Impact of Fluid Retention on Bulk Petroleum Properties in Shale

Abstract

Predicting GOR and petroleum properties within an unconventional resource play has become a paramount concern. Subtle changes in bulk fluid composition are manifested by large differences in phase envelope geometry, meaning that pressure drawdown during production may or may not suit predicted fluid types. In addition, the heterogeneity of sedimentary packages as well as the variability of organic matter composition strongly impact producibility of fluids and thus production allocation. Here we present new insights into the evolution of petroleum properties, relating structures within kerogen to retained and expelled fluid chemistries as a function of maturity. Pyrolysis-gc and Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS) were used to investigate source rock extracts, oils and pyrolysates. While pyrolysis-gc lends itself well to analyzing structural units in kerogens and extracts using small compounds of low polarity (e.g. hydrocarbons), FT-ICR-MS is a perfect tool for rapidly characterizing polar NSO compounds in complex mixtures from pyrolysates, extracts and crude oil. NSO compounds are of high interest because they feature functional groups and thus strongly influence sorption, solubility and partitioning of petroleum compounds within unconventional shale system. Differences in the evolution of the petroleum composition of unconventionally and conventionally reservoired oils are revealed by comparing the polar compound composition of (1) extracts of six Posidonia source rock samples with maturity levels between 0.43 and 1.45% Ro, (2) open-system pyrolysates of those six source rocks, and (3) four Posidonia sourced medium gravity conventional crude oils. The aromaticity and degree of condensation was found to increase much more pronouncedly with increasing maturity for retained NSOs than for oil NSOs. Pyrolysate NSOs hold “intermediate” compositions, pointing to a preferential expulsion of smaller compounds in the crudes and enhanced cyclisation and aromatization processes within retained fluids. The latter process was shown to occur at the cost of aliphatic precursors. A genetic link of the fluids as well as the likely timing of petroleum expulsion was revealed by comparing carbon number distributions in connection with alkyl-chain length distributions. The chemical differences documented here are manifested in the sorptive properties of fluids, and need to be taken into account in formulating production strategies.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015