illed through the reservoir and the pressure conditions are controlled to create a pressure that is lower than the bubble point pressure, the liberated gas expands and drives the oil out of the formation and assists in lifting it to the surface. Reservoirs with the energy of the escaping and expanding dissolved gas as the only source of energy are calledsolution-gas-drive reservoirs.drive mechanism is the least effective of all drive mechanisms; it generally yields recoveries between 15% and 25% of the oil in the reservoir .. Gas-Cap-Drive Reservoirs: reservoirs have free gas existing as a gas cap above the oil. The formation of this gas cap was due to the presence of a larger amount of gas than could be dissolved in the oil at the pressure and temperature of the reservoir. The excess gas is segregated by gravity to occupy the top portion of the reservoir. In such a reservoirs, the oil is produced by the expansion of the gas in the gas cap, which pushes the oil downward and fills the pore spaces formerly occupied by the produced oil. In most cases, however, solution gas is also contributing to the drive of the oil out of the formation.favorable conditions, some of the solution gas may move upward into the gas cap and, thus, enlarge the gas cap and conserves its energy. produced by the expansion of the gas cap are knownas Gas-cap-drive reservoirs. This drive is more efficient than the solution-gas drive and could yield recoveries between 25% and 50% of the original oil in the reservoir .. Water-Drive Reservoirs: other reservoirs exist as huge, continuous, porous formations with the oil / gas occupying only a small portion of the formation. In such cases, the vast formation below the oil / gas is saturated with salt water at very high pressure. When oil / gas is produced, by lowering the pressure in the well opposite the petroleum formation, the salt water expands and moves upward, pushing the oil / gas out of the formation and occupying the pore spaces vacated by the produced oil / gas. The movement of the water to displace the oil / gas retards the decline in oil, or gas pressure, and conserves the expansive energy of the hydrocarbons.produced by the expansion and movement of the salt water below the oil / gas are known as water-drive reservoirs. This is the most efficient drive mechanism; it could yield recoveries up to 50% of the original oil.
OIL AND GAS EXPLORATION
explained in the previous sections, oil and gas exist in reservoirs located thousands of feet below the Earths surface and ocean floors. These reservoirs would exist only in certain locations depending on the geologic history of the Earth. Therefore, determining the location of petroleum reservoirs is a very difficult task and is, probably, the most challenging aspect of the petroleum industry. Finding, or discovering, a petroleum reservoir involves three major activities: geologic surveying, geophysical surveying, and exploratory drilling activities. The following subsections provide a brief background on each of these activities.Surveysurveying is the oldest and first used tool for determining potential locations where there would be a possibility of finding underground petroleum reservoirs. It involves examination of the surface geology, formation outcrops, and surface rock samples. The collected information is used in conjunction with geologic theories to determine whether petroleum reservoirs could be present underground at the...
