Gas production methodology is a function of the type of gas reservoir and the production stage in the life of the particular reservoir. A simple oil and gas reservoir may initially produce high volumes of oil relative to gas, but as the oil production and reservoir pressure decline, an increasing amount of gas may be produced. This increases the gas/oil ratio (GOR) of the produced hydrocarbons. Reservoir management may dictate producing oil first, using the natural gas pressure to increase oil recovery rates. As the GOR increases, reservoir pressure may be too low for natural production, and without secondary production methods, production will eventually cease from the reservoir. In this case, the total recovery factor could be as low as 20%–40%, leaving 60%–80% of the original hydrocarbons—both oil and gas—in place.
Field development plans incorporate reservoir models derived from all available reservoir information. Petroleum engineers use these models to plan the location of field development wells. The figures below show a typical onshore and offshore field development. The main difference between the two developments is that offshore development wells are usually tied to a fixed platform with a set number of slots or well bores. An offshore platform may have a large number of slots, each leading to a well deviating away from the platform, draining a specific portion of the reservoir.
The process of drilling a gas well is quite easy to understand. The sequence is shown below.
Frame 1 shows the drilling pipe with the drill bit penetrating virgin rock. Drilling mud, indicated by arrows, is pumped inside the drill pipe. Drilling mud allows rock cuttings to be brought to surface, cools the drilling bit, and maintains hydrostatic pressure in the well bore, preventing reservoir fluid from rising to the surface. After drilling to a certain depth, the drill pipe is removed, and casing pipe is inserted into the well and cemented in place. Frame 2 shows a smaller drill bit drilling beyond the depth of the initial casing. At a deeper point, the drill pipe and bit are removed, and a smaller diameter casing is inserted and cemented in the newly drilled section. This process is repeated (in Frame 3) with a smaller drilling bit and smaller casing diameter until the desired reservoir depth is reached. At that point, the well is perforated to allow reservoir fluids to enter the well bore. Smaller pipes, called tubings, are usually inserted in the well bore to channel the flow of hydrocarbons from the subsurface to the surface valves and pipelines.