d.). Following this period of exploration one must tackle the seismic interpreters with their predictions and drill exploration wells. If these wells are on-shore, then the cost can be modest, but if the prospected reservoir is off-shore in ultra deep water, drilling a well is very expensive and it becomes an interesting strategy game to balance the risk of drilling a dry well against the risk of missing a big cat. Seismic data gives the wide contours of the reservoir but with low data motion. Near the exploration wells one can remove a very detailed picture of the reservoir rock and fluids, using down-hole logging tools that use quite advanced methods like gamma-rays, NMR and electrical resistivity, in order to map-out the reservoir properties very close to the well. Obviously there remains a lot of uncertainty in the reservoir properties even after combining seismic data, well-log data and educated guesses from experienced geophysicists and geologists (Vink, n.d.).

Reservoir engineers become involved when a reservoir has been found and its location has been roughly mapped out using the seismic data and data from the exploration wells. The duty of reservoir engineers is to use this information to make a field development plan, which explains in suitable detail where future production wells must be drilled and what type of production strategy will be employed. In the beginning of oil and gas recovery, reservoir engineering was simple. The process was to simply drill a hole and at some point there would be oil gushing out. If that did not happen, one would try again a bit farther out. These wells characteristically were in easily reachable locations and drilling depths were very reserved so the cost of drilling these wells was small. Currently a lot of the oil is offshore and drilling depths can broaden to extreme depths, the current depth record is close to 8 kilometers. It is more and more a condition to produce as much as technically possible from the subsurface oil and gas. With the simple strategy of primary depletion, it is only the intrinsic reservoir pressure that presses the oil to the surface, but this pressure declines rapidly and only a small fraction, 15-20%, of the oil can be recovered. In order to reach a higher ultimate recovery (UR), one must re-pressurize the reservoir, for example by injecting water or gas (Vink, n.d.).

Water and gas injection to re-pressurize the reservoir and push the oil towards producing wells are examples of secondary recovery. With secondary recuperation the UR can be significantly higher at almost 30-60%. However, the ambition nowadays is to reach ultimate recoveries of 70-80% and this requires even more enhanced oil recovery (EOR) techniques. Chemicals can be inserted that dissolve oil and wash-out the rock much more successfully than plain sea water does. Or chemicals can be used that make the oil less viscous so that it flows more easily to the producing wells. This viscosity decline is mandatory when attempting to recover very heavy oil or bitumen. This kind of hydrocarbon looks more like the material that is used to make a hockey puck as it essentially is a solid unless it is heated up significantly. Yet an additional quite advanced recovery technique uses air insertion. The oxygen that is in the air act in response with the heavy oil, and this burning manufactures heat and gases that helps to push the oil forward. At the same time, the burning change heavy hydrocarbons into lighter ones and a small percentage of coal-type remains. If such a process can be controlled at the field scale, the UR could be as high as 80-90% (Vink, n.d.).

With the need for these more advanced field development concepts, the role of the reservoir engineer has become more important and also the need grows for tools to help developing such plans, preferable finding the options with the largest chance of a high ultimate recovery on an economically attractive time scale and with the least environmental impact. Reservoir flow reproduction is the main quantitative tool that allows exploring option development concepts and can give forecasts with uncertainty ranges for the various options. Also the distinctiveness of new or complex EOR methods can be investigated, for example the result of injecting steam, or polymers that can dissolve oil, or other chemicals or even bacteria. By uniting lab scale experiments with field-scale reservoir simulations, the margins of doubt around applying such novel and usually expensive improved recovery methods can be reduced (Vink, n.d.).

Natural gas processing...

The composition of the raw natural gas extracted from producing wells depends on the type, depth, and location of the underground deposit and the geology of the area. Oil and natural gas are often found together in the same reservoir. The natural gas produced from oil wells is generally classified as associated-dissolved, which means that the natural gas is associated with or dissolved in crude oil. Natural gas production absent any association with crude oil is classified as non-associated. In 2004, 75% of U.S. wellhead production of natural gas was non-associated. Most natural gas production contains, to varying degrees, small hydrocarbon molecules in addition to methane. Although they exist in a gaseous state at underground pressures, these molecules will become liquid at normal atmospheric pressure. Collectively, they are called condensates (Cohen, 2006).
An ideal field would be one with a very high porosity and permeability reservoir with great continuity, like a pay zone in a well on one side of the field looks like a pay zone in a well on the other side of the field, and there is one continuous, high porosity and high permeability reservoir. Our ideal field could be developed with a minimum number of wellbores, with low decline rates per well. A less than ideal field would be a series of discrete reservoirs, with little or no continuity between the discrete reservoirs. To fully develop our less than ideal field would require far more wells than the ideal field. Also, given the limited volume in each reservoir, the decline rate per well would be fairly high (Cohen, 2006).

Petroleum, commonly referred to as oil, is a natural fuel formed from the decay of plants and animals buried beneath the ground, under tremendous heat and pressure, for millions of years. Formed by a similar process, natural gas often is found in separate deposits and is sometimes mixed with oil. Because oil and gas are difficult to locate, exploration and drilling are key activities in the oil and gas extraction industry. Oil and natural gas furnish about three-fifths of our energy needs, fueling our homes, workplaces, factories, and transportation systems. In addition, they constitute the raw materials for plastics, chemicals, medicines, fertilizers, and synthetic fibers (Oil and Gas Extraction, n.d.).

Using a variety of methods, on land and at sea, small crews of specialized workers search for geologic formations that are likely to contain oil and gas. Sophisticated equipment and advances in computer technology have increased the productivity of exploration. Maps of potential deposits now are made using remote sensing satellites. Seismic prospecting, a technique based on measuring the time it takes sound waves to travel through underground formations and return to the surface has revolutionized oil and gas exploration. Computers and advanced software analyze seismic data to provide three-dimensional models of subsurface rock formations. This technique lowers the risk involved in exploring by allowing scientists to locate and identify structural oil and gas reservoirs and the best locations to drill. Four-D, or "time-lapsed," seismic technology tracks the movement of fluids over time and enhances production performance even further. Another method of searching for oil and gas is based on collecting and analyzing core samples of rock, clay, and sand in the earth's layers (Oil and Gas Extraction, n.d.).

After scientific studies indicate the possible presence of oil, an oil company selects a well site and installs a derrick or tower like steel structure in order to support the drilling equipment. A hole is drilled deep in the earth until oil or gas is found, or the company abandons the effort. Similar techniques are employed in offshore drilling, except that the drilling equipment is part of a steel platform that either sits on the ocean floor, or floats on the surface and is anchored to the ocean floor. Although some large oil companies do their own drilling, most land and offshore drilling is done by contractors (Oil and Gas Extraction, n.d.).

In rotary drilling, a rotating bit attached to a length of hollow drill pipe bores a hole in the ground by chipping and cutting rock. As the bit cuts deeper, more pipe is added. A stream of drilling mud which is a mixture of clay, chemicals, and water, is continuously pumped through the drill pipe and through holes in the drill bit. Its purpose is to cool the drill bit, plaster the walls of the hole to prevent cave-ins, carry crushed rock to…