This sudden loss also causes particulates within this region to migrate thereby causing a kind of vacuum for the formation to cave in. Aerobic and anaerobic bacteria have the tendency of reacting with these liquids and forming polysaccharide which reduce porosity in the region. (Thomas F. B and Bennion D. B, 1994) These defects can be overcome to an extent if a quality underbalanced drilling is performed. What is underbalanced drilling? Underbalanced drilling is carried out when the pressure of the downward circulating drilling fluid is less than the inherent pressure in the rock formation.
While in some areas these conditions occur naturally, in others, such a state can be created by the use of hydrocarbon fluids of lesser densities. (Thomas F. B and Bennion D. B, 1994) This underbalanced condition can also be maintained by circulating along with the nitrogen or natural gases to keep the density at low value. Sometimes tubing strings are utilized to inject the gas and keep the circulating fluid density in check. Using such non-condensable gases to lower the density of circulating fluid is also called as ‘artificial’ underbalanced drilling. The Process Before proceeding to the advantages and disadvantages of underbalanced drilling it is very important to decipher what exactly this process entails and how it varies with overbalanced drilling operations in the actual execution of the process. Source: Engevik Oma Mari, Illustration of bottom hole pressure during OBD & UBD operations, 2007 Use of different densities of liquid creates different pressure at the bottom hole.
It is noted from the figure that with the use of low density fluids with top side pressure, the required bottom hole pressure is obtained at the same time keeping a slender drilling window which basically means that higher pressures are obtained keeping the bore hole window at a minimum.
(Engevik Oma Mari, 2007) This is clearly an advantage since narrow bottom hole windows means the chances of the rock formation capsizing are minimum. Source: Engevik Oma Mari, Pressure margins in UBD & OBD system, 2007 During overbalanced drilling operations the bottom hole pressure should be lower than the fracture pressure of the bottom rock formation since pressure in excess can lead to it capsizing and drill fluid being lost into the fractures.
(Engevik Oma Mari, 2007) However this bottom pressure achieved is usually higher than the pore pressure. As this pressure goes below the pore pressure, the oil from the rock formation slowly makes its way into the wellbore.
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