LUKOIL-AIK Hydraulic Fracture Mapping in Western Siberia
Alexander Kalugin, Geology and Drilling Department, LUKOIL-AIK
Iskander Kazbulatov, Wireline Engineer, Weatherford
Anna Rubtsova, Borehole Seismic Geophysicist, Weatherford
Microseismic monitoring is one of the most efficient methods for hydraulic fracturing mapping. Its principle is based on monitoring micro-earthquakes in the strike-slip fault zones at the fracture propagation direction, or in the zones of natural fault activation. Microseismic events are recorded by a borehole geophone array located in a nearby monitoring well. Good planning and pre-job preparation are very important. Based on geological data and monitoring distance estimations, borehole seismic geophysicists select well candidates for microseismic monitoring, after that they start to prepare monitoring and treatment wells for operations.
An operation performed by Weatherford as part of services provided in Western Siberia for LUKOIL-AIK can serve as a successful example of microseismic monitoring. Maximum distance from the monitoring to the treatment well was 500 m. The well was cased and cemented, but not perforated. A tractor or hard cable can be used to run tools in the hole if the well’s deviation is more than 50 degrees. This well was not producing before being monitoring; but after that it was completed for oil production. Any producing or injection well can be used for monitoring, if it is within the detection range. In this case subsurface pumping equipment should be pulled out of hole and the packer should be set at least 50 m above the perforated interval. It is very important to lower fluid level at the monitoring well at least 400 m from the wellhead to minimize noise from pumps and surface equipment.
Velocities of P-wave and S-wave (compressional and shear waves) are required for locating microseismic events. The following methods can be used for velocity model generation: 1) vertical seismic profiling (VSP) and 2) cross-dipole sonic logging. Using both of these methods yields even better results.
Wave propagation time from the surface to geophones in the well is recorded during VSP. Geophone depth changes for each measurement. Impulse source gives good results in permafrost conditions, as it generates a high amplitude signal. VSP configuration is determined for each well based on the well profile, its location and the geological section. For the LUKOIL-AIK project, the source was located 50 m away from the wellhead at a certain angle to the bottom-hole. 10 m deep holes were drilled for charges placement. If the monitoring well is S-shaped, several different locations for detonation may be required.
Prior to microseismic monitoring in Western Siberia, Weatherford performed cross-dipole sonic logging in the monitoring well with a patented Compact CXD tool, the industry’s only 2.25-in. monopole/CXD tool that can be used to acquire compressional and fast shear wave direction, velocity and transit time. A very precise velocity model for microseismic monitoring was built after collecting information on both the VSP and cross-dipole acoustics.
After running in hole, the geophones have to be oriented. Knowing the geophone depth and well deviation, we can determine the tools’ position. As each sensor can rotate along longitudinal axis, they need to be preliminarily oriented. For this purpose, the bottomhole projection point is identified at the surface and three shots are plotted at the azimuth of 120 degrees between one another. Upon determination of shot coordinates, 10-m deep holes are drilled to place the charges. After that, signals from the sources with known locations are recorded to orient the geophones. Besides, strong seismic signals are recorded when the balls are seated in frac port seats, and it results in a further improvement of the measurement’s accuracy.
Due to the specifics of operations in Western Siberia, monitoring wells are often located at the same pad as the treatment well. The pad is often functioning, with drilling and pumping operations in nearby wells. Moreover, in this project, the drilling rig was 10 m from the monitoring well, and the treatment well was located only 5 m away. In spite of these complications, after 4 hours on site with the crane and a wireline truck, all the tools were run in the borehole. The drilling rig’s and heavy equipment operations in the monitoring area resulted in considerable increase of background noise. To minimize this noise, the drilling rig and workover operations were suspended within 2 km of the monitoring well, truck movements were restricted and electrical submersible pumps (ESP) in nearby wells were shut down for the time of monitoring.
The first microseismic monitoring in Russia was performed to map 4-stage hydraulic fracturing in a horizontal well with the intervals between ports from 100 to 150 meters. The target was a J1 formation. 149 microseismic events were located. The monitoring showed that the azimuth of microseismic events propagation was around 151 degrees and was consistent for all stages. The operating company thus got an understanding of the stress direction in the formation which would be helpful for construction of future wells: for better production, they should be drilled perpendicular to fracture propagation azimuth. According to Weatherford’s specialists, horizontal wells in this area should be drilled at the azimuth of 241 degrees. Performed microseismic monitoring allowed the determination of the induced fracture parameters: the fracture half-length was 230 m which exceeded the designed estimation of 92 m by 2.5 times; the fracture height was 43 m, which means that the fracture stayed in the formation and did not grow towards the underlying water zone. This data was used to change the hydraulic fracturing design. Also, the monitoring revealed that during the mini-frac, the whole stimulated rock volume was opened, and the main stage just filled the same volume with proppant.
So far, microseismic monitoring of hydraulic fracturing has been applied to the Jurassic, Bazhenov and Achimov formations in Western Siberia. The method showed good results with proper pre-job planning. In particular, as rocks of different formations have different properties, geomechanical modeling and assessment of density and cross-dipole logs should be performed before choosing well candidates for monitoring. This helps to make conclusions about the brittleness of rocks (based on Young’s modulus and Poisson’s ratio), and to estimate the maximum monitoring distance for the particular formation.
Microseismic monitoring is a fairly new method for Western Siberia. However, it yielded promising results for improvement of exploration and development efficiency, including hard-to-recover reserves. The method will provide operators with reliable data on the effect of local geological conditions on efficiency of well interventions, drainage zones, three-dimensional hydrofrac geometry, etc.