The formation fluid pressure, or pore pressure, is the pressure exerted by the fluids within the formations being drilled. The sedimentary rocks, which are of primary importance in the search for, and development of oilfields, contain fluid due to their mode of formation. Most sedimentary rocks are formed as accumulations of rock debris or organic material, underwater. Since over two thirds of the earth’s surface is covered with oceans, the vast majority of sedimentary rocks are laid down as marine sediments in the shallow seas around the land areas.
In general, areas of the earth’s surface which are above sea level are affected by the processes of erosion (breaking up and wearing down of the land masses). The debris is washed down into the shallow sea basins where it settles out onto the sea floor, the coarser material generally settling out closer to the shore than the fine silts and clays.
Explanation Sedimentary Process For Formation Fluid Pressure
This process may continue for long periods as the earth’s surface slowly moves, some areas being pushed up to provide fresh surfaces for erosion, with adjacent sea basins slowly deepening to allow great lengths of sediment to build up. Thus sedimentary rocks contain water, usually sea-water, as an integral part of their make-up. As the depth of sediment increases, the rocks are compacted, squeezing water out. The water contained within the rocks becomes progressively more salty as the relatively small molecules of water move through the pore spaces of the rock, while the larger salt molecule is retained. The result of this is that the formation fluid pressure, or pore pressure, exerted by the water in a normal, open, sedimentary sequence is equivalent to that Produced by a free-standing column of salt water, which is rather saltier and heavier than typical sea water. An average figure for normal formation pressure gradient in marine basin sediment determined some years ago in the U.S. Gulf Coast area is 0.465 psi/foot. This is the pressure gradient produced by a column of water of approximately 100,000 ppm chloride. In comparison, a typical value for seawater is 23,000 ppm chloride.
This gradient of 0.465 psi/foot or, expressed as an equivalent mud weight, 8.94 ppg is generally accepted as a representative figure for normal pore pressures in marine basins. There is some evidence that, worldwide, this figure is a little on the high side and evidence in the North Sea generally supports this view. Overall, this results in a slight over-estimate of anticipated pressure which is the safer option. Thanks for reading formation fluid pressure or pore pressure find more article for drilling engineering on our site and bookmark this site by type ctrl + D.