Changes

==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 [[length::298 ft]] [[depth::(100 m]]) of water column..]]

[[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..]]

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.

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.