The Application of Well Site Isotopic Analysis for Reservoir Evaluation

Abstract

The primary productive intervals for deep Jurassic wells being drilled in Kuwait are Middle Marrat; secondary productive intervals are the Najmah and Sargelu carbonates. These HPHT reservoirs, with several fractures and faults, are being drilled with high mud weight with the range of 14-18 ppg with high risk of H2S bearing zones specially for Middle Marrat, thus make the wells complicated for drilling, logging and testing. As a result minimal sets of data can be obtained after the drilling phase.

Analyses of propane and butane were performed in laboratories since the '70, in order to deduce the origin and complex history of gases from their compositional variations. In particular, the carbon isotopic compositions of methane, ethane, and propane and carbon dioxide have been found useful for recognition of the origin of a gas. However, never in the past it has been possible to carry out such isotopic analysis at the wellsite, on a continuous sample, while drilling.

The objective of the current study was to establish a generally applicable genetic characterization of gases released from formation and extracted on surface while drilling, from the models already existing in literature, utilizing Advanced Mudlogging Services such as Carbon isotopes ratio analysis of methane and ethane along with high resolution Gas Chromatography for reservoir fluid characterization. This system provided well site real-time continuous measurements of the concentration of formation gases, including isotopic ratios of δ¹³C on C1 and on C2.

The primary components of the system utilized are:

1. High resolution Cavity Ring-Down Spectroscopy (CRDS) laser spectroscope for isotopic analysis, analyzing carbon isotopic ratios of δ¹³C on C1 and Δ¹³C on C2.
2. High resolution Flame Ionization Detection (DUAL FID) for gas chromatography, analyzing carbon components up to C5.
3. A Quantitative Gas Extraction system, and a gas sample flow control system.

Formation Fluid Characterization was performed while drilling via a series of Isotopic Ratios of methane and ethane, which were used to identify the fluid origin, maturity, connectivity between formations and possible migration trends. Fluid components ratios on the gas species from Methane to Pentane were also used to identify sweet spots and potential vertical reservoir flow barriers.

Following analysis of data from a number of three wells in a different three structures, the Well Site Isotopic Analysis along with high resolution chromatography were confirmed as a powerful tool for reservoir evaluation while drilling the HPHT reservoirs with H2S bearing zones, where utilization of conventional downhole tools were found crucial. In contrast to laboratory based geochemical analyses, the Advanced Mudlogderived data is obtained in near real time and can influence data collection while the exploration rig is on site such as pressure points, fluid sample points, sidewall core or conventional core intervals. In particular, the analysis of the data obtained has enabled the identification of reservoir sweet spots, fluid maturity and fluid origin. This enabled precise and optimized sampling points for further laboratory analysis.