Understanding the water front pattern and the sweep efficiencies in water injection programs is critical to improve the recovery efficiency and to model the reservoir for better simulation studies. 4D seismic surveys have proven to be powerful tools for locating the water front and therefore estimating the macroscopic sweep efficiency. In this paper, we introduce a new method to estimate the microscopic sweep efficiencies in a water injection process using seismic data.
We use some earlier theories and relate the P- and S-wave velocities to the following reservoir parameters: matrix bulk and shear modulus, matrix density, porosity, fluid saturation, water salinity, oil API gravity, gas specific gravity, pressure and temperature. According to some simple assumptions, only the water saturation and pore pressure change after the water injection. Having two equations for seismic wave velocities, we calculate these changing parameters solving a problem of two equations and two unknowns. We model a reservoir and simulate a water injection into it. The synthetic seismograms generated before and after the injection are compared and the water saturation values after the injection are calculated using the discussed approach. The calculated values show a good match with the simulated ones.
Most of the hydrocarbon reserves around the world have been discovered and are in production. They are being depleted rapidly. On the other hand, the worldwide demand for oil is increasing. To overcome the problem, the reservoirs must be managed so that the recoveries from these reservoirs increase. The recovery of most reservoirs is less than 40%. So, we need to improve our management strategies.
The repetition of seismic surveys over a period of time at a specific location can monitor the drainage pattern of fluid flow in the subsurface efficiently. The formations outside the reservoir generally change only slightly during production, but reservoir properties such as pore pressure and fluids saturations change significantly.
The subtraction of two 3D seismic data acquired at different time from a single site will generate a seismic difference image. This seismic difference image can indicate the effects of changes of reservoir properties on the seismic data. Such timelapse seismic analysis can be used to understand fluid flow patterns and is useful in defining the EOR strategies. The ultimate goal of time-lapse seismic monitoring is to detect fluid migration in the subsurface, but in practice, there are several difficulties in achieving this task. Acceptable timelapse seismic monitoring requires consistent seismic acquisition and processing. Only in this case, the seismic difference image can represent reservoir properties changes.
The changes in reservoir properties during production include pore pressure and pore fluid content. Reservoir temperature during production is quite constant. However, in some EOR processes such as thermal recoveries, the temperature also changes. Reservoir pore pressure declines near producing wells as fluid are extracted and increases in the vicinity of injection wells as gases or liquids are injected which will change the seismic wave velocities.
