An analysis of the response of naturally fractured reservoirs to thermal recovery processes is presented, utilizing a suite of dual continuum reservoir simulation models (dual porosity, multiple interacting continua, vertical matrix refinement and dual permeability). The effects of the different models as well a, many fracture and matrix properties on aspects of stearn cycling, steam drive and gravity drainage processes are discussed in some detail.
While some factors are consistent with the isothermal response of naturally fractured reservoirs (in particular fracture spacing and the primary effect of matrix permeability), thermal phase behavior and heat flow effects in these reservoirs impart significantly different more complex behavior.
Most of the naturally fractured reservoirs which are produced by using thermal processes contain very low mobility oil and therefore heat conduction plays a very important role at the initial stages of production. With increasing oil mobility, convective gravity and capillary forces lake over if the matrix permeability is fairly high or the reservoir is fractured extensively. During a production cycle in a stearn stimulation process, heal is conducted from matrix rock to fracture fluid which can increase the fluid's energy tremendously. Depending on the fracture fluid (water/oil) volatility, the additional energy can cause different phase behavior responses.
Fractured reservoirs occur worldwide in the Middle East, Iran, Iraq, France, USA, Venezuela, Canada (Saidi (1987), van Golf-Racht, (1982)), and hold extensive hydrocarbon reserves. The presence of a large number of fractures throughout the reservoir provides extended area, of high permeability, where the fluid flows more easily. However, the productivity of these reservoirs depends on the porosity and permeability of the matrix, which stores most of the fluids in place. Production will cease in reservoirs with very Light matrix rock after the fracture network is depleted, because fluids arc not able to flow at reasonable rates from the matrix to the fracture. Reservoirs with fair matrix permeability will sustain production, because fluids from the matrix will flow into and replenish the fractures. Although fractured reservoirs have been known and produced for decades, a wide variety of production levels and reservoir responses have been observed. This, in turn has given impetus to more recent indepth analysis (both experimental and theoretical) of the underlying mechanisms. The utility of reservoir simulation models in decoupling and quantifying contributing factors has been recognized.
Initially, the behavior of fractured reservoirs was simulated by "single porosity" models with two different approaches:
Fraclure and matrix properties were averaged. With this approach the oil recovery is usually overpredicted, (Chen, et al (1987), Dean and Lo (1986)) especially in situations where the fracture spacing is large.
Fracture and matrix were represented by separate grid blocks. This case has two major drawbacks for field simulation studies:
a very large number of grid blocks is needed to represent the whole reservoir
numerical difficulties arise due to great differences between fracture and matrix properties
Later, Barenblalt, et al (1960) and Warren and Root (1963) introduced a simple dual continuum concept (the dual porosity model) for single phase flow.