Subsurface Fluid Extraction



Because the Mississippi River Delta was created by the sedimentation of source and reservoir rocks, it has become an area of intensive oil and natural gas extraction (Louisiana Coastal Wetlands Conservation and Restoration Task Force, 1993). Hydrocarbon and groundwater extraction have been linked to changes in the rates in which compaction occurs (Nunn, 2003). When natural processes in the Mississippi River delta are altered by human activity the rate at which land subsides can increased.






 
Flood control structures and works on the Mississippi River. Modified from Jim Addison, Army Corp of Engineers, 2004


 


Long term or rapid production of oil, gas, and formation water causes subsurface formation pressures to decline (3). The lowered pressures (3) increase the stress of the overburden (4), causing compaction of the reservoir rocks and may cause formerly active faults (1) to be reactivated (5). Compaction of the strata or downward displacement along faults can cause land-surface subsidence (6). Where subsidence and fault reactivation occur in wetland areas, the wetlands are reverted to open water (7). Figure is not to scale. D, down; U, up.

Modified from USGS 2001


When large volumes of oil, natural gas, and the associated formation of water are extracted from the subsurface, pressures in the reservoirs are reduced and stresses around the reservoir increase. Increases in stress from the overburden cause reservoir compaction that can lead to surface subsidence in localized areas (Morton, 2001).

Since the 1920’s coastal Texas and Louisiana has produced nearly 20 billion barrels of oil and more than 4.2 trillion m3 of natural gas. Although fluid production is generally concentrated within the field areas the effect of reservoir pressure decline extends far beyond the individual fields.



Kreitler et al
. has found that in areas where more then one field is producing in the same strata, regional depressurization can cause subsidence resulting in wetland loss in the areas between the fields (1988).

   
   
Wetland loss results from the loss of vegetated marsh that reverts to open water. As relative seal level rises, wetlands sink below the intertidal zone and barrier island systems move shoreward and become thinner. This causes the wetlands formed behind the water to revert to open water.
 
Evolution of a barrier island system in the Mississippi River Deltaic Plain.
Modified from Penland et al. 1988.

Today, marshes are prevented from receiving the overflow of freshwater and sediments during spring floods because of levees and navigational works on the Mississippi River (Morton, 2002). Instead of nearby marshes receiving these inputs, the freshwater and delta building sediments are deposited into deep water at the edge of the Gulf of Mexico Continental Shelf. The combination of the Mississippi river deltaic plain not receiving an amount of sediment sufficient to sustain land building and declines in subsurface reservoir pressures is, again, an important component of coastal habitat loss in Louisiana.