WELL-BASED MONITORING OF A CO_2 PLUME IN A CARBON DIOXIDE STORAGE PROJECT

摘要

Carbon dioxide (CO_2) capture and storage is thesubject of ongoing research in which severaltechniques are being evaluated to monitor theprogress of the injected CO_2 through the selectedformation(s). Of the formation propertiesdesirable for CO_2 storage, one of the mostimportant is the presence of an effective trappingmechanism to ensure the CO_2 remains in place anddoes not escape to neighbouring formations or tothe surface. Therefore, to confirm that the CO_2plume can be detected and leaks outside the targetformation identified, various monitoringtechniques must be assessed. One such techniquewas tested at the CO_2CRC Otway ResearchFacility in southwest Victoria, and it is the subjectof this paper.Pulsed neutron logging has been used for reservoirmonitoring in the oil business for over 40 years.Conventional techniques rely on the measurementof the thermal neutron capture cross section(Sigma) in saline formation water of known orassumed salinities and/or the salinity-independentcarbon/oxygen (C/O) ratio in oil densities andporosities of sufficient magnitude to enablecalculation of the oil and water saturations.Prior to 2004, the limited sensitivity of Sigma tooil vs. gas and of C/O measurements to water vs.gas precluded quantitative gas saturation and thedifferentiation of gas from oil and fresh water.Recently developed interpretation techniquesusing pulsed neutron gamma ray ratio-basedmeasurements overcome these limitations andenable quantification of gas saturation in fresh,brackish or saline water, oil and three-phaseenvironments.This paper presents the results of a programmeconducted in late 2015 and early 2016 to monitoran injected CO_2 plume. The workscope compriseda base log and a post-injection log in the injectionwell and a post-injection log in the neighbouringmonitoring well. The results show that thecombination of the pulsed neutron capturemeasurement, Monte Carlo tool responsemodelling, and the interpretation technique is ableto locate and quantify the CO_2 plume. In theinjection well, this was accomplished in thepresence of complex completion hardware. Thedata also show that the measurements enable thelocation of leaks within the completion, in thiscase, by detecting CO_2 trapped under a packer.