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Sensitivity analysis and model type evaluation for subsidence above offshore gas reservoirs
This paper describes the results of a sensitivity study conducted to understand how the prediction of subsidence due to gas extraction from offshore gas fields depends on a few key parameters, such as the connection to the adjacent aquifers and the material mechanical properties. The analysis has been performed using an axi-symmetric Finite Element model. A specific gas production field, the Naomi-Pandora gas field in the Northern Adriatic basin, has been assessed in detail. For each of the layers considered in the sand/shale stratification a low value, an intermediate and a high value for the soil stiffness were applied, as determined from oedometer tests, radioactive markers and vertical seismic profiling respectively. The reservoir constitutive behavior has been modeled using different approaches, namely: a linear-elastic, a power law and a Modified Cam Clay model. The study has been performed for different pressure scenarios representing different levels of interaction with adjacent aquifers. The results show the sensitivity of the subsidence bowl as a result of the imposed conditions. These results are compared with the predictions obtained using 3D non-linear elastic and hypoplastic subsidence models of the same gas fields, demonstrating a good agreement. The stiffness of the reservoir is the main factor affecting the surface subsidence. For a gas pressure reduction less than 50 bars the observed seabed subsidence hardly varied for different reservoir material models. The maximum extent of the predicted subsidence bowl (2 cm contour) in 2030 remains far from the Po di Goro parallel and far from the coastline.
Subsidence from geodetic measurements in the Ravenna area
The derivation of subsidence due to a specific cause from geodetic measurements is, in principle, simple, but assumptions implied in the standard approach are never fully correct in practice. Over or underestimation by a factor of up to two may occur. Geodetic measurements alone cannot differentiate between different causes of subsidence. This article describes a modified approach that avoids assumptions on reference point stability and exploits a-priori knowledge of spatial and temporal subsidence patterns. The present integral approach recognizes that geodetic measurements reflect differential, not absolute, vertical displacement of the benchmarks, not of the 'surface', within the area surveyed and recognizes errors that are, or are not correlated in time and/or subsidence in the Ravenna area from the Comune di Ravenna for the period 1982-2002 were revisited.
Environmental effects of land subsidence induced by gas withdrawal along the Ravenna Coast, part I : available information
An overview is given of the expected environmental impact related to predicted land subsidence due to planned gas-withdrawal in the Ravenna coastal zone. This region is a typical example of a deltaic area with soft sediments, susceptible to land subsidence caused by tectonic movements and natural compaction, water withdrawal from shallow aquifers, and gas withdrawal from deep reservoirs. Subsidence may lead to effects on the natural and built environment and on the local economy. This paper presents Part I of the findings of an international multi-disciplinary team of experts, which has performed a comprehensive study to investigate the gas-withdrawal effects on land subsidence in the Ravenna area. It focuses on relevant and available information. For their implication and final conclusions reference is made to Part II of the combined paper.
Environmental effects of land subsidence induced by gas withdrawal along the Ravenna Coast, part II : environmental impact
An overview is given of the expected environmental impact related to predicted land subsidence due to planned gas withdrawal in the Ravenna coastal zone. This region is a typical example of a deltaic area with soft sediments, susceptible to land subsidence caused by tectonic movements and natural compaction, water withdrawal from shallow aquifers, and gas withdrawal from deep reservoirs. Subsidence may lead to effects on the natural and built environment and on the local economy. This paper presents part II of the findings of an international multi-disciplinary team of experts, which has performed a comprehensive study to investigate the effects of gas withdrawal on land subsidence in the Ravenna area. These findings were supported by site visits to inspect coastal and inland environments. This paper discusses the impact of subsidence particularly on coastal dynamics and on mitigation measures. A comparison is made with a similar situation in The Netherlands. Effects of gas withdrawal induced subsidence on the coastal dynamics in the Ravenna Province are marginal. In the area above the Angela-Angelina gas field additional shore protection measures are recommended in order to suppress potential beach erosion. Public safety and risk of damage is assessed and it is concluded that the subsidence due to proposed gas production will cause no risk to public safety and no disproportionate increase in the risk of damage.
Subsidence induced by gas production : an integrated method
An integrated subsurface modelling study was performed in order to predict the amount and the lateral extent of subsidence in an area of the Northern Adriatic region. A number of selected gas fields were independently modelled using an approach in three steps. First, static geological and petrophysical subsurface models were built that were large enough to include the wider area surrounding the gas fields. Secondly, the pressure changes were modelled using a 3D subsurface reservoir simulator, through history matching using production and pressure data. Modelling extended to a few decades after the expected end of the gas production in order to capture all significant effects in the reservoirs and aquifers. The third phase of modelling consisted of finite element geomechanical modelling, taking into account the geometry, vertical heterogeneity and geomechanical rock properties of the subsurface and the pressure fields obtained in the second phase. Finally, the resulting subsidence grids obtained for the individual fields were added together to provide regional contour maps representing the combined effects.
Some fundamental properties of the simultaneous flow of fresh and salt groundwater in horizontally extended aquifers
In this paper we discuss some fundamental properties of solutions of a nonlinear partial differential equation which arises in the study of the simultaneous flow of fresh and salt groundwater in horizontally extended aquifers. This differential equation is an approximate version of the interface motion equation derived by de Josselin de Jong in a previous paper. There he gives a general treatment of the movement of fresh and salt groundwater and he obtains an equation describing the interface between the two fluids as a function of position and time for the case that the horizontal flow component is constant over the height. Here we make a number of additional assumptions in order to simplify the mathematical description.
Non-darcy flow : a state of the art
Recent developments in oil and gas production, in groundwater recovery and management, rockfill dam and rubble mound structure design and lately the use of granular beds for the storage of solar energy, have demanded a more accurate determination of the hydraulic conductivity of porous media. Much excellent work has been done and most of it has been extensively reported in various textbooks. This paper reviews some of the prominent contributions to the study of the hydrodynamic aspects of this socalled non-Darcy flow. Attention is given to practical application using numerical methods.
Piping due to flow towards ditches and holes
The problem of piping is becoming more and more a topic of common interest. A fundamental model to predict its appearance is not yet available. A better understanding of the piping mechanism or at least the behaviour of the mechanism is necessary. Therefore, the "Delft Soil Mechanics Laboratory" under the direction of the "Netherlands Water Defenses Research Centre" is carrying out an extensive investigation on the phenomenon of piping. In the laboratory model tests are run for some typical geometries. Different types of sand are used at various porosities. During a test the discharge and a large number of pore pressures are measured. For the present only the results on homogeneous sand are discussed. Sand mixtures show much more scatter in the results than more homoqeneous material. The degree of mixing seems to play an important role, to be studied in a later stage.
Hydraulic resistance of steel sheet pile joints
Steel sheet pile joints are investigated to determine their permeability behavior. A suitable definition is obtained for the permeability. A testing rig is described and tests on 10 different joint filler materials are reported. The results show significant time-dependent and joint-dependent features. These are explored and conclusions on both water swelling and bituminous materials are noted. A case study gives a design example as well as suggestions on how a safety factor can be implemented.
Anisotropic peat model
A two-dimensional anisotropic stress strain model to descibe the behaviour of peat is introduced. It is composed of an elasto-plastic component for the filling and an axial fibre stiffness.The fibres are orientated in one direction. The strain fields of filling and fibres are supposed to coincide. Calibration of the model with experiments on artificial peat is discussed.