Sampling of fluids in deep boreholes is challenging because of the necessity to minimize external contamination and maintain sample integrity during recovery. The U-tube sampling methodology was developed to collect large volume, multiphase samples at in situ pressures. As a permanent or semi-permanent installation, the U-tube can be used for rapidly acquiring multiple samples or it may be installed for long-term monitoring applications. The U-tube was first deployed in Liberty County, TX to monitor crosswell CO2 injection as part of the Frio CO2 sequestration experiment. Analysis of gases (dissolved or separate phase) was performed in the field using a quadrupole mass spectrometer, which served as the basis for determining the arrival of the CO2 plume. The presence of oxygen and argon in elevated concentrations, along with reduced methane concentration, indicate sample alteration caused by the introduction of surface fluids during borehole completion. Despite producing the well to eliminate non-native fluids, measurements demonstrate that contamination persists until the immiscible CO2 injection swept formation fluid into the observation wellbore.

X-ray computed tomography (CT) is an established technique for nondestructively characterizing geologic cores. CT provides information on sediment structure, diagenetic alteration, fractures, flow channels and barriers, porosity, and fluid-phase saturation. A portable CT imaging system has been developed specifically for imaging whole-round cores at the drilling site. The new system relies upon carefully designed radiological shielding to minimize the size and weight of the resulting instrument. Specialized x-ray beam collimators and filters maximize system sensitivity and performance. The system has been successfully deployed on the research vessel Joides Resolution for Ocean Drilling Program's Leg 204 and 210, within the Ocean Drilling Program's refrigerated Gulf Coast Core Repository, as well as on the Hot Ice #1 drilling platform located near the Kuparuk Field, Alaska. A methodology for performing simple densiometry measurements, as well as scanning for gross structural features, will be presented using radiographs from ODP Leg 204. Reconstructed CT images from Hot Ice #1 will demonstrate the use of CT for discerning core textural features. To demonstrate the use of CT to quantitatively interpret dynamic processes, we calculate 95 percent confidence intervals for density changes occurring during a laboratory methane hydrate dissociation experiment. The field deployment of a CT represents a paradigm shift in core characterization, opening up the possibility for rapid systematic characterization of three-dimensional structural features and leading to improved subsampling and core-processing procedures.

Hydrological and geochemical monitoring are key components of site characterization and CO2 plume monitoring for a pilot test to inject CO2 into a brine-bearing sand of the fluvial-deltaic Frio formation in the upper Texas Gulf Coast. In situ, injected CO2 forms a supercritical phase that has gas-like properties (low density and viscosity) compared to the surrounding brine, while some CO2 dissolves in the brine. The pilot test employs one injection well and one monitor well, with continuous pressure and flow-rate monitoring in both wells, and continuous surface fluid sampling and periodic down-hole fluid sampling from the monitor well. Pre-injection site-characterization includes pump tests with pressure-transient analysis to estimate single-phase flow properties, establish hydraulic connectivity between the wells, determine appropriate boundary conditions, and analyze ambient phase conditions within the formation. Additionally, a pre-injection tracer test furnishes estimates of kinematic porosity and the geometry of flow paths between injection and monitor wells under single-phase conditions. Pre-injection geochemical sampling provides a baseline for subsequent geochemical monitoring and helps determine the optimal tracers to accompany CO2 injection. During CO2 injection, hydrological monitoring enables estimation of two-phase flow properties and helps track the movement of the injected CO2 plume, while geochemical sampling provides direct evidence of the arrival of CO2 and tracers at the monitor well. Furthermore, CO2-charged water acts as a weak acid, and reacts to some extent with the minerals in the aquifer, producing a distinct chemical signature in the water collected at the monitor well. Comparison of breakthrough curves for the single-phase tracer test and the CO2 (and its accompanying tracers) illuminates two-phase flow processes between the supercritical CO2 and native brine, an area of current uncertainty that must be better understood to effectively sequester CO2 in saline aquifers.

Measurements on hydrate-bearing laboratory and field samples are necessary in order to provide realistic bounds on parameters used in numerically modeling the production of natural gas from hydrate-bearing reservoirs. The needed parameters include thermal conductivity, permeability, relative permeability-saturation(s) relationships, and capillary pressure-saturation(s) relationships. We have developed a technique to make hydrate-bearing samples ranging in scale from core plug-size to core-size in the laboratory to facilitate making these measurements. In addition to pressure and temperature measurements, we use x-ray computed tomography scanning to provide high-resolution data providing insights on processes occurring in our samples. Several methods are available to make gas hydrates in the laboratory, and we expect that the method used to make the hydrate will impact the behavior of the hydrate sample, and the parameters measured.