An inverse method for estimating thickness and volume with time of a thin CO2-filled layer at the Sleipner Field, North Sea

Migration of CO 2 through storage reservoirs can be monitored using time lapse seismic reflection surveys. At the Sleipner Field, injected CO 2 is distributed throughout nine layers within the reservoir. These layers are too thin to be seismically resolvable by direct measurement of the separation between reflections from the top and bottom of each layer. Here we develop and apply an inverse method for measuring thick ness changes of the shallowest layer. Our approach combines differences in traveltime down to a specific reflection together with amplitude measurements to determine layer thicknesses from time lapse surveys. A series of synthetic forward models were used to test the robustness of our inverse approach and to quantify uncertainties. In the absence of ambient noise, this approach can unambiguously resolve layer thickness. If a realistic ambient noise distribution is included, layer thicknesses of 1–6 m are accurately retrieved with an uncertainty of ±0.5 m. We used this approach to generate a thickness map of the shallowest layer. The fidelity of this result was tested using measurements of layer thickness determined from the 2010 broadband seismic survey. The calculated volume of CO 2 within the shallowest layer increases at a rate that is quadratic in time, despite an approximately constant injection rate into the base of the reser voir. This result is consistent with a diminished growth rate of the areal extent of underlying layers. Finally, the relationship between caprock topography and layer thickness is explored and potential migration pathways that charge this layer are identified

Controls on the 87Sr/86Sr ratios of carbonates in the Garhwal Himalaya, Headwaters of the Ganges

The episodic variation of the seawater 87Sr/86Sr ratio has been attributed to either variations in the Sr flux or the Sr-isotopic composition of the riverine-dissolved load derived from weathering of the continental crust. The discovery that Himalayan rivers are characterized by high concentrations of dissolved Sr concentrations with high 87Sr/86Sr ratios has raised the possibility that collisional orogens play a critical role in moderating the variations in seawater 87Sr/86Sr ratios. Here we describe Himalayan carbonates and calc-silicates from Garhwal, the headwaters of the Ganges, with extreme 87Sr/86Sr ratios (>1.0). Elevated Sr-isotope ratios result from exchange with Rb-rich silicate material during both Himalayan and pre-Himalayan metamorphic episodes, and the carbonates contribute a significant fraction to the Ganges 87Sr flux. Particularly elevated 87Sr/86Sr ratios are found in calc-silicates from the Deoban Formation of the Lesser Himalaya. A detailed traverse of shales and calc-silicates from this unit confirms that carbonate horizons have increased 87Sr/86Sr ratios as a result of isotopic exchange over length scales of 1030 cm. We conclude that metamorphism of carbonates may cause elevation of their 87Sr/86Sr ratios and that uplift of metamorphosed carbonates may be a consequence of collisional orogens, which contributes to the elevation of seawater 87Sr/86Sr ratios

Benchmarking of vertically-integrated CO 2 flow simulations at the Sleipner Field, North Sea

Numerical modeling plays an essential role in both identifying and assessing sub-surface reservoirs that might be suitable for future carbon capture and storage projects. Accuracy of flow simulations is tested by benchmarking against historic observations from on-going CO2 injection sites. At the Sleipner project located in the North Sea, a suite of time-lapse seismic reflection surveys enables the three-dimensional distribution of CO2 at the top of the reservoir to be determined as a function of time. Previous attempts have used Darcy flow simulators to model CO2 migration throughout this layer, given the volume of injection with time and the location of the injection point. Due primarily to computational limitations preventing adequate exploration of model parameter space, these simulations usually fail to match the observed distribution of CO2 as a function of space and time. To circumvent these limitations, we develop a vertically-integrated fluid flow simulator that is based upon the theory of topographically controlled, porous gravity currents. This computationally efficient scheme can be used to invert for the spatial distribution of reservoir permeability required to minimize differences between the observed and calculated CO2 distributions. When a uniform reservoir permeability is assumed, inverse modeling is unable to adequately match the migration of CO2 at the top of the reservoir. If, however, the width and permeability of a mapped channel deposit are allowed to independently vary, a satisfactory match between the observed and calculated CO2 distributions is obtained. Finally, the ability of this algorithm to forecast the flow of CO2 at the top of the reservoir is assessed. By dividing the complete set of seismic reflection surveys into training and validation subsets, we find that the spatial pattern of permeability required to match the training subset can successfully predict CO2 migration for the validation subset. This ability suggests that it might be feasible to forecast migration patterns into the future with a degree of confidence. Nevertheless, our analysis highlights the difficulty in estimating reservoir parameters away from the region swept by CO2 without additional observational constraints

