【正文】 illing fluid column in the annulus the more useful the drilling fluid is for controlling high pore pressure (the arrow points downward to increasing capability to control high pore pressure). There are limits to how heavy a drilling mud can be. As was discussed above, too heavy a drilling mud results in overbalanced drilling and this can result in formation damage. But there is a greater risk to overbalanced drilling. If the drilling mud is too heavy the rock formations in the openhole section can fracture. These fracturescould result in a loss of the circulating mud which could result in a blowout. 論文翻譯 12 In the past decade it has been observed that drilling with a circulation fluid that has a bottomhole pressure slightly below that of the pore pressure of the fluid deposit gives near optimum results. This type of drilling is denoted as underbalanced drilling. Underbalanced drilling allows the formation to produce fluid as the drilling progresses. This lowers or eliminates the risk of formation damage and eliminates the possibility of formation fracture and loss of circulation. In general, if the pore pressure of a deposit is high, an engineered adjustment to the drilling mud weight (with additives) can yield the appropriate drilling fluid to assure underbalanced drilling. However, if the pore pressure is not unusually high then air and gas drilling techniques are required to lighten the drilling fluid column in the annulus. Figure 114 shows a schematic of the various drilling fluids and their respective potential for keeping formation water out of the drilled borehole. Formation water is often encountered when drilling to a subsurface target depth. This water can be in fracture and pore structures of the rock formations above the target depth. If drilling mud is used as the circulating fluid, the pressure of the mud column in the annulus is usually sufficient to keep formation water from flowing out of the exposed rock formations in the borehole. The lighter drilling fluids have lower bottomhole pressure, thus, the lower the pressure on any water in the exposed fracture or pore structures in the drilled rock formations. Figure 114 shows that the heavier drilling fluids have a 論文翻譯 13 greater ability to cope with formation water flow into to the borehole(the arrow points downward to increasing control of formation water). Flow Characteristics A parison is made of the flow characteristics of mud drilling and air drilling in an example deep well. A schematic of this example well is shown in Figure 115. The well is cased from the surface to 7,000 ft with API 8 5/8 inch diameter, lb/ft nominal, casing. The well has been drilled out of the casing shoe with a 7 7/8 inch diameter drill bit. The parison is made for drilling at 10,000 ft. The drill string in the example well is made up of (bottom to top), 7 7/8 inch diameter drill bit, ~ 500 ft of 6 3/4 inch outside diameter by 2 13/16 inch inside diameter drill collars, and ~ 9,500 ft of API 4 1/2 inch diameter, lb/ft nominal, EUS135,NC 50, drill pipe. 論文翻譯 14 The mud drilling hydraulics calculations are carried out assuming the drilling mud weight is 10 lb/gal (75 lb/ft3), the Bingham mud yield is 10 lb/100 ft2, and the plastic viscosity is 30 centipose. The drill bit is assumed to have three 13/32 inch diameter nozzles and the drilling mud circulation flow rate is 300 gals/minute. Figure 116 shows the plots of the pressures in the inpressible drilling mud as a function of depth. In the figure is a plot of the pressure inside the drill string. The pressure is approximately 1,400 psig at injection and 6,000 psig at the bottom of the inside of the drill string just above the bit nozzles. Also in the figure is a plot of the pressure in the annulus. The pressure is approximately 5,440 psig at the bottom of the annulus just 論文翻譯 15 below the bit nozzles and 0 psig at the top of the annulus at the surface. The pressures in Figure 116 reflects the hydrostatic weight of the column of drilling mud and the resistance to fluid flow from the inside surfaces of the drill string and the surfaces of the annulus. This resistance to flow results in pressure losses due to friction. The total losses due to friction are the sum of pipe wall, openhole wall, and drill bit orifice resistance to flow. This mud drilling example shows a drilling string design which has a open orifice or large diameter nozzle openings in the drill bit. This is reflected by the approximate 700 psi loss through the drill bit. Smaller diameter nozzles would yield higher pressure losses across the drill bit and higher injection pressures at the surface. The air drilling calculations are carried out assuming the drilling operation is at 論文翻譯 16 sea level. There are two pressors capable of 1,200 scfm each, so the total volumetric flow rate to the drill string is 2,400 scfm. The drill bit is assumed tohave three open orifices (~ inches diameter). Figure 117 shows the plots of the pressures in the pressible air as a function of depth. In the figure is a plot of the pressure inside the drill string. The pressure is approximately 260 psia at injection and 270 psia at the bottom of the inside of the drill string just above the bit orifices. Also in the figure is a plot of the pressure in the annulus. The pressure is approximately 260 psia at the bottom of the annulus just below the bit orifices and psia at the end of the blooey line at the surface (top of the annulus). As in the mud drilling example, the pressures in Figure 117 reflects the hydrostatic weight of the column of pressed air and the resistance to air flow from the inside surfaces of the drill string and the surfaces of the annulus. This resistance to flow results in pressure losses due to friction. In this example the fluid is pre