New Development of Multi-Detector Pulsed Neutron Instrument for Quantitative Gas Saturation Monitoring in Air- or Gas- filled Boreholes: Physics, Modelling and Application
التفاصيل البيبلوغرافية
العنوان:
New Development of Multi-Detector Pulsed Neutron Instrument for Quantitative Gas Saturation Monitoring in Air- or Gas- filled Boreholes: Physics, Modelling and Application
One important application of a multi-detector pulsed neutron instrument is to detect gas response for determining quantitative gas saturation in the formation. Obtaining this saturation value is achieved by differentiating the gas response from the liquid response. The maximum formation sensitivity to gas and liquid is observed when the wellbore is filled with liquid. When the wellbore is filled with gas, however, the separation between formation gas and liquid responses is reduced. This reduced separation results in increased uncertainty in the log analysis. The current operational solution is to set a plug and the fill the borehole with water. Although this technique helps to obtain the desired formation gas-liquid sensitivity, it incurs increased operational expenses, potential damage to fragile formations, reinvasion effects, etc. To overcome these challenges, a sleeve has been designed to deploy with the pulsed neutron instrument. The sleeve imitates the presence of water in the borehole around the tool and increases the dynamic range to the levels seen with the boreholes filled with water and oil-based fluids. This technique extends the application of multi-detector pulsed neutron instrument to provide solutions in airor gas-filled boreholes. It avoids the potential damage to the formation, diminishes HS&E hazards, and reduces operational expenses by eliminating the operation of loading the borehole with water or brine. The sleeved pulsed neutron instrument has been evaluated using Monte Carlo nuclear modelling for design and validation. Prior to any deployment, a sensitivity analysis is conducted using Monte Carlo N-particle (MCNP) transport code to evaluate the formation sensitivity improvement due to the presence of the sleeve. The impact of various tool-sleeve positions and the performance of various sleeve materials have also been studied. A case study is presented to demonstrate how the sleeved pulsed neutron tool significantly improves formation gas sensitivity as compared to the standard tool in gas-filled boreholes.
