Abstract

The paper deals with focused sampling in two reservoirs of a carbonate field off the west coast of India. It was imperative to characterize the fluid through down hole optical spectroscopy prior to trapping the PVT samples. The fluid type in both the reservoirs was known. In one case gaseous phase was predominant for a long time even though the reservoir is oil bearing. The near well bore permeability was very low of the order of 1 md/cp due to drilling induced damage. Because of low permeability high draw down was applied resulting in low flow line pressure which was below the bubble point pressure of the reservoir and therefore the gas fraction was mainly being pulled into the flow line. The well was drilled with oil based mud and fluorescence channel was indicating high fluorescence. The possibility of the fluorescence due to oil based mud was ruled out after the flow line pressure was considerably reduced resulting in no fluorescence. This indicated that fluorescence seen earlier was due to liquid fraction along with gas, an effect similar to that seen with an atomizer. More over the fluorescence values were high which further strengthened the assumption that oil droplets were in the flowing media. With meticulous analysis of the down hole optical spectra from the fluid in the tool flow line and subsequent pressure adjustments oil started to flow after pumping for about six hours and oil PVT sample was trapped

The other case is related to trapping an uncontaminated gas sample. The sample purity was essential as the same was to be matched with leaking gas at the surface and thereafter to track down the gas reservoir that was contributing to the leakage. The reservoir from which this sample was being trapped is a depleted reservoir and no PVT sample was available for this reservoir. Sample studies for other gas reservoirs in the field were available. Due to pressure depletion there was considerable mud loss and after eleven hours of pumping and simultaneous analysis of the down hole optical spectra, the gas sample was trapped with contamination less than 9%

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