Subsidence is the sinking or gradual lowering of the earth's surface. It is found worldwide in a variety of environments on land and the seafloor. Subsidence can result from either natural geologic and/or man-made causes. Natural geologic causes are basin-downwarp, fault movement, sediment compaction, and relaxation of deep earth stresses. Man-made causes include groundwater pumping, mining, oil and gas production, river channelization, and surface loading. A subsided area can vary in size from a few acres to thousands of square miles. Elevation losses can be from a fraction of an inch to tens of feet. Damage can range from minor land elevation loss to costly infrastructure disruption and long-lasting environment damage.
Long Beach was once known as the "Sinking City". Oil and gas production from the giant Wilmington Oil Field, where 3.75 billion barrels (42 gallons per barrel) have been produced, created a land surface "subsidence bowl" of up to 29 feet deep in and around the Port of Long Beach (Port) and along the coastal strand of the City of Long Beach. Over 20 square miles have been affected adjacent to the shoreline from the Port to Seal Beach. Early 1940's groundwater pumping contributed to the land sinking, but the majority of the subsidence resulted from oil and gas extraction. Damage to public and private property and the rebuilding of Port facilities have cost billions in today's dollars.
Although subsidence has been arrested, constant monitoring and control by the Long Beach Oil and Gas Department is ongoing and will continue into the future. Stable land surfaces are critical for continued regional economic growth that cannot be jeopardized by the effects of oil and gas production.
Approximate Extent of the Subsidence Bowl
Subsidence, the sinking of the land surface, began in the 1940's with the pumping of under ground water at Terminal Island Naval Shipyard. The area sank more than four feet by 1945, far more than attributed to groundwater withdrawal. In 1951, the rate of subsidence exceeded two feet per year. By 1958, the affected area was 20 square miles and extended beyond the Harbor District. Total subsidence reached 29 feet in the center of the "Subsidence Bowl". The ocean inundated wharves, rail lines and pipelines were warped or sheared, while buildings and streets were cracked and displaced. Ninety-five oil wells were severely damaged or sheared off by underground slippage. Oil, gas and water production caused pressure losses and the weight of the overburden compacted the oil sands. The surface sank in response to this underground compaction.
Harbor Wharf Flooded
Results from Harbor Area Subsidence
In the 1950's, Oil Properties showed that water injection (water flooding) would repressure the oil formations, stop the underground compaction as well as surface subsidence, and increase oil recovery. In order to conduct effective coordinated water flood operations, the various fault blocks needed to be "unitized". In 1958, after the State passed the California Subsidence Act, the City and numerous private owners began creating four Fault Block Units. Each Fault Block Unit was operated by one owner and the revenues and expenses were shared on proportion to each participant's ownership percentage. By 1966, subsidence had stabilized and in some areas, later rebounded (rose) by up to 2 feet. Damage and remediation costs reached an estimated $100 million.
Many oil experts had recognized for years that the oil pools extended eastward under the City and offshore to Seal Beach. Development was not started in this area until 1965 because of drilling restrictions placed by the City in order to protect against subsidence. After it became apparent that water injection was stopping subsidence and the easterly oil field extension could be developed safely, the City (by vote of the people) lifted the drilling ban. A lesson learned, the citizens of Long Beach required that the eastern offshore extension of the field be unitized prior to development and water injection be started immediately. At the same time, restrictions were placed on the development to ensure that subsidence would not occur and that the natural beauty of the shoreline would be protected.
Island Water Injection Schematic
By a California Legislative vote, a complicated formula for division of oil revenue was worked out. Under the Act, the City maintains control as Unit Operator and receives a small portion of the revenue, while the major portion goes to the State of California. All operations are conducted in the new development by the field contractor, THUMS Long Beach Company formed by combining five major oil companies. These operations are closely controlled by the City to ensure that subsidence caused by oil extraction will not occur again.
Four ten-acre drilling island were built offshore and landscaped in such a manner as to enhance the beauty of the shoreline. The beautification of these drilling islands is a prime example of how industrial development can be made compatible to any area when properly supervised.