Brazil pre-salt, santos basin: Feasibility study for the application ofborehole gravity to improve reservoir monitoring

Brazilian Pre-Salt reservoir characterization and monitoring is a challenge for surface geophysical methodsdue to the inherently minute observable signals from the reservoirs, located below the massive salt, in depthranges > 5000 m. The status of a program to develop a wireline deployed 3-axis gravity sensor with a target sensitivity of~ 5 μGal is firstly introduced. This is followed by a feasibility study for the potential deployment as a time-lapse gravity survey (4D gravity monitoring) within the pre-salt. A conceptual pre-salt reservoir model forthe Libra field, Santos Basin, the offshore of Brazil, is developed, built on available pre-salt knowledgeprovided from seismic imaging and well log data, including reservoir and production characteristics. Themodel is based on water substituting oil over 6 months within and through the reservoir, with adoption ofan oil production rate for the field published in 2017 by the ANP. These data are used to forward modelgravity to understand the potential for a detectable signal and thereby establish a baseline for a time-lapsegravity survey (4D gravity) that could be used to monitor the Libra Field. A clear gravity response >80 μGal is observed over a six month period in the reservoir, due to waterreplacing oil, at the defined oil production rate. In a three-axis measurement the vertical axis is directlyrelated to the magnitude of the fluid substitution and the two horizontal axes are sensitive to the fluidmovement directions. Together, these suggest that an annual survey with a limited well stock could beeffective in monitoring this type of reservoir and that a wireline deployed 3-axis gravity tool is likelyto provide significant additional surveillance to constrain a reservoir production strategy through betterappreciation of the direction of water movement. The follow-on step would be to model further field dataand run a baseline survey to develop a novel reservoir surveillance method within the Pre-Salt

The organic carbon dynamics of a moorland catchment in N. W. England

The carbon cycle was quantified in the catchment of Doe House Gill, which drains high-relief moorland, with thin organic-rich soils (leptosols and podzols) 10–25 cm deep, in northern England. The soil C pool of 8,300 g m-2 is due mainly to humic acid and older humin. If steady state is assumed, and a single soil C pool, the average 14C content of the whole soil (93% modern) yields a mean carbon residence time of 800 years, although this varied from 300 to 1,600 years in the four samples studied. Stream water fluxes of dissolved and particulate organic carbon (DOC, POC) were 2.5 and 0.4 g m−2 a−1 respectively in 2002–2003, lower than values for some other upland streams in the UK. The C pool, flux, and isotope data were used, with the assumption of steady state, to calibrate DyDOC, a model that simulates the soil carbon cycle, including the generation and transport of DOC. According to DyDOC, the litter pool (ca. 100 gC m−2) turns over quickly, and most (>90%) of the litter carbon is rapidly mineralised. The soil is calculated to gain only 16 gC m−2 a−1, and to lose the same amount, about 80% as CO2 and 20% as DOC. From the DO14C content of 107.5% modern (due to “bomb carbon”) the model could be calibrated by assuming all DOC to come directly from litter, but DOC is more likely a mixture, derived from more than one soil C pool. The seasonal variability exhibited by stream water DOC concentration (maximum in September, minimum in January) is attributed mainly to variations in rainfall and evapotranspiration, rather than in the metabolic production rate of “potential DOC”. The model predicts that, for a Q 10 of 2, the total soil organic C pool would decrease by about 5% if subjected to warming over 200 years. DyDOC predicts higher DOC fluxes in response to increased litter inputs or warming, and can simulate changes in DOC flux due to variations in sorption to soil solids, that might occur due to acidification and its reversal