Fluid Contact Monitoring Using Pulsed–Neutron Logging in Challenging Gas–Filled Boreholes for Production Planning in a Mature Carbonate Gas Field

Abstract

Fluid contact logging using pulsed neutron (PND) tool in challenging gas-filled boreholes is essential for gas field life monitoring, especially following sustained production. The acquired data is used to identify the remaining gas column and pore fluid contact movements for future planning to optimize production. In this field, wells were logged with the PND tool to identify the current GWC.

The gas-filled boreholes with unknown liquid levels was logged with the PND tool in sigma mode. Sigma, near and far count, and its ratio were used to identify the gas-water contact. A minimum of 2 passes were required for quality control using repeatability, which increased the confidence in the interpretation. Ideally, it is always recommended to run PND tool in a liquid-filled borehole to establish confident gas and liquid differentiation. Due to operational constraints, the wells could not be filled up with water. Thus, the PND tool was run in challenging conditions to detect gas/water contact movement after many years of production.

Sigma data revealed the fluid contact changes in the target carbonate reservoir regardless of the borehole fluid variation in all 4 wells, allowing mapping of the remaining gas level. Due to fluid level uncertainty in the borehole, liquid level variation in each well was observed for all wells based on early time period, burst measurement. Despite the variation of borehole fluid, the great depth of investigation of the sigma measurement enabled the fluid contact to be determined. Also, the top of the carbonate could be determined from the high to low deflection, which indicates the transition from shale to carbonate in the upper gas zone. The fluid contact could be identified by the increasing trend from the low sigma gas zone to a higher sigma in the liquid zone. The near and far count also suggest a similar response, though with some effect of gas borehole fluid on the measurement. Repeatability between 2 passes increases the confidence in the interpretation, even in challenging wells where borehole fluid-level changes has been observed. The fluid contact could be determined in all 4 wells that were logged using both measurements (sigma, near and far count). In addition, by running a gamma ray sensor with this tool, the gamma ray log suggests the possibility of scale build up from produced water, which is essential input for future reservoir management.

The fluid contact could be determined despite a challenging logging environment for the pulsed neutron tool using sigma measurements without the need to fill up the gas well with water for ideal logging conditions. The current identified contact, together with possible scale buildup, are then used by the reservoir management team to facilitate future production planning.