Fluid Densities

The gradients may be caloulated from surface fluid densities, or may be directly measured by downhole pressure measurements using the repeat formation testing tool (RFT). The interfaces predicted can be used to confirm wireline measurements of fluid contact. 

The formation density log is the main tool for measuring porosity. It measures the bulk density of a small volume of formation in front of the logging tool, which is a mixture of minerals and fluids. Providing the rock matrix and fluid densities are known the relative proportion of rock and fluid (and hence porosity) can be determined. 

The bulk density measured by the logging tool is the weighted average of the rock matrix and fluid densities, so that ... 

The formation bulk density (p ) can be read directly from the density log (see Figure 5.51) and the matrix density (p J and fluid density (p,) found in tables, assuming we have already identified lithology and fluid content from other measurements. The equation can be rearranged for porosity ((])) as follows ... 

Measurement Requirements. Any analysis of measurement requirements must begin with consideration of the particular accuracy, repeatabihty, and range needed. Depending on the appHcation, other measurement considerations might be the speed of system response and the pressure drop across the flow meter. For control appHcations repeatabihty may be the principal criterion conversely for critical measurements, the total installed system accuracy should be considered. This latter includes the accuracy of the flow meter and associated readout devices as well as the effects of piping, temperature, pressure, and fluid density. The accuracy of the system may also relate to the required measurement range. 

Momentum Flow Meters. Momentum flow meters operate by superimposing on a normal fluid motion a perpendicular velocity vector of known magnitude thus changing the fluid momentum. The force required to balance this change in momentum can be shown to be proportional to the fluid density and velocity, the mass-flow rate. 

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 ... 

Sohd salt, ground and packaged in several particle size grades, can be used in saturated salt brines to increase the fluid density (28). However, sized salt is most often used as a water-soluble material for bridging or sealing porous formations. At one time the sized salt systems were used primarily for completion or workover operations, but use has increased as ddU-in fluids for horizontal wells (29). 

FIG. 6-9 Fanning Friction Factors. Reynolds niimher Re = DVp/ i, where D = pipe diameter, V = velocity, p = fluid density, and i = fluid viscosity. (Based on Moody, Trans. ASME, 66, 671 [1.944].)... 

Porous Media Packed beds of granular solids are one type of the general class referred to as porous media, which include geological formations such as petroleum reservoirs and aquifers, manufactured materials such as sintered metals and porous catalysts, burning coal or char particles, and textile fabrics, to name a few. Pressure drop for incompressible flow across a porous medium has the same quahtative behavior as that given by Leva s correlation in the preceding. At low Reynolds numbers, viscous forces dominate and pressure drop is proportional to fluid viscosity and superficial velocity, and at high Reynolds numbers, pressure drop is proportional to fluid density and to the square of superficial velocity. 

D = diameter of cylinder or effective width of objecl V = free-stream velocity p = fluid density [L = fluid viscosity... 

A = projected area perpendicular to the flow p = fluid density V = free-stream fluid velocity... 

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