Obaiyed Field Fluid Geochemical Analysis
Authors: Mohamed Mohamed Samy 
Published: December 2007

In the last 2–3 years, MDGC analysis (Rijswijk) and HRGC (Houston) have been improved significantly mainly in areas of resolution and reproducibility. Key dates are 6/96 for HRGC and 1/98 for MDGC. Prior to these dates, data quality was somewhat lower for each of the respective analysis. Earlier studies utilizing these techniques are still considered valid, but work over the last 1–2 years has demonstrated that we are now able to get more out of our data sets. For MDGC, we don't recommend mixing old and new data sets.

Comparison of HRGC and MDGC Methods

The strengths and weakness of MDGC and HRGC analysis are based on some configuratins. Details are discussed in the following sub-sections.

Reproductivity: refers to our ability to work with analytical data collected over time. Excellent reproductivity is one of the primary advatages of MDGC relative to HRGC. With HRGC data, a set of oils has to be anal - yzed at one time because of column aging and variable co-elution of peaks. This is sometime referred to as the "throw away problem". In contrast, all MDGC data can be compared and used in fingerprinting studies, regardless of when the analysis were done. Although the problem for HRGC in practice isn't too bad, this benefit of the MDGC method significantly reduces both costs and turn-around time for application projects.

Effectiveness: since MDGC measures the relative amounts of mono-aromatics, it is not surprising that this technique has proven to be so useful in discriminating slight compositional differences in oils and condensates.

Availability: This issue is concerned with access to the analytical capability for Shell OU customers. The advantage here goes to the HRGC method. This technique is available at Shell labs in both Rijswijk and Houston. In contrast, MDGC is only available through SEPTAR, Rijswijk.

Introduction

Based on a large number of appllication studies both within and outside Shell, it has been established that petroleum fingerprinting technology (PFT) is a Powerful and cost effective tool for:evaluating reservoir compartmentalization anddetermining production allocation for commingled wells/flowlines.

Over the last 4–6 years, researchers in Rijswijk and Houston have significantly improved Shell petroleum fingerprinting capabilities, mainly in the areas of analytical tools, interpretation strategies, and computer software. Two new fingerprinting tools for liquid oil and condensate samples were established in Shell PFT. The first method is called High- Resolution gas chromatography (HRGC) while the second is called Multi-dimensional Gas Chromatography (MDGC).

HRGC is routinely used in petroleum geochemistry studies, both in exploration and production applications. Detailed information on "HRGC" analysis and instrumentation is beyond the scope of this paper, but the common output is a GC trace showing the distribution of peaks/compounds in the oil or condensate sample from C5 to C35. In petroleum fingerprinting work, the main peaks/compounds of interest are the minor peaks that exist in great abundance between nC8 and nC18. Interpreted "HRGC" traces typically provide peak height data on 200- 500 peaks/compounds, including numerous undentified peaks.

MDGC was developed by Shell labs in Rijswijk in the mid 90's, and this analytical method is more advanced than HRCG. Unlike HRGC, which detects all types of hydrocarbon compounds (e.g. straight chained, branched and cyclic alkanes, as well as aromatics), MDGC separates and quantifies only the mono-aromatic compounds in the C8 to C10 range. The technique give greatly enhanced peak separation, enabling precise quantification and much better reproducibility through time.