Changes

Theoretically, fracturing occurs when the pore pressure reaches P_f. However, P_f increases as pore pressure increases. Although the theory generally is described in terms of the pore pressure needed to overcome the horizontal stress keeping the fractures closed, in practice the pore pressure must approach the lithostatic pressure for brittle failure to occur.<ref name=ch10r52>Lorenz, J., C., Teufel, L., W., Warpinski, N., R., 1991, Regional fractures: a mechanism for the formation of regional fractures at depth in flat-lying reservoirs: AAPG Bulletin, vol. 75, no. 11, p. 1714–1737.</ref>

Theoretically, fracturing occurs when the pore pressure reaches P_f. However, P_f increases as pore pressure increases. Although the theory generally is described in terms of the pore pressure needed to overcome the horizontal stress keeping the fractures closed, in practice the pore pressure must approach the lithostatic pressure for brittle failure to occur.<ref name=ch10r52>Lorenz, J., C., Teufel, L., W., Warpinski, N., R., 1991, Regional fractures: a mechanism for the formation of regional fractures at depth in flat-lying reservoirs: AAPG Bulletin, vol. 75, no. 11, p. 1714–1737.</ref>

The following figure 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|10-42}}See text for explanation.]]

[[file:evaluating-top-and-fault-seal_fig10-42.png|thumb|{{figure number|1}}See text for explanation.]]

==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==

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. The figure below compares the fracture gradient pressure for the case of a well on land and the same well with an additional [[length::298 ft]] [[depth::(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 [[length::298 ft]] [[depth::(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.

Similarly, facies changes within a basin can alter the density distribution in the sediment column and seal risk. A facies change from dense carbonates to less dense siliciclastics changes the overburden stress gradient. A higher pore pressure is required to fracture a top seal in the denser sediment column. Seal risk is greater in the less dense sediment column. The overburden stress gradient and seal risk similarly change with progressive subsidence and compaction.

Similarly, facies changes within a basin can alter the density distribution in the sediment column and seal risk. A facies change from dense carbonates to less dense siliciclastics changes the overburden stress gradient. A higher pore pressure is required to fracture a top seal in the denser sediment column. Seal risk is greater in the less dense sediment column. The overburden stress gradient and seal risk similarly change with progressive subsidence and compaction.

==See also==

==See also==