Heavy oil contained in naturally fractured reservoirs is becoming an important resource as conventional oil reserves are depleted. However, maximizing recovery from such reservoirs is problematic due to the low flow rate of oil and the poor understanding of recovery mechanisms. One EOR method that is of particular interest is Thermally Assisted Gas-Oil Gravity Drainage (TA-GOGD). In this process, steam is injected into the reservoir. This heats the rock matrix blocks through the higher permeability fracture network and improves oil recovery principally by reducing the oil viscosity and thus increasing the rate of gravity drainage through the matrix. This paper shows that there is a critical steam injection rate for TA-GOGD in fractured reservoirs. If the injection rate is below this critical rate, the time to heat the matrix will increase and oil recovery increases with the square of steam velocity in the fracture. If the steam injection rate is greater than the critical rate then there is no significant increase in oil recovery with rate. A simple formula for calculating the critical steam rate in the fractures is derived analytically. We show that it correctly predicts the critical injection rate for detailed simulations of steam injection into an element of a fractured reservoir containing live oil. The predictions are shown to be correct for all fluid and rock properties investigated.
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