There are many oil reservoirs, usually shallow, that contain oil which is too viscous for effective production. Nevertheless, economic recovery from such reservoirs is still possible through the application of heat. Heavy crude oils, which may have a viscosity up to one million times that of water, will show a reduction in viscosity by a factor of 10 for each temperature increase of 50 °C. The most successful way to raise the temperature of a reservoir is by the injection of steam. In the most widespread method, called steam cycling, a quantity of steam is injected through a well into a formation and allowed time to condense. Condensation in the reservoir releases the heat of vaporisation that was required to create the steam. Then the same well is put into production. After some water production, heated oil flows into the well bore and is lifted to the surface. Often the cycle can be repeated several times in the same well. A less common method involves the injection of steam from one group of wells while oil is continuously produced from other wells.
An alternate method for heating a reservoir involves in situ combustion—the combustion of a part of the reservoir oil in place. Large quantities of compressed air must be injected into the oil zone to support the combustion. The optimal combustion temperature is 500 °C. The hot combustion products move through the reservoir to promote oil production.
The process of steam injection into the reservoir or in situ combustion creates thermal expansion and changes in the structure of the enclosing rocks, leading to mechanical deformations of the reservoir and adjacent formations. Subsequently, the microseismic activity in these zones is significantly increased relative to the general background seismicity.
Microseismic emission arising in the heat-affected zone propagates to the day surface and can be recorded using surface-based highly sensitive equipment. Using its Full-Wave Location technology, TenzorGEO can monitor these processes from the surface, locate position and evaluate the direction and size of the steaming zone, confirm breakthrough channels for vapor filtration or the position of the combustion front. Armed with this information, the operator can effectively plan reservoir development and adjust recovery technics in the process.
DR ROY BITRUS, PhD
DIRECTOR OF SALES