Changes

[[:file:evaluating-top-and-fault-seal_fig10-42.png|Figure 1]] charts P_f vs. pore pressure for a range of overburden stress gradients (0.5–1.0). The pore pressure equals or exceeds P_f only when the pore pressure is equal to or greater than the lithostatic stress. Fracture occurs when P_f equals the pore pressure.

[[:file:evaluating-top-and-fault-seal_fig10-42.png|Figure 1]] charts P_f vs. pore pressure for a range of overburden stress gradients (0.5–1.0). The pore pressure equals or exceeds P_f only when the pore pressure is equal to or greater than the lithostatic stress. Fracture occurs when P_f equals the pore pressure.

[[file:evaluating-top-and-fault-seal_fig10-42.png|thumb|{{figure number|1}}Chart of P_f vs. pore pressure for a range of overburden stress gradients (0.5–1.0).]]

[[file:evaluating-top-and-fault-seal_fig10-42.png|thumb|300px|{{figure number|1}}Chart of P_f vs. pore pressure for a range of overburden stress gradients (0.5–1.0).]]

==Stress and poisson's ratio variation==

==Stress and poisson's ratio variation==

==Effect of water depth, stratigraphy, and facies changes==

==Effect of water depth, stratigraphy, and facies changes==

[[file:evaluating-top-and-fault-seal_fig10-43.png|thumb|{{figure number|2}}Comparison of the fracture gradient pressure for the case of a well on land and the same well with an additional 298 ft (100 m) of water column..]]

[[file:evaluating-top-and-fault-seal_fig10-43.png|thumb|300px|{{figure number|2}}Comparison of the fracture gradient pressure for the case of a well on land and the same well with an additional 298 ft (100 m) of water column..]]

Pore pressure alone does not control hydraulic fracturing. Changes in the overburden stress change the theoretical fracture pressure and seal risk. For example, water depth alters the overburden stress and therefore P_f. [[:file:evaluating-top-and-fault-seal_fig10-43.png|Figure 2]] compares the fracture gradient pressure for the case of a well on land and the same well with an additional 298 ft (100 m) of water column. The water column substitutes low-density water for high-density rock. The result is a shift of P_f to lower values. If the water depth were sufficiently great, there would be an increased likelihood of hydraulic fracturing with no change in pore pressure.

Pore pressure alone does not control hydraulic fracturing. Changes in the overburden stress change the theoretical fracture pressure and seal risk. For example, water depth alters the overburden stress and therefore P_f. [[:file:evaluating-top-and-fault-seal_fig10-43.png|Figure 2]] compares the fracture gradient pressure for the case of a well on land and the same well with an additional 298 ft (100 m) of water column. The water column substitutes low-density water for high-density rock. The result is a shift of P_f to lower values. If the water depth were sufficiently great, there would be an increased likelihood of hydraulic fracturing with no change in pore pressure.