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Optimum Well Spacing and In-Fill Schedule in Cold Production

by Franklin L. Foster, Ph.D.
   - based on a presentation by Bernard Tremblay, Saskatchewan Research Council

           When developing a field, one of the key decisions involves the placement of wells.  Research shows that developing a field on smaller spacing will lead to higher oil recovery factors because of the nature of the wormhole network which develops during the cold production process. At larger well spacing this network does not drain as efficiently the reservoir leaving more oil behind. On the other hand, smaller well spacings by definition require more wells to be drilled which increases the production costs. An optimum placement of wells exists which depends on the price of heavy oil.

          The basic mechanisms involved in cold production are: solution-gas drive and sand production. When the pressure in a reservoir is decreased below a certain pressure, methane comes out of solution in the form of bubbles. These bubbles then grow into neighbouring pores and displace the oil towards the well. Eventually, these bubbles coalesce until the critical gas saturation is reached at which point mostly gas is produced leaving behind the oil. Heavy oil contains more natural surfactants and has a higher viscosity than light oil. These two factors imply that the gas bubbles in the reservoir will be more stable leading to higher critical saturations and therefore to a higher oil recovery factor. Solution gas drive is, however, not sufficient by itself to produce heavy oil at economical rates. Allowing sand production leads to the development of high permeability channels, called wormholes, which drain the reservoir.

            Another question is whether the production and recovery rates of an existing field can be increased economically by infilling the existing well placements.  In other words, is there anything to be gained by drilling wells in and among existing producers?  Again, the research suggests caution.  An infill well can block the growth of an existing well.