Bøger i Schriftenreihe des Energie-Forschungszentrums Niedersachsen (EFZN) serien

Hydraulic fracturing is essential technology for the development of unconventional resources such as tight gas. So far, there are no numerical tools which can optimize the whole process from geological modeling, hydraulic fracturing until production simulation with the same 3D model with consideration of the thermo-hydro-mechanical coupling. In this dissertation, a workflow and a numerical tool chain were developed for design and optimization of multistage hydraulic fracturing in horizontal well regarding a maximum productivity of the tight gas wellbore. After the verification a full 3D reservoir model is generated based on a real tight gas field in the North German Basin. Through analysis of simulation results, a new calculation formula of FCD was proposed, which takes the proppant position and concentration into account and can predict the gas production rate more accurately. However, not only FCD but also proppant distribution and hydraulic connection of stimulated fractures to the well, geological structure and the interaction between fractures are determinant for the gas production volume. Through analysis the numerical results of sensitivity analysis and optimization variations, there is no unique criterion to determine the optimal number and spacing of the fractures, it should be analyzed firstly in detail to the actual situation and decided then from case to case.

This research work is on Short- and Long-term Stability of a 100% Renewable Autonomous Power System for a Typical Geographical Region. It is concerned with understanding, modelling, analysing and mitigating power system stability problems associated with 100 percent renewable electrical power system. The complexity of power systems is continually increasing because of the growth in asynchronous interconnections and use of new power electronic based technologies for solar and wind power integrations. At the same time, regulatory constraints have forced utilities to increase the renewable energy share on the power systems. This research mainly deals with maximizing the Non-Conventional Renewable Energy (NCRE) share in the power system. Implementation of 100 percent renewable electric power system is no longer a myth. However, the system requires a large energy storage capacity for full supply with renewable energy in the electricity sector and new large scale of synchronous inertia shall be added to the system for frequency stability. Furthermore, necessary measures have to be taken for maintaining power system oscillations and damping which occur due to frequent power disturbances from wind and solar energy sources. However, the implementation of 100 percent renewable electrical power system depends on the many other things such as the financial capability, resources availability, etc.

Injection into geological formations is seen by many as a short to medium term measure to reduce emissions of CO2 to the environment and as such to slowdown the pace of global warming. The injection process requires that the fluid flows effectively into the host formation. To this end it is very important to accurately predict the pressure and temperature of the fluid along the well and especially at the bottom of the hole. In the present dissertation a rigorous procedure to estimate fluid pressure and temperature along CO2 injection wells has been developed based on analytical modeling. The procedure accommodates wellbores of varying diameter, varying deviation angles, non-uniform tubing strings and layered formations with different thermal properties and varying geothermal gradients.To test the models, computer codes have been written with Visual Basic.Net language on the Microsoft Visual Studio Platform. The codes are encapsulated in a user-friendly Graphical User Interface.The simulated results are compared with field observed data from a shallow aquifer injection vertical well in Germany (Ketzin) and that from a relatively deeper offshore aquifer injection slanted well in Norway (Snøhvit). The maximum deviation is around 2% for pressure and around 10% for temperature.