FOR many years past geologists have been looking for ideal structures. Unfortunately these are rarely found. Even assuming. an ideal structure is thought to be located, it is evident that this will not be proved until a considerable amount of drilling has been done, and in the meantime production has to be taken without complete knowledge of underground conditions. It is, therefore, exceptionally rare that a field is produced under ideal conditions right from the start, and most projects such as pressure control, repressuring, water drive, etc., are put forward as a cure for rapid decline rather than as a prevention. Structures may be intensely irregular due to complex folding and faulting. Sands may, and nearly always are, of. uneven thickness and porosity. Water may be introduced by faulting and the edge line may be extremely irregular. Some areas may contain loose and fine sand necessitating special well-head control, while others may contain hard porous sands not even requiring liners. On the other hand, occasionally structures approaching the ideal are found. In Trinidad there are the 1enticular sands entirely free of edge water. The Sugarland field appears to be as ideal as possible (as is also its production policy), while in Iraq some structures appear to be "made to specification." STRucTuRE OF RESERvoms. What is applicable to one field will not suit another, and it is therefore proposed to deal only with the ideal rate of withdrawal of oil from the ideal structure. It is assumed, therefore, that the structure is a regular anticline of even thickness and porosity under volumetric control, as illustrated in Diagram 1.
The sands contain oil, gas and water in their correct gravity levels, some of the gas being in solution in the oil. It is usual to find, in fields where the production horizon is up to 4000 feet deep, that there is free gas lying above the oil in the crestal part of the structure, but it is possible that under sufficient pressure all the gas is in solution in liquid form.
The gas and oil are trapped in the top of the structure and kept under pressure by a head of water. It will be seen from the typical pressure gradient diagram that the maximum pressure is at the bottom of the syncline and it is equivalent to the total water head "H." The decrease in pressure is constant for every foot of the flank of the anticline-until the water/oil contact is reached and at this point is equivalent to "H-h." Gas is absorbed in oil under pressure, depending on the constitution of the gas and oil. According to Henry's Law, "if temperature is kept constant the amount of gas dissolved by a liquid is proportional to the absolute pressure exerted by the gas on the liquid."