THE EFFECT OF GAS COLUMN THICKNESS ON PRIMARY OIL RECOVERY FROM A HORIZONTAL RESERVOIR (GAS-CAP, SIMULATION, MODEL STUDY)

A numerical model is used to study the depletion performance of horizontal oil reservoirs overlain by gas zones varying in size from 0 to 3 times the thickness of the oil zone. The depletion is studied and described in terms of a gas coning stage for which the oil recovery is correlatable with dimensionless rate and time and a drainage stage for which the oil recovery and time are correlatable with a dimensionless gravity number. Specific cases studied include a low API gravity oil and a high API gravity oil with production taken from the bottom ten percent of the oil zone.;The results indicate that oil recovery can vary from 5 to 60 percent of the original oil in place depending on the size of the gas-cap, the nature of the fluid, the dimensionless drainage radius, the abandonment pressure and the production rate. At rates less than the initial maximum gas-free production rate, recovery increases with the size of the gas-cap. At higher rates, recovery decreases to a minimum, which depends upon the nature of the fluid, the dimensionless drainage radius and the abandonment pressure, and then generally increases as the size of the gas-cap increases. Recovery is significantly greater when the oil has a high API gravity. The recovery-rate relationships developed have important implications for current production and regulatory practice.