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Oil-Cracking Kinetic Parameters Play a Crucial Role in the Assessment of Shale Gas Plays


It is generally accepted that the kinetics of cracking reactions within the kerogen – bitumen – petroleum transformation control the thermal evolution of hydrocarbons in the subsurface. Pyrolysis experiments analyzing gas generation from natural oils and model compounds have been conducted to ascertain the kinetic parameters and estimate gas yield under varying conditions. Results from these kinetic studies are utilized in petroleum systems modeling to generate estimates of gas yields. Unfortunately, depending upon the experimental method, composition of oil used, and whether ramped or isothermal heating is used to crack the oil to gas; the kinetic parameters vary dramatically. As an example, a comparison between 7 different gas generation pyrolysis experiments produced Ea (activation energy) parameters that vary between 48 – 88 kcal/mol and A (frequency factor) values that range from 4.5 × 1010 to 3.2 × 1022 s-1. Application of these various kinetic parameters in modeling gas yields will produce estimates with variations in excess of 270%. Prior to development of shale gas plays, initial assessments on gas yields typically underestimate the volumes of OGIP significantly. This is in part due to the difficulty in assessing gas generation from in-situ oil cracking (both adsorbed and free oil within source interval). Using the Utica/Pt. Pleasant shale play as an analog, modeling scenarios were created to assess gas generation utilizing a range of oil-cracking kinetic parameters. The range of Arrhenius parameters were risked – ranking determined from experimental methodology and applicability to burial history fundamentals. These risked values were input into a probabilistic (Monte Carlo) simulation to determine the statistical (P10, P50, P90) range of gas generation. In the Utica/Pt. Pleasant shale play, a most likely yield assessment of 145 bcf/mi2 is similar to yields determined from drilling and petrophysical parameters. Utilizing screened oil-cracking kinetic parameters will greatly reduce the significant under-estimation of gas yields from shale plays prior to development.