In the frame of the DGMK-Project 741-2 we contribute to the understanding of the stress dependency of anisotropic seismic velocity as background for seismic forward modeling (e.g. constuction of macro model for imaging) and the interpretation of seismic data with respect to pore pressure, effective stress tensor and lithology. We use the physically based porosity deformation approach for description of compliances in dry or drained anisotropic rocks. We improved the predictability of velocities by e.g. accounting for stiff porosity next to compliant porosity. We include the description of non-diagonal elements using the assumption of anellipticity and of an isotropic piezosensitivity tensor. We performed and explained supporting experiments under uniaxial loading conditions using various types of room dry sedimentary rocks with VTI and originally HTI symmetry. Furthermore we investigated the impacts of pore fluids in a sandstone and a fine grained posidonian limestone. The main effects are the Gassmann effect and local fluid flow. We conclusively explained our measurements in the fully saturated sandstone by combining velocity predictions by means of the porosity deformation approach in drained rocks with classical poroelastic models. Additionally measurements in shales under hydrostatic and triaxial loading conditions have been performed in cooperation with an external company and subsequently evaluated.

In view of the increasing scarcity of energy resources, wells are being drilled to progressively greater depths for the production of liquid and gaseous hydrocarbons. Consequently, the drilling equipment is being subjected to higher temperatures and pressures. Economical exploitation of the reservoir is possible only with the application of drilling and completion methods which do not damage the formation. The damage potential of drilling fluids and treatment fluids is assessed on the basis of return permeability tests. For obtaining more conclusive results from return permeability tests and for increasing the reliability of their interpretation in the future, systematic investigations on the reproducibility of the results were of special importance. In the course of this third project phase, several return permeability tests were performed on each of two different model cores under constant conditions in the circulation system constructed at the institute. On the one hand, Obernkirchen sandstone was employed as model material; on the other hand, a synthetic material consisting of sintered borosilicate glass was used for the other model core. At least five tests were performed on the model core of each material, in order to evaluate the results for their reproducibility. A further important task was the selection of a suitable core material for future tests.

The main objective of the fourth phase of the project 680 was to optimize a complete fracturing operation in horizontal wellbores for maximum productivity. Thus, a workflow and numerical tool chain Frac- and Frac-Produ simulators were developed with consideration of the thermo-hydro-mechanical coupling for the history matching and optimization of stimulation and production, respectively. After the verification of the developed simulators, a full 3D reservoir model was generated based on a real tight gas field in North German Basin. The simulated fracturing results are comparable with the treating pressure measurement, while the simulated gas production rate has the similar tendency to that of in-situ measured data. The numerical sensitivity analysis results show that the influences of proppant type on fracture geometry and fracture conductivity is much larger than that of the fluid viscosity, while the influences of the injection time are the smallest. The optimization simulation results show that the injection rate is not the higher the better. The fracture spacing should also not be too small. Thus, there is no universal criterion to determine the optimal number and spacing of the fractures, should be analyzed and decided from case to case.

This report covers the results obtained during the second phase of the research project DGMK 746/2 titled “Experimental and Numerical Analysis of Polymer Flooding Processes using Micromodels”. The overall project aims to provide new insights into two-phase displacement processes - in particular polymer EOR processes - using micro visual studies supported by advanced micro models („chips"). The main targets for the second project phase were defined based on several challenges encountered during the first project phase, such as (a) the extension of the laboratory setup to enable flooding experiments at typical reservoir temperature conditions, (b) the design and creation of new advanced micro models based on micro-CT images with specified wetting conditions, (c) the validation of the micro model flooding results by comparison with core flooding as well as (d) a further extension of the polymer rheological database. An improved microfluidics setup with heating capabilities was successfully realized during the project phase, which enabled suitable experimental conditions matching the typical reservoir oil temperatures observed in German oil fields. Also, a core-flooding setup was created at the department which enabled the performance of core-plug experiments at equivalent flooding conditions. For the generation of advanced micro models, micro-CT images were obtained from Bentheimer core plugs, and a workflow was created that aims to convert the 3D flow paths of the real rock to the pseudo-2D flow patterns obtainable in the sandwich micro-models generated in this research project. Instead of a quarter of a 5-spot pattern, these new chips were using a linear flow pattern mimicking the core-plug flooding geometry. The wettability of the micro models was successfully modified using chemicals. The comparison of flooding results from micro models and the core plug is documented in the project report, as well as the numerical simulation of the chips and the core plug experiment. The rheological database was extended by various HPAM and xanthan polymers. Particular focus was given to viscoelastic properties and the visualization of stream lines during the polymer flow through the chips.

A drilling test rig for model milling was designed and optimized. Extensive tests were conducted with the aid of the model scale drilling rest rig. These tests showed that the common cutting theories of metal machining can also be used for optimizing the casing milling process in the field. The first tests were performed with conventional inserts. Care was taken to the fact that the geometry of the inserts attached on the laboratory milling tool were almost identical to those used in the field. In addition to the common design, even inserts with a modified rake angle and made from special tungsten carbide were tested. All tests were documented by using a high speed camera to evaluate the cutting process and chip development in detail. Tens of casing milling tests have been successfully conducted with the casing milling test rig at the institute using conventional and FHP-inserts. Test results confirm that longer chips are formed and produced at high rotational speeds. According to the available test results, shorter chips were observed at lower rotational speeds compared to higher rotational speeds. The trend of the insert wear increases with increasing the forward velocity for all rotational speeds. The FHP-insert with the cobalt content of 10 % showed to be war resistant compared to the conventional insert. Furthermore, innovative ideas for improving the milling process have been described and feasibility studies have been started.