Subsidence from Oil and Gas Production in Louisiana

Summary and Conclusions

There are many situations in Louisiana in which OGE does cause surface subsidence, but the amount of surface deformation is minimal or negligible. Surface subsidence from OGE is probably far more widespread than everyone realizes, but in the cases in which it does exist, it is so minimal that nobody observes it. Since it is not something known to cause surface or above ground structural problems in Louisiana, few people in the state are aware that it even exists. Since OGE subsidence is not really on many people's minds, what visible subsidence that does exist is not even noticed.

Not being noticed and not occurring are two different things. I interviewed a number of geologists and engineers with significant experience in the oil and gas industry in the state (References 6-14), and only two of them, Boudreaux⁶ and Kumar1¹ had any knowledge of OGE subsidence in the state, though nearly all were aware of the more well known cases of OGE subsidence in other parts of the U.S. A search of the technical literature on the subject provides significant discussion and documentation on how, when, and to what extent OGE subsidence is likely to occur. Information on actual case studies in Louisiana, though, is very limited in the literature. What I found generally indicates that when subsidence from OGE does occur in Louisiana, it is either negligible to minimal;³ or, in the case of coastal marsh areas, not enough is known to quantify and separate OGE effects from other concurrent subsidence mechanisms such as salt water intrusion, soil consolidation, etc.¹

Background

Groundwater withdrawal, drainage of organic soils, and underground mining of coal are the principal causes of subsidence. In the U.S., these activities have caused approximately 10,000, 3,600, and 3,100 square miles of surface subsidence, respectively, from these three activities. Additionally, about 18 percent of the conterminous U.S. surface is underlain by cavernous limestone, gypsum, salt, or marble, making the surface in these areas susceptible to catastrophic collapse into sinkholes.⁴ Though small in overall scale because of the limited distribution of deposits, the solution mining of sulfur via the Frasch process can create significant surface disturbances. Additional mechanisms from natural processes are active in South Louisiana, primarily in the marshes and nearby coastal zone, that cause subsidence from the consolidation of thick sediments.¹

 

With a few exceptions, surface subsidence from oil and gas extraction (OGE) is relatively insignificant. Those few exceptions, however, can be quite spectacular; the literature on OGE subsidence deals mainly with a few notable examples such as the Wilmington field near Long Beach, California which has experienced 30 or more feet of subsidence; the Goose Creek oil and gas field in Harris County, Texas; oil reservoirs near Lake Maracaibo, Venezuela where huge dykes had to be built to protect the coastal area from flooding; and the Groningen gas fields in The Netherlands.² In an extensive study of Louisiana oil and gas reservoirs, Martin and Serdengecti³ concluded that some surface subsidence probably occurs over all hydrocarbon reservoirs that experience a pressure decline during production, but in only a few of the tens of thousands of developed oil and gas fields has subsidence actually been detected. Their study of fields in Louisiana indicated that for most cases the potential subsidence is insignificant. Geertsma² concluded the same thing for oil and gas fields in general throughout the world.

Discussion of Louisiana and Conditions Conducive to Subsidence^{2,3,7,11,13,14}

According to studies by Boesch et.al.,¹ apparent sea level along Coastal Louisiana is currently rising more than five times faster than global sea level due to subsidence of thick unconsolidated sediments. This rapid subsidence is a natural process related to the accretion and consolidation of sediment deposited by the various lobes of the Mississippi River Deltaic Plain. Prior to this century, the accretion process pretty much kept up with or exceeded the consolidation process across Coastal Louisiana. The channelization of the Mississippi River and its distributaries combined with dredging of canals, has upset the natural balance, disrupted the supply of active sediment and subjected the wetlands to wave exposure, saltwater intrusion, and other processes that cause deterioration of the wetlands. This accelerates the natural subsidence in the marshes from the consolidation of the previously deposited sediments. Any additional subsidence that may be caused from the extraction of subsurface fluids such as oil and gas is masked by this natural subsidence until someone devises a method to delineate the subsidence caused be each process.

 

Sharp and Hill⁵ attempted to separate the subsidence contribution from OGE through a comparative study of areas along the Texas coast where different subsidence mechanisms are predominant. They investigated the Big Hill and Fannett fields of the Frio Formation in the northeastern portion of the Texas Gulf coastal plain. They stated that these reservoirs are similar in depth, amount of reservoir depressurization, and geology to many others in the Texas and Louisiana Gulf coastal region. The data they analyzed indicates that regional subsidence caused by petroleum production "may be widespread along the Texas and Louisiana Gulf coasts." More study is required to make a definitive determination. One of their conclusions is that regional global positioning satellite / releveling studies over the region for a period of up to ten years may be needed to accurately assess the magnitude and extent of regional subsidence.

 

Surface subsidence results from the combined effects of the subsurface reservoir compressing or compacting a sufficient amount that it causes a displacement in geological formations above that are not strong enough to withstand the underlying structural force changes. If the changes in reservoir conditions are small enough or the upper geological structures are strong enough, surface displacement will not occur or will be only negligible. Reservoir compaction is a function of the extent of pressure reduction, the thickness of the producing zone, and the compressibility of the formation structure. Most reservoirs in which OGE has caused significant compaction consist of a sequence of unconsolidated sands and shales. Studies have shown that the amount of surface subsidence caused by reservoir compaction decreases with increasing depth of the reservoir from the surface and increases with the effective diameter of the reservoir and thickness of the reservoir. In other words, large scale subsidence results from special conditions such as large pressure declines in shallow, thick, large, highly compressible reservoirs. Production practices that include artificial pressure maintenance by gas drive or water injection counteract the reservoir compacting that would otherwise occur.

