Understanding the current pressure of unperforated gas reservoirs is critical for field development. Perforation of depleted reservoirs results in low hydrocarbon production, unwanted water production, or no flow. Thus, a through-casing well-based reservoir pressure surveillance is essential before perforating the target zones to assess if pressure depletion occurs due to the production of connected reservoirs from offset wells.
Pulsed neutron (PN) logging in wells filled with produced gas can reduce some measurements’ formation sensitivity. For example, ratio-based measurements required for gas saturation and pressure depletion analysis using inelastic gamma-ray count rates can be particularly affected when wellbores are gas-filled. As a solution, a sleeved-PN well logging technique was developed to delineate pressure depletion in cased gas-producing wells. The pulsed neutron source and gamma-ray detectors of the sleeved-PN tool are covered with a layer of hydrogen-rich material, such as fiberglass, thereby improving the measurements’ formation sensitivity compared to ones from a regular PN tool in a gas-filled wellbore. The pressure depletion evaluation workflow includes nuclear measurements from a sleeved-PN tool, the Monte Carlo N-Particle (MCNP) method-based forward modeling of tool responses, and an iterative analysis algorithm.
We deployed a sleeved-PN logging tool in a gas condensate-producing well in the North Sea. The well was produced from deeper Triassic sands with a section of Jurassic sand bodies above which are not perforated and of uncertain depletion. During PN logging, the wellbore was filled with gas from the deeper perforated sands. Pressure depletion analysis was performed using three key measurements: inelastic and capture gamma-ray count rate ratios and a macroscopic thermal neutron capture cross-section (formation sigma). Each measurement revealed distinctive characteristics; therefore, comparing these measurements integrated with forward modeling responses allowed for determining reservoir pressure depletion. Accurate MCNP modeling was a crucial factor in the pressure depletion evaluation.
Furthermore, the current in-situ gas density estimation was based on a series of MCNP modeling results. An iterative method of comparing measured and modeled data with reference to original water saturation was used to calculate the current gas density. The analysis showed indications of pressure depletion in the lower sand section but not in the upper formation. Ratio-based PN measurements were effective in indicating depletion. Formation sigma was practical to compute the current water saturation as it is relatively insensitive to changes in reservoir pressure and gas density.
Evaluating pressure depletion before perforating sands in gas-producing wells, especially when gas is present in the borehole, is challenging. However, advances in PN logging technique and an innovative data analysis method enabled cost-effective monitoring of formation pressure in a cased-hole environment and enhanced confidence in reservoir management decision-making.
