Sourceless Porosity Estimation in Gas Reservoirs Using Integrated Acoustic and NMR Logs

Abstract

Abstract Due to environmental concerns, the use of radioactive chemical sources is discouraged. This presents great challenges for conventional practices. Source-free logging tools, such as Nuclear Magnetic Resonance (NMR) and acoustic tools, have also been used for porosity determination. However, sometimes uncertainties in lithology, mineralogy, and/or fluid properties can result in unacceptably large error bars in porosity by either NMR or acoustic logging alone. Integrating acoustic and NMR measurements for gas-zone porosity estimation has been reported in relatively clean sandstones. The existing method in literature has not been extended to shaly sandstone reservoirs or carbonate formations where clay is also present. We developed a new, practical sourceless porosity-estimation method which uses NMR log to calibrate the acoustic porosity model and which corrects shale effect on acoustic porosity. To apply this method, both clean and shaly water-bearing intervals are selected for calibrating acoustic model parameters. By applying the calibrated model, dry matrix slowness on each depth in the gas-bearing interval can be obtained. Porosity can then be determined by introducing the computed dry matrix slowness to Raymer-Hunt-Gardner transform for the gas-bearing interval. This method has been successfully tested on a number of wireline and LWD logs. In this paper are two case histories; one case study involves a shaly sand reservoir, and the other, a low-porosity carbonate reservoir. For both cases, the sourceless porosity is cross-validated with other approaches for both the water-bearing and gas-bearing sections.