Reducing Uncertainties in Formation Evaluation through Innovative Mud Logging Techniques
Authors: Blanc P. , Brevière J. , Laran F. , Chauvin H. , Boehm C. , Fréchin N. , Capot M. , Benayoun A. 
Published: December 2007

Mud logging and LWD services provide the earliest data available while drilling. If the acquisition of gas in mud data while drilling is a universal practice, the integration of such data into formation evaluation studies remains the exception in the petroleum E&P industry. An historical joint Total / ENI research project started in 1997 has developed a new interpretation method called Gas While Drilling (GWD™). This method, based on the integration of surface gas data, PVT and geochemical analyses, logs, etc., enables us to improve reservoir characterization, in terms of hydrocarbon-bearing levels, fluid variations, fluid contacts, probable biodegradation, gas diffusion or leakage, seal efficiency, compartmentalization, lateral extension and so on.

The variety and potential of these applications have led to: considerable improvements in the quality of existing measurementsthe development of new data processing and interpretation processes

In 1996, a continuous and fast measurement of C1 to C5 components, named Reserval, was developed by Geoservices. This new analysis system led to an improvement of the GWD™ method on formation evaluation studies, compared to older systems (such as the Gaslogger). The improvement of both the gas chain equipment and the interpretation method has led to technical specifications for gas data acquisition in order to have the best gas data as possible available to perform a GWD™ interpretation; these recommendations to Mud Logging companies were set up jointly by Total and ENI in 2001.

In this paper the necessity of using good quality gas systems to derive valid GWD™ interpretations with the added value of gas while drilling data for early formation evaluation is demonstrated. Real case studies show the use of GWD™ reduces uncertainties in hydrocarbon-bearing reservoir identification where electric logs remain doubtful, and even yields additional pay zones not seen by logs. This is particularly the case in a multi-layered context, associated with thin beds.

Furthermore, a good quality gas chain (from the gas trap to the analyzer together with data management) is compulsory for hydrocarbon fluid behavior differentiation. It is shown that it is impossible to get a valid interpretation for thin, hydrocarbon-bearing, reservoir identification, with determination for fluid units, if the gas chain does not fit the technical specifications required by GWD™. Not only does the gas chromatograph need to combine a fast time cycle with sensitivity and accuracy, but the gas trap has to be truly, constant volume. This is of prime importance for providing accurate fluid sampling recommendations.

The use at the rig site of a gas chromatograph coupled with a mass spectrometer (FLAIR system by Geoservices) instead of the current single gas chromatograph analyzer is a major technical evolution for gas chain acquisition, and brings extra scope for added-value interpretation for formation evaluation from gas while drilling data. Preliminary results show the great potential of this new way of monitoring the mud gases while drilling. A fundamental understanding of the thermodynamic and geochemical behavior of light hydrocarbons in the C1 to C8 range will lead to the emergence of an innovative geochemical log at the well site.