<P> At the bottom of the tubing is the down - hole pump . This pump has two ball check valves: a stationary valve at bottom called the standing valve, and a valve on the piston connected to the bottom of the sucker rods that travels up and down as the rods reciprocate, known as the traveling valve . Reservoir fluid enters from the formation into the bottom of the borehole through perforations that have been made through the casing and cement (the casing is a larger metal pipe that runs the length of the well, which has cement placed between it and the earth; the tubing, pump, and sucker rod are all inside the casing). </P> <P> When the rods at the pump end are travelling up, the traveling valve is closed and the standing valve is open (due to the drop in pressure in the pump barrel). Consequently, the pump barrel fills with the fluid from the formation as the traveling piston lifts the previous contents of the barrel upwards . When the rods begin pushing down, the traveling valve opens and the standing valve closes (due to an increase in pressure in the pump barrel). The traveling valve drops through the fluid in the barrel (which had been sucked in during the upstroke). The piston then reaches the end of its stroke and begins its path upwards again, repeating the process . </P> <P> Often, gas is produced through the same perforations as the oil . This can be problematic if gas enters the pump, because it can result in what is known as gas locking, where insufficient pressure builds up in the pump barrel to open the valves (due to compression of the gas) and little or nothing is pumped . To preclude this, the inlet for the pump can be placed below the perforations . As the gas - laden fluid enters the well bore through the perforations, the gas bubbles up the annulus (the space between the casing and the tubing) while the liquid moves down to the standing valve inlet . Once at the surface, the gas is collected through piping connected to the annulus . </P> <P> Pumpjacks can also be used to drive what would now be considered old - fashioned hand - pumped water wells . The scale of the technology is frequently smaller than for an oil well, and can typically fit on top of an existing hand - pumped well head . The technology is simple, typically using a parallel - bar double - cam lift driven from a low - power electric motor, although the number of pumpjacks with stroke lengths 54 inches (137 cm) and longer being used as water pumps is increasing . An example can be found here . </P>

Which type of simulation would be used for gas exploration