Fluid Inclusion Stratigraphy (FIS)

Application

FIS is a patented technique (available only through Fluid Inclusion Technologies – FIT) that involves the rapid analysis of volatiles trapped as fluid inclusions in cuttings, core or outcrop samples.

Fluid inclusions may be the freshest samples of reservoir fluids available especially if no or little present day stain is observed as they remain even after pore fluids change or are flushed. The technique has become very popular due in part to the rapid turnover (preliminary data in 1–2 weeks and final report after another similar period), the large numbers of samples from a single well that can be screened and the relatively low cost for the amount of information obtained.

FIT claim that their cumulative experience and database of documented case histories are the most extensive in the world for this type of analysis. Details of the method can be found at: http://www.slb.com/resources/other_resources/product_sheets/core_pvt_labs/fluid-lab-services-fluid-inclusion-stratigraphy.aspx

NB we cannot add diagrams and representative graphs because FIT, now owned by Schlumberger, would not give copyright permission for any material to be reproduced.

A typical well programme will provide almost continuous depth profiles of present and past pore fluid chemistry, which can be mapped in one, two or three dimensions, depending on sample coverage. The applications can be many and include:

Specific objectives of the FIS analysis for a particular well, together with a request that the interpretation is focussed on these factors, should be communicated to FIT. A major advantage is that the FIS results aid selection of the best samples/zones for detailed analysis of fluid inclusions.

If samples can be analysed while drilling is under way, FIT believe that it is possible to estimate the column height, product type and location of GWC before the logging runs and production tests are completed. Alternatively, similar data can be collected on wells near prospects to assess product type (e.g. H2S risk).

Sample requirements

Any core, cuttings or outcrop samples cleaned of surface contamination are suitable for FIS Analysis. For routine well studies a sampling interval of 10 m is recommended.

However, FIT quote a single price “per well” for up to a specified number of samples (contact FIT directly for this information) so it is worthwhile sending up to this total number if enough samples are available from the well. If there are more samples than the “limit”, reservoir horizons should be prioritized with as tight a sampling density as possible.

Analytical procedure

Samples are cleaned of surface contamination and introduced via an automated system into a high vacuum chamber. They are then sequentially impacted with pneumatic rams causing fracturing of the grains and liberation of entrapped fluids.

This volatile fraction is dynamically pumped through multiple quadrupole mass spectrometers where individual molecules are ionized, separated according to their m/z ratio and detected with electron multipliers. The signal is then processed, creating a mass spectrum for each sample (log of response vs m/z over the range 2–180).

The technique documents the presence and relative bulk abundance of geologically important inorganic species as well as organic species with 13 C atoms or less.

Selected ions for all samples from a well are plotted vs depth to create fluid inclusion chemical profiles throughout the penetrated section. The cleaning of the samples is a critical aspect as fluid inclusions represent very small amounts of hydrocarbons and any hydrocarbons remaining on the samples prior to crushing will severely distort the picture.

Potential problems

The main potential pit-fall with any fluid inclusion work is establishing the cleanliness/purity and representability of what is considered in-situ and reliable.

Effective washing of samples is essential to avoid contamination and erroneous/misleading results.

Some of the most likely problems are:

  • oil based mud may occasionally affect data
  • reworked inclusions
  • presence of mature source fragments
  • worn bits causing localised heating and cracking of organic mud additives, generating false inclusions and hence anomalous proximity to pay indications (Ohm & Haneferd 2011).

References

Ohm S.E., Haneferd H. (2011) Drilling conditions making wells unsuitable for fluid inclusion studies on drill cuttings. AAPG Bulletin 95, 537–558.