Three of the key tasks performed by reservoir engineers are the estimation of the Stock Tank Oil Originally in-Place, STOOIP, in an oil reservoir or drainage volume, estimation of the stabilized production rate from a well, forecast future performance of an oil reservoir or well.
Two common methods for calculating the STOOIP in an oil accumulation are:
The Volumetric Method
The Material Balance Method
Drive mechanisms are the physical phenomena that keep reservoir pressures high and cause fluids to move to lower pressure areas in the reservoir. There are five drive mechanisms in an oil reservoir:
Rock and fluid expansion
Solution gas drive
Gas cap drive
Natural aquifer drive (or water encroachment)
The major flow regimes experienced by vertical production wells occur in sequence due to the pressure disturbance caused by production propagating radially outward from the well. These flow regimes are:
Well dominated flow
Late transient flow
Boundary dominated flow
The stabilized production rates from a production well occur during the boundary dominate flow regime. These stabilized production rates are governed by the inflow performance relationships.
Well damage or stimulation can occur in all production wells. Well damage/stimulation is quantified by the Skin Factor. A positive (+) skin factor results from well damage, while a negative (-) skin factor results from well stimulation.
Two common methods for making future reservoir or well forecasts are: