The bitumen deposits of Alberta contain an estimated 2.6 trillion barrel of hydrocarbon in place and therefore compare to the giant oil fields of the Middle East (ref 3). Unfortunately, the bitumen has viscosities & ranging from 1,000 to 4,000,000 centipoise and will not flow at virgin reservoir temperatures. Despite this, bitumen production rates in Canada are on the rise (seee Figure 1) and are projected to become a substantial portion of the total petroleum output in the future. (ref. 5) A large percentage of the bitumen occurs at burial depths exceeding 300 meters which excludes the possibility of mining. In most cases recovery is by steam injection which lowers the bitumen viscosity steam injection is an expensive proposition so economic bitumen production must rely on two factors - efficient production techniques and a strong oil price.
For thermal processes to be successful, it is important that intervals that contain mobile fluids - gas or water - be readily identified. Mobile fluid intervals, if present, have a major impact on the thermodynamics and the resulting operating efficiency of the stimulation process.
The identification of mobile water is usually attempted by interpretation of resistivity logs. However resistivity devices respond to both mobile and immobile water.
The Nuclear Magnetism Log provides a measurement of the Free Fluid Index (FFI) of the formation. FFI represents those fluids that are free to move within the pore space of the rock. In a shaly sandstone reservoir containing only bitumen and water, the FFI measurement does not include the volume of bitumen, clay bound and capillary bound water. At this point it is important to note that the Free Fluid Index is not mobile water (ie. producible water) in the classical reservoir engineering sense, as water that is free to move within the pore space may not be able to move through the pore throats. However, the results of previous work (ref 7), have indicated that mobile water zones are characterized by FFI readings greater than 5 to 6 porosity units (p.u.).
This paper will discuss some of the problems associated with bitumen production. The Nuclear Magnetism log will be introduced and the Free Fluid Index (FFI) will be used to define a petrophysical model. This model will then be implemented in the interpretation of well log data on five wells.
Although various thermal recovery techniques are available, cyclic steam stimulation is one of the few effective methods of recovering bitumen, Economically the process offers operators the benefits of short term production revenue with minimal risk.
The cyclic steam stimulation process on a single well involves three distinct stages, as illustrated in Figure 2.
Steam Injection. Typically steam is injected at rates in the order of 200 cubic meters per day for 30 to 50 days. Steam temperatures in the wellbore are approximately 200 degrees centigrade. In order to achieve these temperatures steam pressures are high. Bitumen zones generally have low injectivity due to low permeability to water, in which case steam pressures are usually sufficiently high to cause formation fracturing.