Improved horizontal well logging porosity calculation for a gas reservoir in the Northern Ordos Basin, China

The porosity calculation is the basis for the estimation of permeability and fluid saturation, which plays a significant role in hydrocarbon production. Acoustic logging is a critical method for porosity calculation but it suffers from partial distortions in horizontal wells. In addition, owing to high costs and imperfect technology, the horizontal logging data are always insufficient in the Northern Ordos basin, China. Therefore, porosity calculations based on partially distorted acoustic logs and insufficient horizontal logging data complicate the well logging interpretation. In this study, the abnormal acoustic log phenomena were analyzed based on the stratigraphic correlation and several influencing factors resulting in abnormal acoustic slowness in horizontal wells were summarized. Based on a detailed analysis, we proposed a novel porosity calculation model in which the normal acoustic log was combined with the compensated neutron log. Based on accurate porosity estimations, a fluid saturation model and a permeability model obtained from core analysis were developed and validated by three cases. A comparison with production data indicates that our models are effective and accurate for well logging interpretation, especially in horizontal wells. Therefore, the proposed models can be applied to the formation evaluation of highly deviated and horizontal wells for better hydrocarbon prediction in the future.