The first methods of oil exploitation were very primitive. The tools of the oilman at that time were a shovel and a bucket. Very little oil was used and it was applied to grease the axles of carts, for lighting and to heal sheep. These methods of exploitation fully satisfied the market at the time. Today, of course, this is not enough. The figures show how far man has come in extracting oil: in 1955, 763 million tonnes of oil were extracted. To extract such huge quantities of oil, which is often extracted from various depths, very efficient and powerful equipment is needed. There are natural seeps, where the oil comes out like spring water. However, there are also very deep wells. In 1955, a depth of 6640 metres was reached in the Mississippi River delta near New Orleans.
[001] Wooden pumpjack at Fairbank Oil Fields.
It is worth to note that oil does not form underground lakes where a well is drilled. The oil is in the pores between the grains of sand. An oil field has industrial value, when it is isolated by impermeable rock layers. Otherwise, the oil would not persist in the sandstone. Isolation layers are usually clay and shale, but may also be siliceous rock, quartzite, marl and limestone. Isolation layers also separate the oil field from lower or higher water horizons. An oil field is most valuable when it consists of only sand grains. The sand grains are sometimes bound together by a binder and form rocks called sandstones. The binder can be clay, limestone or silica. The type of binder determines the hardness of the sandstone. The spaces between the grains of sand are connected by tiny channels. Through these channels, oil flows under pressure. The size of these channels determines how much and how fast the oil can flow through them. The oil field can never be fully exploited. Under favourable conditions, the amount of oil extracted reaches 80%, while the rest remains in the soil, inaccessible to man. 
[002] The pumpjack - diagram. 1 - oil field 2 - anchor of extracting pipes 3 - suction-discharge pump with valve balls 4 - extracting pipes 5 - wellhead 6 - oil outflow 7 - connecting rod 8 - pumpjack 9 - pump engine 10 - counterweight 11 - pullers 12 - nodding beam Oil in a field behaves completely differently from oil on the surface. The reason lies in the different temperature and pressure at depth. The average temperature increases by 1 °C for every 30 metres of depth. The higher the temperature, the lower the viscosity of the oil. The oil field is under considerable pressure. This is called the formation pressure. Its magnitude is determined by the height of the water column. Since oil is always accompanied by natural gas, which can be dissolved or bubbled up, a higher pressure means more gas in the oil. The more dissolved gas in the oil, the lower its viscosity. The viscosity of the oil is critical to the performance of the oil field. Maintaining a low viscosity forces the temperature and pressure to be as high as possible. The temperature of the reservoir is reduced by the expansion of the gas caused by the drop in pressure. The pressure decreases due to the exploitation of the field. With proper exploitation, the formation pressure remains constant. The channels that the oil has left are filled with water. If it doesn't have enough time to fill them, the pressure of the field will drop, thus decreasing its efficiency. 
[003] Pumpjacks on the oil field.
In order to extract the oil, specially designed pumps were developed. They are called pumpjacks. The motor driving the pump is on the surface and the piston is driven by special rods of a length corresponding to the depth of immersion of the pump. The walls of the borehole are reinforced with casing pipes. This protects the borehole from backfilling and flooding. Inside the casing, the pump pipes are inserted. These in turn carry the oil to the surface. Usually seamless pipes with diameters ranging from 1 1/2 to 4 inches are used. The pipe sections are joined together in a column by special couplings with tapered threads. The pump pipes must have considerable strength, their test pressure is 200 bars. The first section of the pump pipes is a submersible pump with ball valves. Usually the diameter of this pump is the same as the diameter of the pipes. The construction principle of this pump is similar to that of any other suction-pressure pump with ball valves. The limitation is the diameter of the pipeline. The length of the cylinder can be up to 1500 mm to achieve the greatest possible capacity. The pump cylinder is made of cast iron with a high surface quality. In the lower part of the cylinder there is a suction valve, which consists of a steel ball and valve seat. The pump piston is usually made of machine tool steel. At the top of the piston is the discharge valve, which also consists of a steel ball and valve seat. The pump is designed so that both the valve and the piston can be pulled to the surface without having to pull out the pump cylinder. This is very helpful when repairing and cleaning the valves. The final component is the rods connecting the pump piston to the main sway bar. 
[004] Working principle of the valve ball pump.
In addition to oil, most wells also yield natural gas. Just below the pump there is a gas separator, which ensures that only crude oil enters the pump pipeline. Natural gas is transported through the space between the pump pipes and the casing (outer) pipes. The gas cannot come into contact with the atmosphere and passes straight from the well into the sealed pipes, where it is transported on to the user. Similarly, crude oil does not come into contact with the atmosphere. The wellhead is a part that performs several functions. It contains the gas connection, and a separate oil line connection. The head also contains a seal that prevents oil from leaking into the atmosphere. The drive of pumpjacks is divided into two main types: group drive and single drive. A group drive involves supplying power to the pumps of several wells simultaneously. Single drive, on the other hand, drives one pump per single well. 
[005] A directional wheel used to propel a group of pumpjacks. The group drive is usually carried by means of cables and a directional wheel. The wheel operates in an oscillating motion. The disadvantage of this solution is the low accuracy in determining the production parameters and the same stroke for each node, regardless of the conditions in the well. The single drive is more adapted to the individual needs of the wellbore, both in terms of lifting capacity and the length and number of strokes. The group drive has a limited stroke length, matching the average conditions of the entire group of wells. For this reason, single drives are becoming increasingly popular in the oil industry. The entire equipment, whether for single or group pumping of wells, must be properly balanced. During one pump cycle, the load on the motor should be as constant as possible. During the upward movement of the piston, in addition to the arcia and inertia forces, the motor has to overcome the weight of the rod and the fluid to be pumped. During the downward movement of the piston, the engine does not work, and is even driven by the pump. 
[006] Oil field in the USA. The pumpjack does not pump the oil directly in the ground. Oil from the oil field flows into the well due to the pressure difference between the oil field and the bottom of the well. The pressure difference should remain as high as possible for as long as possible. This influences the percentage of exploitation of the oil field, which cannot be exploited completely. Modern methods of reviving oil fields, aimed at maintaining the pressure difference for as long as possible and even increasing it, mean that fields that are already known to be inactive, can still provide valuable raw material. These methods are irrigation and gasification of deposits. In the first case, water is pumped into the oil field through specially prepared boreholes; in the second case, gas taken from other deposits is pumped in. However, part of the oil, a tribute to the earth, will always remain in the depths. • • •
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