On the possible contribution of clayey inter-layers to delayed land subsidence above producing aquifers

Abstract. In recent years, measurements of land subsidence above pumped aquifers by permanent GPS and InSAR have exhibited some delay relative to drawdown ranging from months to years. The current modeling approaches accounting for water fluid dynamics and porous medium geomechanics may fail to predict such a delay and may underestimate the land settlement after the well shutdown. In the present communication, an investigation is made on the residual compaction of the intervening clayey formations as a possible contribution to retarded land subsidence. The pore pressure variation within the aquifer and its propagation in the clay are simulated by a finite element flow model, with the resulting pore pressure decline used as input data in a hypo-plastic geomechanical model. A proper sensitivity analysis on (i) aquifer depth, (ii) ratio between the sandy and the clayey layers thickness and hydraulic conductivity, (iii) oedometric compressibility in first and second loading cycles, is performed for a typical geology of a Quaternary sedimentary basin. The results show that a certain fraction, up to 20 % of the overall land subsidence, can take place after the shutdown of the producing wells depending on actual basin, litho-stratigraphy and parameter values

An engineering approach to quantify geomechanical safety factors in UGS programs

Abstract. Underground Gas Storage (UGS) has become one of the most widely used practices to cope with seasonal peaks in energy consumption. The planning of any new UGS facility, or its upgrading to increase the working gas volume and reservoir performance, must be supported by an evaluation of possible induced effects on the environment. From a geomechanical point of view, storage activity results in a cyclic change in stress and deformation in the reservoir rock and the surrounding formations. The main environmental issues to be accounted for when natural fluid pore pressure is planned to be exceeded are the following: (a) the differential displacements at the land surface possibly mining the integrity of ground structure; (b) the integrity of the reservoir and caprock; (c) the possible reactivation of faults, if the target reservoir is located in a faulted basin; and (d) the vertical upheaval and land subsidence that can impact on the surface drainage network in low lying coastal areas. We present an original methodology for evaluating the geomechanical safety of UGS activities using an approach derived from what is traditionally applied in the structural design of buildings. A safety factor, a margin of security against risks, is defined for each of the geomechanical issues listed above. First, a 3D FE-IE numerical model is developed to reproduce the stress and displacement due to the UGS program under evaluation. Then the reservoir pressure is increased until the "failure" condition is reached allowing to evaluate how far the project designed condition is from the above limit. The proposed approach is applied to Romagna, a depleted gas reservoir in Northern Italy converted to UGS, with the aim of investigating the safety of the project to increase the reservoir pressure up to 120 % pi, where pi is the original reservoir pressure before the start of primary production. The 3D geomechanical model has been developed using recent 3D seismic data, land displacements by InSAR, lab tests on reservoir and caprock samples, in-situ Modular Formation Dynamic Tester (MDT) stress tests, and large background information acquired from other UGS reservoirs located in the same sedimentary basin. The analysis outcome has revealed that the investigated scenario is safe, with safety factor larger than 1, in the range from 1.2 to 4. The most critical condition (the smallest safety factor) has been obtained in relation to the mechanical integrity of the reservoir formation, under very conservative conditions (cohesion = 0, friction angle = 30∘)

Estimate of a spatially variable reservoir compressibility by assimilation of ground surface displacement data

Abstract. Fluid extraction from producing hydrocarbon reservoirs can cause anthropogenic land subsidence. In this work, a 3-D finite-element (FE) geomechanical model is used to predict the land surface displacements above a gas field where displacement observations are available. An ensemble-based data assimilation (DA) algorithm is implemented that incorporates these observations into the response of the FE geomechanical model, thus re- ducing the uncertainty on the geomechanical parameters of the sedimentary basin embedding the reservoir. The calibration focuses on the uniaxial vertical compressibility c M , which is often the geomechanical parameter to which the model response is most sensitive. The partition of the reservoir into blocks delimited by faults moti- vates the assumption of a heterogeneous spatial distribution of c M within the reservoir. A preliminary synthetic test case is here used to evaluate the effectiveness of the DA algorithm in reducing the parameter uncertainty associated with a heterogeneous c M distribution. A significant improvement in matching the observed data is obtained with respect to the case in which a homogeneous c M is hypothesized. These preliminary results are quite encouraging and call for the application of the procedure to real gas fields

Characterization of earth fissures in South Jiangsu, China

Abstract. The Suzhou-Wuxi-Changzhou (known as "Su-Xi-Chang") area, located in the southern part of Jiangsu Province, China, experienced serious land subsidence caused by overly exploitation of groundwater. The largest cumulative land subsidence has reached 3 m. With the rapid progress of land subsidence since the late 1980s, more than 20 earth fissures developed in Su-Xi-Chang area, although no pre-existing faults have been detected in the surroundings. The mechanisms of earth fissure generation associated with excessive groundwater pumping are: (i) differential land subsidence, (ii) differences in the thickness of the aquifer system, and (iii) bedrock ridges and cliffs at relatively shallow depths. In this study, the Guangming Village Earth Fissures in Wuxi area are selected as a case study to discuss in details the mechanisms of fissure generation. Aquifer exploitation resulted in a drop of groundwater head at a rate of 5–6 m yr−1 in the 1990s, with a cumulative drawdown of 40 m. The first earth fissure at Guangming Village was observed in 1998. The earth fissures, which developed in a zone characterized by a cumulative land subsidence of approximately 800 mm, are located at the flank of a main subsidence bowl with differential subsidence ranging from 0 to 1600 mm in 2001. The maximum differential subsidence rate amounts to 5 mm yr−1 between the two sides of the fissures. The fissure openings range from 30 to 80 mm, with a cumulative length of 1000 m. Depth of bed rock changes from 60 to 140 m across the earth fissure. The causes of earth fissure generation at Guangming Village includes a decrease in groundwater levels, differences in the thickness of aquifer system, shallow depths of bedrock ridges and cliffs, and subsequent differential land subsidence.</p

