Human Induced Subsidence

 

The impact of humans on the rate of subsidence has been noted in many regions around the world, especially in those that are undergoing hydrocarbon and groundwater extraction. In Venice, Cassiani and Zoccatelli are assessing the risk of subsidence in offshore gas fields to determine what impact removing large volumes of natural gas might have on the already sinking city. Risk assessments like the ones being done in Italy are being applied to planned gas production projects in order to predict the extent of subsidence in coastal regions (2000).

Evidence that extraction of hydrocarbons could cause subsidence was first noted by Pratt and Johnson in the Goose Creek Oil field near Houston, Texas (1926). Shortly after the oil field was developed in 1917 oil extraction was identified as the medium that was responsible for induced subsidence. Since the subsidence occurred in such a localized area it lead to the conclusion that the withdrawal of subsurface fluids from shallow reservoirs could lower the land elevation, cause earthquakes, and activate faults around the edges of producing fields (Pratt and Johnson, 1926).

In Long Beach, California at the Wilmington Oil field land subsidence, caused by oil and natural gas production lead to flooding and structural damage to bridges, railroads, and other harbor facilities. As the oil reserves were depleted, sand compaction caused almost 9 m of land subsidence (Poland and Davis, 1969). Groundwater extraction has had similar effects on land subsidence in Louisiana.
Over the last 75 years industry and population growth has increased and in that time so has the demand for freshwater. Since the early 1950’s groundwater withdrawals in East Baton Rouge Parish have averaged 100 million gallons per day. This has led to an average decline in water levels of 0.3 meters per year in all of the areas major aquifers. The consequences of such long term groundwater extraction drains water from interbedded clay confining layers increasing the potential for sediment compaction and surface subsidence. As the water is pumped from the aquifer the pressure from the overburden increases and causes the clay to become more compact (Nunn, 2003). This is the same general principle that can be applied to hydrocarbon extraction. In both cases the pressure in the reservoir or aquifer decreases as it is pumped out causing the stress from the overburden to increase and consolidated the sediments below.
  Population Densities for East Baton Rouge showing that it is a region of Louisiana that is highly populated. Modified from U.S. Census Bureau
 

Because we know that geologic subsidence in areas of thick Holocene sediments occur at a much smaller rate, a correlation has been made, by Morton, between areas that exceed historical geologic subsidence rates and subsurface fluid extraction (2002). The highest geologic subsidence rates have been less then 3 mm. per year. These rates correspond with the reactivation of fault zones and oil and gas fields (Morton, 2002). Hydrocarbon production is known to cause natural subsidence of land and has been indicated as a cause of wetland loss. However, natural subsidence from hydrocarbon production and compaction should decrease over time as production slows and compaction decreases (Morton, 2002). In Madison Bay this is not the case and in recent years increases in subsidence rates and wetland loss have been linked to the extraction of subsurface resources
 

Source of Estimate

Period

Ave. Subs. Rate

Reference

C14core P-1-90

4740 BP

2.7 mm/yr

Roberts et al. (1994)

C14peat sample 2067

425 BP

1.4 mm/yr

Frazier (1967)

Houma tide gauge

1946 - 1962

0.7 mm/yr

Penland et al. (1988)

Houma tide gauge

1962 - 1982

19.4 mm/yr

Penland et al. (1988)

Petit Caillou relevel line

1966 - 1993

9.3 mm/yr

Morton et al., 2002

SET measurements

1992 - 1997

22 mm/yr

Cahoon et al., 1999

Cores and water levels

1969 - 1999

23 mm/yr

This study

 Average geological and historical rates of subsidence for the Terrebonne delta plain region near Madison Bay.  Modified from Morton et al 2003

 

Subsidence rates in Madison Bay have averaged 23 mm a year since the 1960’s. Madison Bay is surrounded by wells used to extract hydrocarbons from deep subsurface reservoirs (Morton et al, 2003). Lapeyerouse, Lirette, and Bay Baptist contain the hydrocarbon fields that are closest to Madison Bay. Within these fields hydrocarbon production took place at low to moderate rates in the 1940’s and 1950’s and peaked in the 1970’s (Morton 2003). After peak production in the 1970’s subsidence rates should have decreased. However, the rates of subsidence in Madison Bay have not followed this trend. The three fields mentioned above have combined to produce 2 trillion cubic feet of gas and 154 million bbls of oil and water (Morton 2003). This has been a site of intense subsurface hydrocarbon extraction and is an important component of accelerated rates of subsidence and wetland loss.

   

Graph showing wetland loss continued well after oil, gas, and water production decreased in Lapeyrouse Oil field.. Wetland loss should have decreased following decreases in hydrocarbon production. Thus, subsidence associated with the Madison Bay area can be correlated with subsurface fluid extraction. Modified from Morton et al. 2002


Graph showing wetland loss continued well after oil, gas, and water production decreased in Lirette Oil field.. Wetland loss should have decreased following decreases in hydrocarbon production. Thus, subsidence associated with the Madison Bay area can be correlated with subsurface fluid extraction. Modified from Morton et al. 2002