Australian National Low Emissions Coal Research and Development

Abstract This paper presents and discusses the distribution of fluid and leak-off pressure data from the subsurface of onshore and offshore Netherlands in relation to causes of formation fluid overpressure and the permeability framework. The observed fluid pressure conditions demonstrate a clear regional difference between the southern and the north and north-eastern part of the study area. In the southern area, formation fluid pressures are close to normal and well below measured leak-off pressures. In the north, formation fluids are overpressured and may locally even approach the measured leak-off pressures. The regional differences in fluid overpressure can, in large part, be explained by differences in geologic framework and burial history. In the south, relatively low rates of sedimentary loading and the presence of relatively permeable sedimentary units have led to the currently observed normally pressured conditions. In the northern area, relatively rapid Neogene sediment loading plays an important role in explaining the observed overpressure distributions in Cenozoic mudstones, Cretaceous Chalk and Rijnland groups, and probably also in Jurassic units. The permeability framework of the northern and north-eastern area is significantly affected by Zechstein and Triassic salt deposits and structures. These units are characterised by very low permeability and severely restrict fluid flow and pressure dissipation. This has created hydraulically restricted compartments with high overpressures (for example overpressures exceeding 30 MPa in the Lower Germanic Trias Group in the Terschelling Basin and Dutch Central Graben).

ABSTRACT Alternative fiber configurations have been tested in an attempt to improve the sensitivity of surface-deployed distributed acoustic sensing (DAS) fiber cables for the purpose of recording steep-angle P-wave reflections. Four alternative fiber configurations were deployed at the Aquistore CO2 storage site to record 401 dynamite shots during a 3D vertical seismic profiling survey. The test cable comprised horizontal configurations (straight fiber, helixes, and asymmetric helixes) buried in a shallow trench and vertical configurations (straight fiber and helixes) deployed in 3.5 m drillholes. Evaluation focused on deep reflections with two-way traveltimes of 0.8–1.8 s. All of the alternative fiber configurations increased the sensitivity relative to the horizontal straight fiber. Sensitivity was highest for the vertical straight fiber configurations and the asymmetric helixes with sensitivity increases of more than 10 and 5 dB, respectively, and amplitude-variation-with-offset behavior similar to that of a vertical-component geophone for reflections with incidence angles of 0°–15° at the surface and 0°–34° at the reflector. Modeling of the DAS responses explains the general pattern of sensitivity variability among the different configurations, but it does not explain the large range of observed sensitivities.