Modelling ground rupture due to groundwater withdrawal: applications to test cases in China and Mexico

Abstract. The stress variation induced by aquifer overdraft in sedimentary basins with shallow bedrock may cause rupture in the form of pre-existing fault activation or earth fissure generation. The process is causing major detrimental effects on a many areas in China and Mexico. Ruptures yield discontinuity in both displacement and stress field that classic continuous finite element (FE) models cannot address. Interface finite elements (IE), typically used in contact mechanics, may be of great help and are implemented herein to simulate the fault geomechanical behaviour. Two main approaches, i.e. Penalty and Lagrangian, are developed to enforce the contact condition on the element interface. The incorporation of IE incorporation into a three-dimensional (3-D) FE geomechanical simulator shows that the Lagrangian approach is numerically more robust and stable than the Penalty, thus providing more reliable solutions. Furthermore, the use of a Newton-Raphson scheme to deal with the non-linear elasto-plastic fault behaviour allows for quadratic convergence. The FE – IE model is applied to investigate the likely ground rupture in realistic 3-D geologic settings. The case studies are representative of the City of Wuxi in the Jiangsu Province (China), and of the City of Queretaro, Mexico, where significant land subsidence has been accompanied by the generation of several earth fissures jeopardizing the stability and integrity of the overland structures and infrastructure.</p

Monitoring and Modeling Farmland Productivity Along the Venice Coastland, Italy

AbstractThe southern portion of the Venice coastland is a very precarious environment and salt contamination of land and groundwater is a severe problem that is seriously impacting the farmland productivity. Geophysical surveys, lab testing and continuous monitoring of hydrological parameters together with crop yield distribution were performed and acquired from 2010 to 2012 in a 21ha basin cultivated with maize crop and representative of the area. The dataset is here used to set-up a numerical model of soil moisture dynamics coupled with plant transpiration, photosynthesis and growth. The hydraulic model is linked to the atmosphere by the calculation of the stomatal conductance which is optimized for maximum carbon gain. The model is applied to the field site to understand the impact of land elevation, soil heterogeneities, and seawater contamination on land productivity

New scenarios in secondary hyperparathyroidism: etelcalcetide. Position paper of working group on CKD-MBD of the Italian Society of Nephrology

Bone mineral abnormalities (defined as Chronic Kidney Disease Mineral Bone Disorder; CKD-MBD) are prevalent and associated with a substantial risk burden and poor prognosis in CKD population. Several lines of evidence support the notion that a large proportion of patients receiving maintenance dialysis experience a suboptimal biochemical control of CKD-MBD. Although no study has ever demonstrated conclusively that CKD-MBD control is associated with improved survival, an expanding therapeutic armamentarium is available to correct bone mineral abnormalities. In this position paper of Lombardy Nephrologists, a summary of the state of art of CKD-MBD as well as a summary of the unmet clinical needs will be provided. Furthermore, this position paper will focus on the potential and drawbacks of a new injectable calcimimetic, etelcalcetide, a drug available in Italy since few months ago

New scenarios in secondary hyperparathyroidism: etelcalcetide. Position paper of working group on CKD-MBD of the Italian Society of Nephrology

Bone mineral abnormalities (defined as Chronic Kidney Disease Mineral Bone Disorder; CKD-MBD) are prevalent and associated with a substantial risk burden and poor prognosis in CKD population. Several lines of evidence support the notion that a large proportion of patients receiving maintenance dialysis experience a suboptimal biochemical control of CKD-MBD. Although no study has ever demonstrated conclusively that CKD-MBD control is associated with improved survival, an expanding therapeutic armamentarium is available to correct bone mineral abnormalities. In this position paper of Lombardy Nephrologists, a summary of the state of art of CKD-MBD as well as a summary of the unmet clinical needs will be provided. Furthermore, this position paper will focus on the potential and drawbacks of a new injectable calcimimetic, etelcalcetide, a drug available in Italy since few months ago

Delayed subsidence of the Dead Sea shore due to hydro-meteorological changes

Many studies show the sensitivity of our environment to manmade changes, especially the anthropogenic impact on atmospheric and hydrological processes. The effect on Solid Earth processes such as subsidence is less straightforward. Subsidence is usually slow and relates to the interplay of complex hydro-mechanical processes, thus making relations to atmospheric changes difficult to observe. In the Dead Sea (DS) region, however, climatic forcing is strong and over-use of fresh water is massive. An observation period of 3 years was thus sufficient to link the high evaporation (97 cm/year) and the subsequent drop of the Dead Sea lake level (− 110 cm/year), with high subsidence rates of the Earth’s surface (− 15 cm/year). Applying innovative Global Navigation Satellite System (GNSS) techniques, we are able to resolve this subsidence of the “Solid Earth” even on a monthly basis and show that it behaves synchronous to atmospheric and hydrological changes with a time lag of two months. We show that the amplitude and fluctuation period of ground deformation is related to poro-elastic hydro-mechanical soil response to lake level changes. This provides, to our knowledge, a first direct link between shore subsidence, lake-level drop and evaporation