Basic Research Needs for Geosciences - Energetics Meetings and ...

PRIORITY RESEARCH DIRECTION: TRANSPORT PROPERTIES AND IN SITU CHARACTERIZATIONOF FLUID TRAPPING, ISOLATION, AND IMMOBILIZATIONCourtesy of Jesus CarreraFigure 43. Dissolution of CO 2 in brine increases its density and creates an unstable condition (bottom brine lighterthan top brine) that promotes downwards fingering. As a result, dissolution goes from being diffusive (ratedecreasing as t -1/2 ) to advective (constant rate). The time to onset of instability and actual dissolution is sensitive tomodeling details (Hidalgo and Carrera 2007).Because seal properties are variable under different fluid and stress conditions, measurementsmay perturb seal properties. Tests of seal properties in the laboratory or from the wellbore areinherently limited in terms of their spatial coverage and validity when extrapolated to in situpressure and temperature conditions. Rock samples retrieved from the field have been unloaded,cooled, and their fluid conditions radically altered. Seal rocks near wellbores are highlyperturbed during well construction. Measurements of small flows or small changes in propertieswithin or across seals may be dominated by well properties (casing, cement, damaged zone)rather than the true properties of the seal. These inaccuracies, extrapolated to basin scale andlong time frames, lead to potentially large errors in prediction of migration paths and inaccurateassessment of seal continuity and integrity.Improved field methods are required for accurate determination of the properties of flow barriers.If these measurements could be made in situ without major modification of the naturalenvironment, then observations of the seal properties—fluid composition, pressure, and stresschange—during anthropogenic perturbations could be made. The temporal changes in these sealproperties during fluid injection must be measured to obtain reliable information about theperformance of the seal in the presence of changing fluid properties.Identification and characterization of local flaws in sealing structuresIdentification of impermeable strata and to a lesser extent prediction of impermeable faults arerelatively mature techniques. However, techniques to locate and to quantify the properties of theweak points or flaws in these seals are poor or nonexistent. Where a buoyant fluid such as oil orgas has been trapped, this trapping is a priori evidence of the quality of the seal (although theexact sealing mechanism may not be described). However, in other cases, such as introduction ofbuoyant CO 2 or assessment of the dynamics of another fluid system, new conceptualization,approaches, and techniques need to be developed. Especially challenging is the need to assesssmall features (fractures, sand-on-sand contacts) at the millimeter to a few meter scale over areasof hundreds of square kilometers.134 Basic Research Needs for Geosciences: Facilitating 21 st Century Energy Systems