New development of sensitivity improvement for compensated neutron porosity tool in gas-filled boreholes

Porosity is one of the most important indicators used for hydrocarbon reservoir evaluation and extraction. However, obtain accurate formation porosity measurements in gas-filled boreholes can be challenging, this is because gas cannot slow down neutrons as effectively as water due to its lower hydrogen index, which results in measurement sensitivity reduction and consequently impacts the obtained porosity. One way to address this issue is loading water into the borehole which is commonly referred to as water loading. However, this process significantly increases operational time and cost, as well as shielding perforated gas zones. Therefore, a new strategy is proposed to improve the sensitivity of porosity measurement in gas or air-filled boreholes without the need for water loading. The key to this strategy is to incorporate a sleeve design outside the tool to enhance its performance in gas-filled intervals. The sleeve is composed of a downhole applicable material whose neutron slowing down ability is close to water. The optimized deployment orientation of the sleeve could then be determined through simulation. In this work, three potential sleeve materials are identified and compared based on an existing neutron porosity tool. The impacts of different tool-sleeve-borehole orientations are analyzed to determine the most effective design strategy. A series of test pit experiments is conducted to validate the feasibility of the design.