmuir Isotherm Curves Underestimate OGIP and Production by 20%, a Barnett Case Study

Southwest Section AAPG Annual Convention

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How Typical Langmuir Isotherm Curves Underestimate OGIP and Production by 20%, a Barnett Case Study


Description of the Material

Adsorption in kerogen pores in organic-rich shale and tight rocks is often quantified by methane Langmuir isotherms up to typically 1200 psia. However, past experiments have found that at reservoir condition, which is usually much higher than 1200 psia, mono-layer assumption might be inaccurate, especially for a multiple components system. Additionally, the pore size distribution of the shale rock can further compound errors. This paper presents a novel optimized Simplified Local Density (SLD) algorithm to quantify the hydrocarbon in place with experiment validation using Barnett core samples that captures how methane isotherms may be affected significantly at a higher pressure. The new finding may influence petroleum production and resource substantially in organic-rich unconventional reservoirs.


Adsorption model for unconventional oil and gas is imperative to evaluate resource in place and to estimate production performance. By properly modeling adsorption in organic-rich formations under reservoir condition, we may rectify underestimation for reserves and original fluid in place to a significant degree, and impact operational decisions concerning productions in unconventional reservoirs.

Results, Observations, and Conclusions

A multiple components system is modeled using SLD-Peng Robinson Equation of State with pore size distributions obtained from Barnett formations. Results show that adsorption behaves significantly different at reservoir condition than the lab condition, owing primarily the pressure applied. The new method not only parallels pressure and the amount being adsorbed, but also demonstrates a significant difference in volumetric as reservoir depletes. Experiments at reservoir conditions using several Barnett core samples confirm that difference, and show that the cumulative production could be 20% or higher than the case when multi-layer adsorption is not properly accounted for, at the same reservoir pressure.

Significance of Subject Matter

  1. This paper proposes a new adsorption modeling algorithm that consistently evaluates adsorption effect for reservoir fluid density evolution using industrial accepted equation-of-state model and reservoir pressure depletion.
  2. A substantial increase in oil production and/or reserves is observed when multi-layer adsorption is properly modeled, as compared with using conventional Langmuir isotherms.
  3. The new method may bring additional insight to greatly mitigate uncertainties for productivity and EUR evaluation for unconventional reservoirs.