Primary & Secondary Recovery

Hydrocarbon production can be divided into several phases, depending on the methods used. Some of these methods and technologies fall under Class II UIC regulations.

Primary Oil Recovery

In most conventional hydrocarbon reservoirs, either the expansion of natural gas at the top of the reservoir or the formation pressure forces hydrocarbons into the well and up to the surface. Valves at the surface control the flow as the fluids are prepared for storage and sale. Pump jacks or other forms of equipment used to create artificial lift are considered primary oil recovery.

Pumpjacks create artificial lift during primary recovery.

Secondary Oil Recovery

As production continues, formation pressure falls under the level required to force hydrocarbons to the surface. Operators can add fluids to the reservoir to increase the formation pressure and thereby extract more hydrocarbons. The most common secondary recovery techniques are gas injection and waterflooding. The secondary recovery stage reaches its limit when the injected fluid (water or gas) is produced in considerable amounts from the production wells and the production is no longer economical. The successive use of primary recovery and secondary recovery in an oil reservoir produces about 15% to 40% of the original oil in place. 

THUMS Islands Case Study

California’s headline-making 1921 oil discovery at Signal Hill launched a drilling boom. So many derricks sprouted it became known as “Porcupine Hill.” By the early 1930s, the massive Wilmington oilfield extended through the surrounding city of Long Beach.1

Signal Hill covered in derricks, California
Aerial view of Signal Hill’s oil field in 1930. A sea of oil wells almost cover the city, which is completely surrounded by the city of Long Beach.

Although Californians had experience dealing with groundwater induced subsidence and the building damage it caused, by 1951 Long Beach was sinking at the alarming rate of about two feet each year.2 

Location of THUMS Islands near Long Beach

During the 1950’s, experts showed that water injection would repressure the oil-bearing units, stop underground compaction as well as surface subsidence, and increase oil recovery.

By the time the THUMS Islands were built in 1965 to tap into the East Wilmington Oil Field, the community of Long Beach had sunk 30 ft.2 The islands were operated by THUMS, a consortium named after the parent companies involved: Texaco, Humble, Union Oil, Mobil and Shell.

The rim of the islands were made of boulders from Catalina Island, and the islands were then filled with dredged material from the bay. In 1967 the islands were named Grissom, White, Chaffee and Freemen in honor of lost NASA astronauts.3

Island Grissom, one of the THUMS Islands

Landscaping, a waterfall and tall structures conceal drilling rigs.

A peak of 148,495 BOPD were produced in 1969. By 1992, the pumping volume was 44,444 BOPD through the water injection method of oil recovery, producing low-grade crude oil. The one-billionth barrel of oil was produced in 2011. As with many water flood projects, water production overtook oil production: water cut was 20% in 1965, and by 1994 it was 92%.3  To counter subsidence, water injection pumps were established to offset extracted petroleum, sustain reservoir pressures and extend oil recovery.2 


1. American Oil & Gas Historical Society. (n.d.). Signal Hill boom. Retrieved 11/9/2020 from

2. American Oil & Gas Historical Society. (n.d.). THUMS-California hidden oil islands. Retrieved 11/9/2020 from

3. THUMS Islands. (n.d.). In Wikipedia. Retrieved 11/10/2020 from

Images: “Oil pumps” by huyangshu via Shutterstock; “Pumpjack pumping crude oil” by Anan Kaewkhammul via Shutterstock; “Signal Hill, California” by © Spence, Public domain, via Wikimedia Commons; “Location of THUMS Islands near Long Beach” by OpenStreetMap contributors, CC BY-SA 2.0, via Wikimedia Commons; “Island Grissom, one of the THUMS Islands” by USDOE, Public domain, via Wikimedia Commons