![]() ![]() Lastly, excessive mud densities are a disadvantage because they unnecessarily increase mud costs. A high overbalance pressure also increases the risk of sticking the drill pipe (see Chapter 9). Laboratory experiments and field experience have shown that the rate of penetration is reduced by mud overbalance pressure (the differential between the mud pressure and the pore pressure when drilling in per meable rocks)5'6 7-8 9 and by the absolute pressure of the mud column when drilling rocks of very low permeability. Several methods have been developed for predicting the occurrence of geopressures.1 Knowledge of the expected pore pressure and fracture gradients: 15 usually enable casing to be set at exactly the right depth, thereby greatly reducing the number of disaster wells.Īnother disadvantage of excessive mud densities is their influence on drilling rate ( rate of penetration). Under these circumstances, it is generally necessary to set a string of casing to separate the two zones. The problem of maintaining mud density high enough to control formation fluids, but not so high as to induce a fracture, becomes acute when normally pressured and geopressured formations arc exposed at the same time. In induced fracturing, mud is lost into the fracture so formed, and the level in the annulus falls until equilibrium conditions are reached. This failure is known as induced fracturing. In the first place, excessive mud density may increase the pressure on the borehole walls so much that the hole fails in tension. In the interest of well safety, there is a natural tendency to carry a mud density well above that actually needed to control the formation fluids, but this policy has several major disadvantages. Very rarely is an increase in mud density justified as a means of improving cutting carrying capacity. The buoyant effect of the mud on the drill cuttings increases with its density, helping transport them in the annulus, but retarding settling at the surface. In the case of plastic formations, such as rock salt and unconsolidated clays, the pressure of the mud is crucial The bulk density of partially compacted sediments increases with depth, but an average SG of 2.3 is usually accepted, so that the overburden (or geostatic or litholostatic) pressure gradient is about 1 psi/ft (0.23 kg/cm2/m), and the pore pressure of geopressured formations is somewhere between the normal and the overburden pressure gradients, depending on the degree of compaction.īesides controlling pore fluids, the pressure of the mud column on the walls of the hole helps maintain borehole stability. The hydrostatic pressure gradient of formation fluids varies from 0.43 psi/ft to over 0.52 psi/ft (0.1 to 0.12 kg/cm2/m), depending on the salinity of the water. ![]() Normally pressured formations, which have a self-supporting structure of solid particles (so the pore pressure depends only on the weight of the overlying pore fluids), and abnormally pressured or geopressured formations, which are not fully compacted into a self-supporting structure (so the pore fluids must bear the weight of some or all of the overlying sediments as well as the weight of the overlying fluids). ![]()
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