 

The preceding indicates that oil reservoirs produced by pressure depletion in loose sands or, and extremely large gas reservoirs in unconsolidated sands or friable rocks, are the types most susceptible to subsidence. This is indeed what Geertsma² and Martin and Serdengecti³ found to be true in their studies, leading to the conclusion that other reservoirs are not a real concern in regard to surface subsidence.

 

Production structures in Louisiana are basically from two geological time periods. North Louisiana, where nearly all of the shallow production occurs in the state, is much older geologically, with the deepest basement or base rock lying at about 25,000 feet.^7,11 The geological structures above this are relatively old, hard and consolidated, compared to South Louisiana. The deepest base rock in South Louisiana is at about 50,000 feet. The sediments and structures above this basement are much younger and less consolidated than in North Louisiana. On the other hand, there is little shallow oil and gas production in South Louisiana. The potential for surface subsidence is limited by the fact that generally •the large shallow gas reservoirs are in the stronger consolidated structures of North Louisiana, and •production by pressure depletion in less consolidated structures is in the South Louisiana reservoirs at the much greater depths.

 

Martin and Serdengecti3 in their extensive analysis of data from Louisiana, correlated the characteristics of reservoirs in Louisiana, including:

 

• About 10% of reservoirs are shallower than 1 km (kilometer) or (3281 feet).
• About 78% are deeper than 2 km (6562 feet).
• Almost half are deeper than 3 km (9843 feet).
• Approximately 85% of the reservoirs are less than 10 meters (32.8 feet) thick.
• Louisiana reservoirs have very small thickness to diameter ratios; less than 0.01 for 90% of the reservoirs.
• Approximately 60% of the reservoirs have depth from surface to reservoir diameter ratios of greater than 1.0.

 

From these and other reservoir characteristics, they applied experimental data and correlations to make the following generalizations on the maximum possible subsidence that could normally be expected to occur:

 

• Most normally pressured reservoirs should have a maximum possible subsidence of less than 0.02 meters (0.8 inches).
• A number of deeper Louisiana reservoirs were initially highly over pressured, resulting in a pressure drop in the reservoir from production of twice that of a normally pressured reservoir. For these the maximum possible subsidence should be less than 0.04 meters (1.6 inches).
• Many oil and gas fields consist of a vertical sequence of reservoirs. Subsidence calculations must include the effects of each reservoir.
• Surface subsidence from OGE can be avoided by injecting fluids to maintain pressure. Subsidence in strong water drive reservoirs can be avoided by limiting reservoir withdrawal to allow sufficient water influx to maintain reservoir pressure.

 

Cited References

¹Boesch, Donald F., Douglas Levin, Dag Nummedal, and Kevin Bowles, Subsidence in Coastal Louisiana: Causes, Rates, and Effects on Wetlands, FWS/OBS-83/26, performed by Louisiana Universities Marine Consortium and Louisiana State University Department of Geology for National Coastal Ecosystems Team, Division of Biological Services, U.S. Fish and Wildlife Service: Washington, D.C.; August 1983.

 

²Geertsma, J., Land Subsidence above Compacting Oil and Gas Reservoirs, Journal of Petroleum Technology; June 1973.

 

³Martin, J.C. and S. Serdengecti, Subsidence over Oil and Gas Fields, Reviews in Engineering Geology, Vol. VI, Geological Society of America; 1984, pp. 23-34.

 

⁴Mitigating Losses from Land Subsidence in the United States, Panel on Land Subsidence, National Research Council, National Academy Press: Washington, D.C., 1991, pp. 1-2.

 

⁵Sharp, J.M., Jr. and D.W. Hill, Land Subsidence along the Northeastern Texas Gulf Coast: Effects of Deep Hydrocarbon Production, Environmental Geology, Vol. 25; 1995, pp. 181-191.

 

Individuals Contacted (October 1996)

⁶Boudreaux, F.J. (Phil), Petroleum Engineer Administrator, Engineering Division, Office of Conservation, Louisiana Department of Energy and Natural Resources, Baton Rouge.

 

⁷Broussard, Jim, Engineer and District Manager, Shreveport District, Office of Conservation, Louisiana Department of Energy and Natural Resources, Shreveport, LA.

 

⁸Ghalambor, Ali, Professor of Petroleum Engineering, University of Southwestern Louisiana, Lafayette, LA (Contacted by Billy P. Ramagost).

 

⁹John, Chacko J., Director, Basin Research Institute, Louisiana State University, Baton Rouge.

 

¹⁰Kohler, Joel, Petroleum Engineer, Office of Conservation, Louisiana Department of Energy and Natural Resources, Baton Rouge.

 

¹¹Kumar, M.B., Geologist Supervisor, Geological Division, Office of Conservation, Louisiana Department of Energy and Natural Resources, Baton Rouge.

 

¹²Marsalis, William E., State Geologist and Director, Louisiana Geological Survey, Louisiana Department of Natural Resource, Baton Rouge.

 

¹³McGough, Charles, Engineer and District Manager, Monroe District, Office of Conservation, Louisiana Department of Energy and Natural Resources, Monroe, LA.

 

¹⁴Ramagost, Billy P., Petroleum Engineer, Technology Assessment Division, Louisiana Department of Energy and Natural Resources, Baton Rouge.

November 4, 1996

To: Jack Caldwell, Secretary of Natural Resources

From: Mike French, Director, Technology Assessment Division

Subject: Subsidence from Oil and Gas Production in Louisiana