Complex Fracture Geometry Investigations Conducted on Western-Siberian Oil Fields at Rosneft Company

摘要

The generalization of Hydraulic fracturing in West Siberia and the increase of job size over the recent year can impact the field development strategy. The correct estimation of the fracture dimension is critical to maximize the recovery factor of heterogeneous reservoir developed with water flood. Three main uncertainties exist: fracture height, half-length and azimuth. Commercial fracture models provide length estimate once a reliable estimate of height is known. This is evident for 2D model which requires a direct knowledge of the height but also for p3D model where the height is indirectly obtained from coupling stress profile and fluid flow. Fracture azimuth is traditionally provided by the horizontal stress anisotropy from open hole sonic logging. Unfortunately, in West Siberia at depth of 2500-3000 meters, there is negligible tectonic and open hole sonic dipole did not provide obvious fracture orientation. Fracture height growth affect mostly fracture job size and cost. Height growth has also shown to be a cause of premature wellbore screen out. Fracture half-length and orientation can have a significant impact on the effectiveness of pressure maintenance and flood efficiency. A review of world publication of direct fracture geometry measurement has shown the validity of seismic methods and tilt indicators for tight rock, such as carbonates and tight sandstone. However, all experiments on soft sandstones, such as found in West Siberian, have shown more limited results. Given the uncertainties in effective Fracture geometry and the negative impact that they could have on the field development, Rosneft decided to invest in a field research study denominated Fracture Geometry Investigation, to validate various method offered by the service industry. Two basic methods were tested and combined: wellbore logging and passive seismic. Wellbore logging is used to obtain an estimate of wellbore fracture height. It combines temperature log immediately after Minifrac or after Frac and Cased Hole Sonic Anisotropy (CHSA) which can be run at any time after frac. The direct estimate of fracture height is used to validate the result of a calibrated HF simulator using Net Pressure matching analysis. Passive Seismic monitoring (PSM) is used to obtain direct estimate of height, length and azimuth. PSM Acquisition must be done during hydraulic fracturing from the nearest well. The main goal of this study was to validate each method’s effectiveness and to construct a calibrated fracture model for the particular reservoir under investigation. The results of this investigation will be used to optimize fracture design, pressure maintenance strategy and pattern orientation.