Christopher D. White

Effects of Stress-Dependent Hydraulic Properties of Proppant Packs on the Productivity Indices of the Hydraulically Fractured Gas Reservoirs

Hydraulic fracturing stimulates wells and has enabled exploitation of the vast unconventional hydrocarbon resources in the US and globally. Proppants, which are granular materials that prevent fractures from closing, must provide high fracture conductivities and withstand closure stresses without getting crushed. Advances in imaging technologies and high-performance computing enable calculation of transport and mechanical properties of pore-scale images. Imagebased mechanical and flow simulations can rapidly and accurately estimate the transport properties of proppant packs in fractures at different closure stresses, providing a credible alternative to difficult and expensive physical experiments. This study examines transport properties of a ceramic proppant pack with confining stresses from zero to 20,000 psi. The images of this packing show rearrangement of the packing structure, embedding of the grains at the rock wall, and crushing of individual proppant particles. Lattice Boltzmann (LB) simulation results of this proppant pack indicate that the permeability and inertial flow parameter are sensitive to stress at high stresses (which crush the proppant particles) compared to lower stresses. Predicted stress-dependent permeability and non-Darcy factors corresponding to the effective stress fields around the hydraulic fractured completions are included in a two-dimensional gas reservoir simulator to calculate the productivity indices. Productivity indices with permeability and non-Darcy factors kept constant at initial effective stress (6000 psi) are ca. 0.03 percent higher than those with stress-dependent permeability and non-Darcy factors for a gas rate of 20 MMscf/D.Hydraulic fracturing stimulates wells and has enabled exploitation of the vast unconventional hydrocarbon resources in the US and globally. Proppants, which are granular materials that prevent fractures from closing, must provide high fracture conductivities and withstand closure stresses without getting crushed. Advances in imaging technologies and high-performance computing enable calculation of transport and mechanical properties of pore-scale images. Imagebased mechanical and flow simulations can rapidly and accurately estimate the transport properties of proppant packs in fractures at different closure stresses, providing a credible alternative to difficult and expensive physical experiments. This study examines transport properties of a ceramic proppant pack with confining stresses from zero to 20,000 psi. The images of this packing show rearrangement of the packing structure, embedding of the grains at the rock wall, and crushing of individual proppant particles. Lattice Boltzmann (LB) simulation results of this proppant pack indicate that the permeability and inertial flow parameter are sensitive to stress at high stresses (which crush the proppant particles) compared to lower stresses. Predicted stress-dependent permeability and non-Darcy factors corresponding to the effective stress fields around the hydraulic fractured completions are included in a two-dimensional gas reservoir simulator to calculate the productivity indices. Productivity indices with permeability and non-Darcy factors kept constant at initial effective stress (6000 psi) are ca. 0.03 percent higher than those with stress-dependent permeability and non-Darcy factors for a gas rate of 20 MMscf/D.

Rare Earth Elements Geochemistry and Nd Isotopes in the Mississippi River and Gulf of Mexico Mixing Zone

Rare earth elements (REE) concentrations were measured in surface waters collected across the salinity gradient in the Mississippi River estuary (i.e., Mississippi River plume, MRP), which includes the near-shore portion of the Louisiana Shelf. In addition, the neodymium (Nd) isotope compositions of two river water samples, and the acid leachable fractions of the associated suspended particulate matter (SPM), were quantified to compare Mississippi River water, and SPM with Gulf of Mexico waters. Despite the spatial limitations associated with sampling due to the size of the Mississippi River system, this study provides some important insights into the REE geochemistry of the MRP. The Mississippi River and its estuarine waters are enriched in the heavy REE (HREE) compared to the light REE (LREE) when normalized to shale composites. All water samples from the estuary also exhibit substantial negative Ce anomalies. In contrast to the majority of other estuaries investigated, removal of REE in the low salinity reaches of the Mississippi River estuary is less substantial. For example, approximately 50% of the river borne Nd is removed in the low salinity region (S < 10) of the Mississippi River estuary compared to the estimated global average of ca. 70% removal of Nd for estuaries worldwide. We propose that the relatively low REE removal in the Mississippi estuary reflects the high pH (~8) of the Mississippi River, where REE complexation with carbonate ions and natural organic ligands act to stabilize REE in solution. The Nd isotope composition of Mississippi River water near its outflow to the Gulf of Mexico is eNd(0)= -10.5. The acid leachable fraction of the associated SPM is more radiogenic (-9.95 ≤ eNd(0) ≤ -9.77), and closer to the generally more radiogenic Gulf of Mexico (eNd(0) = -9.0). Sequential extraction of the Mississippi River bank sediment reveals substantially different Sm/Nd ratios for the operationally defined fractions of the sediment, which suggests variations in Nd isotope compositions between the labile fractions and the bulk sediment.

