Drilling fluids hydrostatic pressure

Collapse load originates from the hydrostatic pressure of drilling fluid, cement slurry outside the casing and later on by moving formations e.g. salt... 

Normal pressure regimes follow a hydrostatic fluid gradient from surface, and are approximately linear. Abnormal pressure regimes include overpressured and underpressured fluid pressures, and represent a discontinuity in the normal pressure gradient. Drilling through abnormal pressure regimes requires special care. 

Density. The density of the drilling fluid is adjusted using powdered high density soHds or dissolved salts to provide a hydrostatic pressure against exposed formations in excess of the pressure of the formation fluids. In addition, the hydrostatic pressure of the mud column prevents coUapse of weak formations into the borehole. Fluid densities may range from that of air to >2500 kg/m (20.8 Ib/gal). Most drilling fluids have densities >1000 kg/m (8.33 lb/gal), the density of water. The hydrostatic pressure imposed by a column of drilling fluid is expressed as follows ... 

If the drill string becomes differentially stuck, mechanical methods or spotting fluids can be appHed, or the hydrostatic pressure can be reduced (147). In general, penetration of water- or oil-based spotting fluids into the interface between the filter cake and the pipe accompanied by dehydration and cracking results in reduction of differential pressure across the drill string (147,148). Spotting fluids are usually positioned in the open hole to completely cover the problem area. 

The fluid pressure in the rock at the bottom of a well is commonly defined as pore pressure (also called formation pressure, or reservoir pressure). Depending on the maturity of the sedimentary basin, the pore pressure will reflect geologic column overburden that may include a portion of the rock particle weight (i.e., immature basins), or a simple hydrostatic column of fluid (i.e., mature basins). The pore pressure and therefore its gradient can be obtained from well log data as wells are drilled. These pore pressure data are fundamental for the solution of engineering problems in drilling, well completions, production, and reservoir engineering. 

The yielding of pipe does not occur provided that the equivalent stress is less than the yield strength of the drill pipe. For practical calculations, the equivalent stress is taken to be equal to the minimum yield strength of the pipe as specified by API. It must be remembered that the stresses being considered in Equation 4-54 are the effective stresses that exist beyond any isotropic stresses caused by hydrostatic pressure of the drilling fluid. 

Hydrostatic pressure of the drilling fluid behind the drill string at the packer level is... 

Basically all formations penetrated during drilling are porous and permeable to some degree. Fluids contained in pore spaces are under pressure that is overbalanced by the drilling fluid pressure in the well bore. The bore-hole pressure is equal to the hydrostatic pressure plus the friction pressure loss in the annulus. If for some reason the borehole pressure falls below the formation fluid pressure, the formation fluids can enter the well. Such an event is known as a kick. This name is associated with a rather sudden flowrate increase observed at the surface. 

Assuming the formation fluid does not enter the drillpipe, we know that the SIDPP plus the hydrostatic head of the drilling fluid inside the pipe equals the pressure of the kick fluid (formation pressure). The formation pressure is also equal to the SICP plus the hydrostatic head of the original mud, plus the hydrostatic head of the kick fluid in the annulus. 

When the well is circulated through the choke line, a rapid loss in hydrostatic pressure is seen when the kick fluid begins to enter the choke line. Hydrostatic pressure is lost because low density gas is displacing the drilling mud from the small volume of choke line. Small kick volumes can result in long columns of gas in the choke line. Surface choke response must be rapid enough to prevent new kick fluid from entering the well due to the reduction in bottomhole... 

In a diamond cell, the sample volume is sacrificed for the sake uf higher pressures, and hence, all operations connected with (he cell have lo be performed under a microscope. In preparing the DAC for an experiment, the first step is to indent Ihe metal gasket (hardened stainless steel strip nr Inconel strip) with the anvil diamonds to the correct thickness (5(1 to UK) micrometers) and then drill a IOO- to 200-micrumeter hole as close lo the center of the indentation as possible. The gasket is seated on the face of one of the diamonds in the same orientation as it had when the indentation was made. The sample material and a small chip of ruby for pressure calibration are (lien placed in the hole. Finally, to maintain hydrostatic pressure the hole is tilled with a tiny drop of fluid from a syringe and then (he hole is quickly sealed by (he diamond fucus before Ihe fluid evaporates. 

Hydrostatic Pressure. During drilling, the column of rock originally occupying the hole is replaced by a column of drilling fluid. The... 

The hydrostatic pressure exerted by the drilling fluid on the walls of the borehole must be carefully chosen. If the hydrostatic pressure is too large, then rock formations in contact with the fluid may fracture and large volumes of drilling fluid may be lost to the formation (lost circulation). If the hydrostatic pressure is too small, then the rock formation may fail by tensile failure. The effect of drilling fluid density on the mechanical stability of boreholes has been discussed by several authors (4, 5)... 

Figure 9. Experimental and predicted rheograms for invert emulsion oil-based drilling fluids. Rheograms collected at 600 bar hydrostatic pressure and various temperatures (shown in degrees centigrade). (Reproduced with permission from reference 69. Copyright 1986 Society of Petroleum Engineers.)...

Figure 13 shows the pressure dependence of the Herschel-Bulkley parameters [n, K, r0, and t Hb (eq 14) at the highest shear rate] for a weighted drilling fluid at 40 °C. In contrast to oil-based drilling fluids, the rheology of water-based fluids shows little dependence on hydrostatic pressure. 

Hydrostatic pressure = density of drilling fluid x true vertical depth acceleration of gravity. If hydrostatic pressure is greater than or equal to formation pressure, formation fluid will not flow into the well-bore. 

Similarly as the diameter of the laser-drilled orifice is increased, the hydrostatic pressure within the tablet is decreased and therefore An Ap. Furthermore, the osmotic pressure within the tablet vastly exceeds that of the surrounding biological fluids and thus Art may be reduced to 7t. Under these conditions, equation 27 becomes ... 

There are in fact some bromine applications that are not questioned from an environmental point of view. Bromine has a similar usage to chlorine for water disinfection. Silver bromide is used as a Hght-sensitive substance in photographic emulsions. A quarter of aU bromine used finds its way down into boreholes in oilfields, where calcium and zinc bromide solutions are used as drilling fluids. In high concentrations, the liquids are heavy and exert a high hydrostatic pressure that prevents the very disturbing blow-out situations. 

A kick is a well control problem in which the pressure found within the drilled rock is higher than the mud hydrostatic pressure acting on the borehole or rock face. When this occurs, the greater formation pressure has a tendency to force formation fluids into the wellbore. This forced fluid flow is called a kick. If the flow is successfully controlled, the kick is considered killed. An uncontrolled kick that increases in severity may result in what is known as a blowout. ... 

Three main parameters play a role in a kick development. Two are related to the type of rocks in which drilling is performed permeability and porosity. The third parameter is the difference between the formation fluid pressure and the mud hydrostatic pressure. If the formation pressure is much greater than the hydrostatic pressure, a large negative differential pressure exists. 

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