Evaluation of Heterogeneities and Composition Profiles of Hydrocarbon Mixtures in a Reservoir: A New Analytical Approach
Authors: Causin E. , Consoloni C. , Riva A. 
Published: December 2007

A new laboratory method, based on chemical and geochemical analysis of the residual hydrocarbons extracted by washing the core plugs and/or the drilling cuttings with solvents (toluene or chloroform) before undergoing routine petrophysical analyses, has been developed for evaluating the characteristics of crude oils and gas condensates which saturate a formation. The results of the analyses are expressed in profiles which enable variations in composition and molecular heterogeneity of the hydrocarbons in a reservoir to be clearly detected.

The techniques used are Rock-Eval pyrolysis, which gives indications on the quantity and quality of the residual hydrocarbons present in the rock, gas chromatography coupled with mass spectroscopy (GC-MS) for molecular analysis, and thin layer chromatography with a flame ionization detector (TLC-FID) for subdividing the residual hydrocarbons into different groups (paraffins, aromatics, resins and asphaltenes).

The paper shows the results obtained on analyzing the residual hydrocarbons of cores and cuttings of different wells, including horizontal wells.

Introduction

In the oil industry the study of reservoir rock usually consists of its petrographic, petrophysical and sedimentological characterizations, which are obtained using laboratory methods, well log recordings and from well production tests. It is in this direction that all efforts, aimed at improving knowledge of a reservoir, are being placed. The past few years have witnessed, for example, improvements in the traditional techniques of well logging, and the emergence of others, such as image logs; while many core analysis laboratories have acquired new and more sophisticated equipment, such as tomographers, which enable a more thorough examination of the internal structure of the porous medium and evaluation of the influence which a rock's heterogeneity has on its transport properties.

On the other hand, the only information on the fluids present in a formation has remained that derived from samples taken from well bottom hole or from surface separators.

To extend our knowledge on reservoir hydrocarbons and their distribution in a formation, an integrated series of methods has been developed. These laboratory methods are based on the analysis of the hydrocarbons remaining in core plugs or drilling cuttings, and are quickly able to provide important information concerning the chemical composition and molecular characteristics of the oils or gas condensates present in a reservoir. Despite it being well known that the invasion of drilling mud filtrate can have a negative influence on the quality of these findings, as indeed it can on all other measurements, the small amounts of residual hydrocarbons recoverable are sufficient for the methods which will be described, together several applicative examples, in this paper.

Due to the different nature of the information obtainable and its difficulty of use, this set of methods is based on three different techniques, namely Rock-Eval pyrolysis (RE), gas chromatography coupled with mass spectroscopy (GC-MS) and thin layer chromatography with a flame ionization detector (TLC-FID) (Fig.1).

As a result of these point analyses, it is possible to distinguish between indigenous and migrated hydrocarbons, to investigate their origins and maturity, to establish with precision the existence of any possible gas/oil or water/oil contacts, to draw up molecular composition profiles of hydrocarbons and obtain information concerning their chemical composition, to identify zones in which heavy hydrocarbons (resins and asphaltenes) are present, to establish the real presence of compositional gradients and to obtain indications regarding the productive potential of a formation.

The only real limitation of these methods concerns the use of oil-based mud to drill a well. In fact, the oil used to prepare the drilling mud mixes with the original hydrocarbons, modifying irreversibly the composition of the residual hydrocarbons to be analysed.

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