Statistical modeling of geopressured geothermal reservoirs

Identifying attractive candidate reservoirs for producing geothermal energy requires predictive models. In this work, inspectional analysis and statistical modeling are used to create simple predictive models for a line drive design. Inspectional analysis on the partial differential equations governing this design yields a minimum number of fifteen dimensionless groups required to describe the physics of the system. These dimensionless groups are explained and confirmed using models with similar dimensionless groups but different dimensional parameters. This study models dimensionless production temperature and thermal recovery factor as the responses of a numerical model. These responses are obtained by a Box-Behnken experimental design. An uncertainty plot is used to segment the dimensionless time and develop a model for each segment. The important dimensionless numbers for each segment of the dimensionless time are identified using the Boosting method. These selected numbers are used in the regression models. The developed models are reduced to have a minimum number of predictors and interactions. The reduced final models are then presented and assessed using testing runs. Finally, applications of these models are offered. The presented workflow is generic and can be used to translate the output of a numerical simulator into simple predictive models in other research areas involving numerical simulation. HighlightsScreening models for identifying attractive geothermal reservoirs are presented.The scaling analysis of the model produced fifteen dimensionless numbers.Important dimensionless numbers are found using statistical algorithms.Uncertainty violin plots are used to introduce dimensionless time into the models.The presented workflow is useful for translating simulation results into models.

Ventilation time scales of the North Atlantic subtropical cell revealed by coral radiocarbon from the Cape Verde Islands

We present coral- and sclerosponge-based reconstructions of the 14C content in North Atlantic dissolved inorganic carbon (DIC) during the last ~100 years from the subtropical cells (STCs). These waters are sensitive to the dynamics of the shallow overturning meridional circulation that transports heat and water masses from the subtropics to the tropics. We use these records to investigate the circulation patterns of the off-equatorial upwelling regions of the STCs, which are not well understood. Coral and sclerosponge skeletons provide long time series of ocean DIC 14C content, a tracer of oceanic circulation, effectively extending the observational record back in time. Sclerosponge data from the Bahamas were used to extend the existing subtropical 14C time series to the 21st century. Coral 14C data from the Cape Verde Islands (1890–2002) captured the 14C signature of water brought to the surface in the off-equatorial regions of the STC present near the West African coast. We observe a unique postbomb trend at Cape Verde that is similar to the upwelling regions in the Pacific, and we interpret this trend as the result of the slow penetration of bomb 14C into the interior ocean as part of the STC circulation. Using a multibox mixing model we constrain the time history of bomb 14C in the eastern tropical Atlantic, and we estimate a 20 year time scale for ventilation of the thermocline in this area of the ocean. The similarity between the Atlantic and Pacific 14C-based records of upwelling suggests that both are caused by bomb 14C penetration rather than more complex explanations that invoke changes in thermocline depth (e.g., related to El Nino–Southern Oscillation variability) or changes in the strength of the subtropical cells. Our results offer constraints for models of tropical ocean circulation and anthropogenic CO2 uptake that attempt to reproduce the characteristics of the shallow wind-driven circulation in the Atlantic.

The competing effects of stress and water saturation on in situ Q for shallow (< 1 m), unconsolidated sand, evaluated with a modified spectral ratio method

A publicly available seismic dataset from a lab experiment shows the simultaneous dependence of quality factor (Q) on water saturation and stress in unconsolidated sand. Large Q gradients (e.g., > 10 m−1) necessitate a spectral ratio method modified to assume that Q changes with each ray path, thereby eliminating false Q values (e.g., < 0). Interval Q values (Qint) increase the most with depth (dQ/dz = 43 m−1) and stress (dQ/dσ = 0.0025/Pa) in dry sand and the least in partially saturated sand (dQ/dz = 10 m−1 and dQ/dσ = 0.0013/Pa) where attenuation created by local fluid flow reaches a maximum. Expected Qint values can be extrapolated from dQ/dσ and are bounded by Qint of the dry (Qdry) and partially saturated (Qwet) media (e.g., Qdry ≥ Qint ≥ Qwet). Qint deviations outside this range may be explained by changes in effective stress, attenuation mechanism, or sediment composition. Field values of seismic attenuation in natural settings may be helped by these constraints, although attenuation remains subject to careful consideration of other factors, e.g., grain size, sorting, and shape.