The energy which causes oil and gas to flow from a reservoir formation intoa well depends upon a differential between the formation pressure and thepressure at the well face. This differential pressure may be the result ofanyone of at least three forces, or a combination of them. Gas under pressurein an oil-producing formation (part of the gas may be in solution and part incontact with the oil) is the principal source of energy for producing oil inmost fields. In some fields, however, hydrostatic pressure due to water incontact with the oil and/or the force of gravity may be important factors incausing oil to flow into a well. Still another force affecting oil movement issuggested by some students of the problem; the force dependent upon thephenomenon of capillarity.
Hydrostatic pressure is seldom the only source of energy in an oil sand, but itis commonly an adjunct to the compressed gas in the formation. Where thiscondition prevails, increased production may be obtained provided the waterencroachment is properly controlled. In addition to the water-drive effect, thehydrostatic head may tend to maintain pressure in the formation byprogressively reducing the volume of the reservoir as withdrawals to thesurface are made through the producing wells.
The action of the force of gravity on the oil in a formation is of relativelysmall importance, from an energy standpoint, until the last stage of theproductive life of a well is reached. This force is the only remaining naturalsource of energy in some of the eastern fields, but in most of the deep wellsof the Mid-Continent and California districts the force of gravity can beprecluded as a source of energy, because the wells reach their economic limitof production before the force of gravity becomes an important factor in movingoil to the wells.
Since the majority of fields produce oil chiefly by reason of the gas pressurein the oil horizons, the following treatment deals mainly with factors thatinfluence gas-energy relationships.
Gas-Energy Relationships Formation Pressures The gas pressures existing in oil-producing formations may be the result of acombination of several contributing factors.