Oil and gas fields usually have a lifespan of between 15 and 30 years, from when the first oil is extracted until they are abandoned. Production can last 50 years or more in the case of larger reservoirs. However, deep-sea deposits are only exploited for 5 to 10 years due to very high extraction costs.
Preparatory phase
During the exploration phase, oil and gas companies locate and evaluate the hydrocarbons contained in a reservoir. Before production can begin, they must design a development plan. This involves:
-Calculate the profitability of the field to determine if future oil and gas sales will be sufficient to cover the cost of production for the entire useful life of the field.
-Determine the number of wells to be drilled and select the most suitable facilities for each stage of production. The studies at this point focus on what extraction methods should be used and how the products that are brought to the surface will be processed, temporarily stored and shipped.
-Define the reservoir's production profile to forecast annual production volumes from the start of production to abandonment.
-Next, the company must prepare the production site by leveling the ground, clearing the trees, building roads to transport the equipment and building technical facilities and housing for future site workers. In offshore locations, it will be necessary to build the platform or transport it to the production site.
The different stages of a reservoir's life cycle
The life cycle of oil and gas fields can be divided into three stages:
-Start (two to three years). During this period, production increases gradually as more and more wells are drilled.
-Plateau production, when production stabilizes. This stage also lasts two to three years, or sometimes longer in the case of larger reservoirs.
-Decline, during which production falls at a rate of 1% to 10% per year. When production ends, large amounts of oil and gas remain underground. Therefore, oil and gas companies are constantly seeking to improve recovery rates using enhanced recovery techniques. Oil field recovery rates range from 5% to 50%. The rate is higher (60% to 80%) for fields that produce only natural gas, since their lower density and higher flow make production more efficient.
The ups and downs of production
In oil and gas production, things don't always go as planned. Some fields will produce 10% to 20% more oil or gas than expected, while others may produce much less than originally estimated.
There are many reasons for this unpredictability. Oil and gas fields contain wastewater, which is pumped into the well along with hydrocarbons. Over time, there may be more water and less oil or gas. The cost of extracting and separating water can result in a loss-making operation. In addition, in some fields, extracted natural gas is not intended for sale. However, gas production in these fields can sometimes increase, meaning that less oil is produced.
The global economic climate can also affect the life cycle of oil and gas fields. For example, if oil prices fall over a long period of time, companies may decide to leave an oil field ahead of schedule. Conversely, if oil prices rise, production can continue for longer.
All of these factors affect profitability and, in some cases, force companies to abandon production early, with the risk of losing almost all of their significant initial investment. To reduce this risk, engineers perform periodic evaluations throughout the life cycle of a reservoir.
