Page 43 - Origin and Prediction of Abnormal Formation Pressures

26 G.V. CHILINGAR, J.O. ROBERTSON JR. AND H.H. RIEKE III
73
2~ 72

S 71
-~ 70
~a 69

-- 68
67
.-~ 66

65
,,,,~
64
r~
=
"t= 63
62
0 2 4 6 8 10 12 14 16 18 20 22 24 26
Total dissolved salts, %
Fig. 2-2. Density of brines as a function of total dissolved solids (TDS). (Modified after Frick, 1962, fig.
22.21, p. 22.24.)

pp. If the vertical permeability of sediment allows the pore fluid to move out as the load
is increased, then the pressure (stress) distribution in pore fluids is the same as that of a
continuous column of water extending to the water-table surface (hydrostatic pressure).
Inasmuch as the mineral grains (skeletal structure) support a load equal to the weight
of the overlying water and grains minus the weight transferred to the water by the grains
(buoyant force), then the force responsible for compaction, Fe, is equal to:
Fe = Ft- Fb (2-2)
where Fe is the effective grain-to-grain force on the horizontal surface, A, Fb is buoyant
force, and Ft is total overburden load. The total overburden force, Ft is equal to:
ft = fo + W~ + Wf- (2-3)

where Fo is outside (external) force exerted on the body of the sediment under
consideration.
W~ is weight of solids:

l'{/s --- gs(l -- ~) Vb (2-4)
where ?,~ is the specific weight of solids, 4~ is the fractional volumetric porosity, and Vb
is the bulk volume of sediment.
Wf = weight of interstitial fluids:
Wf = yfq9 gb (2-5)

where ?,f is the specific weight of fluids.