Impact of Total Organic Carbon on Adsorption Capacity, In-Place Hydrocarbons, and Ultimate Recovery: A Case Study of the Duvernay Formation in Alberta, Canada
Authors: Sharma Gaurav , Galvis-Portilla Henry 
Published: November 2018

Unconventional reservoirs, especially shale gas reservoirs, exhibit dual porosity (free fluid porosity and adsorbed fluid porosity). The adsorbed volume is a function of total organic carbon (TOC) and thus, higher organic contents are assumed to be directly related to higher hydrocarbons in place. However, this case study tried to evaluate this concept and found that with higher TOC, though gas in place increases the recoverable hydrocarbons reduces due to the low contribution from adsorbed heavier components.

We thoroughly evaluate the impact of organic contents on adsorbed hydrocarbons and further compare with the petrophysical properties and production behaviors; herein using information from the Devonian aged Duvernay Formation in Western Canada. First, multi-well analysis of core and log-derived TOC revealed that variations in organic contents are a function of the stratigraphy and thermal maturity, particularly increases in carbonate contents seems to correlate with lower organic contents, whereas increases in quartz and clays correlate with higher organic contents. Then, adsorption capacities were analyzed as a function of variations in the TOC. Finally, comparisons of hydrocarbons in-place and production contribution of the adsorbed volume is analyzed for different average TOC wells.

It is observed that TOC impacts relative adsorption of methane which further impacts the fluid characteristics (gas wells have higher average TOC as compared to the oil wells). This observation becomes relevant as we could partially understand well performance from fundamental understandings of the variations in organic contents. Results of Langmuir isotherms indicate a significant increase in adsorption of heavier components compared to the increment in adsorption of methane components with higher TOC. This observation is further analyzed for production data of the multi-fractured horizontal wells which suggested the following: 1) desorption in the oil flowing wells increases as the saturation of the oil phase decreases, or in other words when the relative permeability of the gas increases. 2) In the gas flowing wells, desorption does not follow the trend of the relative permeability, while based on Langmuir pressure initial contribution is significant which declines as reservoir pressure drops. Further, for the gas flowing well, the production forecast from calibrated production model (with measured produced volumes) shows that post-production of 10 years, recovery is 3.66% in which contribution from desorption is about 17.6%. This observation in the production analyses highlights how with different adsorption capacities of heavier components, adsorption contribution in the production varies. Finally, post this study it is found that TOC plays a vital role in adsorption capacity, gas in place and in the production performance. The relation of the TOC with fluid characterization and recoverable reserves is complex and should be analyzed with the variation in adsorption and desorption capacity of lighter and heavier components.