Spatial and temporal evolution of injected CO2 at the Sleipner Field, North Sea

Time-lapse, three-dimensional (3D) seismic surveys have imaged an accumulation of injected CO2 adjacent to the Sleipner field in the North Sea basin. The changing pattern of reflectivity suggests that CO2 accumulates within a series of interbedded sandstones and mudstones beneath a thick caprock of mudstone. Nine reflective horizons within the reservoir have been mapped on six surveys acquired between 1999 and 2008. These horizons have roughly elliptical planforms with eccentricities ranging between two and four. In the top half of the reservoir, horizon areas grow linearly with time. In the bottom half, horizon areas initially grow linearly for about eight years and then progressively shrink. The central portions of deeper reflective horizons dim with time. Amplitude analysis of horizons above, within, and below the reservoir show that this dimming is not solely caused by acoustic attenuation. Instead, it is partly attributable to CO2 migration and/or CO2 dissemination, which reduce the impedance contrast between sandstone and mudstone layers. Growth characteristics and permeability constraints suggest that each horizon grows by lateral spreading of a gravity current. This model is corroborated by the temporal pattern of horizon velocity pushdown beneath the reservoir. Horizon shrinkage may occur if the distal edge of a CO2-filled layer penetrates the overlying mudstone, if the buoyant plume draws CO2 upward, or if the effective permeability of deeper mudstone layers increases once interstitial brine has been expelled. Topographic control is evident at later times and produces elliptical planforms, especially toward the top of the reservoir. Our results show that quantitative mapping and analysis of time-lapse seismic surveys yield fluid dynamical insights which are testable, shedding light on the general problem of CO2 sequestration

Recharge flux to ocean-ridge black smoker systems: a geochemical estimate from ODP Hole 504b

We use the Sr-isotopic composition of rock and anhydrite, as a monitor of fluid composition, in DSDP/ODP Hole 504B to calculate the recharge flux to the axial high-temperature hydrothermal circulation. The fluid and rock Sr-isotope profiles are well fit by a tracer transport mass-balance model that approximates fluid-solid exchange by linear kinetics. The calculated time-integrated flux of 1.7+/-0.2 x 106 kg m-2 is significantly less than the 5 x 106 kg m-2 calculated from thermal models that assume all magma is intruded into a high-level magma chamber at the base of the sheeted dykes. Our low flux is consistent with the observed thermal structure as recorded by secondary alteration minerals in Hole 504B and the intrusion of magma as lenses distributed through the lower oceanic crust. It leaves open the questions as to how the lower oceanic crust cools and how seawater geochemical budgets balanc

Melt generation at very slow-spreading oceanic ridges: constraints from geochemical and geophysical data

We show that there is a strong and consistent correlation between geochemical and geophysical estimates of the amount of melt generated in the mantle beneath oceanic ridges. This correlation holds across all spreading rates and on scales down to the size of individual ridge segments. There is an abrupt decrease in the amount of melt generated at full spreading rates below ~20 mm/a. Our observations are consistent with the conclusion that <10% of the melt is frozen in the mantle before it reaches the crust and that serpentine probably represents only a small percentage of the material above the Moho. The melt is well mixed on a ridge segment scale, probably in high level magma chambers, but the melts remain distinct between segments. The rare earth element concentrations of basalts from very slow-spreading ridges are higher than those from normal oceanic ridges, which is directly indicative of reduced mantle melting, and they show characteristic light rare earth element enrichment, interpreted as caused by a deep tail of small percentage wet melting. The decrease in melt production at rates below ~20 mm/a points to the importance of conductive cooling inhibiting melting of the upwelling mantle at very slow-spreading centres