Month: August 2009

Production-weighted decline rate of oil and gas fields is somewhat different, especially since 1985, compared to the average decline rate. The reason for this change is the introduction of new technologies, most notably horizontal drilling and fracturing techniques, in many major fields. Using new technologies, it was possible to halt the decline in many giants and keep production stable for some time. Eventually the average and the production-weighted declines must follow each other. The currently stable production-weighted decline cannot be expected to continue far into the future, once technology enhanced fields reach the final onset of decline. A limit for the average decline rate of the giant oil fields occur when all the fields in the population have reached the onset of decline. In other words the average decline rate cannot increase monotonously, but will sooner or later reach a limit.

The evolution of decline rates of oil fields over past decades includes the impact of new technologies and production techniques and clearly shows that the average decline rate for individual giant fields is increasing with time. These factors have significant implications for the future, since the most important world oil production base – giant fields – will decline more rapidly in the future. In roughly mid 2004, total world oil production ceased to expand. Instead, new production has only succeeded in keeping world oil production relatively flat. An analysis by Cambridge Energy Research Associates estimated that the weighted decline of production from all existing world oil fields was roughly 4.5%. Andrew Gould, CEO of Schlumberger, stated that an accurate average decline rate is hard to estimate, but an overall figure of 8% is not an unreasonable assumption. The International Energy Agency (IEA) came to the conclusion in 2008 that the average production-weighted decline rate worldwide was 6.7% for post-peak fields which means that the overall decline rate would be less, since many fields are not yet in decline. Modeling future field behavior is done by extrapolating the historical production data with an exponential decline curve. This does not take dramatic deviations into account and assumes that declines will continue approximately exponentially. This leads to a somewhat optimistic extrapolation. The decline rate of a field is affected by introduction of new technology, investments, changes in strategies and other factors affecting production. Decline curves can be made much more detailed and complicated (e.g. hyperbolic). The decline of smaller fields is equal to or greater than those of the giants. A detailed study of fields showed that giants declined at an average of 13%, while the small fields, condensate, and NGL declined at 20% or more. A small field requires fewer wells to fully develop; hence it is more easily depleted. A large field requires many more wells, often widely separated, so it is typically depleted more slowly. High depletion rates, which are common in small fields, have been shown to strongly correlate with high decline rates.

The end of the plateau phase of a field is the point where production enters the decline phase. Production profiles of giant fields generally have a long plateau phase, rather than the sharp peak often seen in smaller fields. Higher decline rates must be applied to giant fields that enter decline in the future as prolonged plateau levels and increased depletion made possible by new and improved technology result in a generally higher decline rates. Indeed, although technology can extend the plateau phase, it is at the expense of more pronounced declines in later years. Since a large number of important giants are subject to enhanced production methods, such as waterflooding, gas injection, fracturing or other measures, it is reasonable to expect relatively higher declines after those fields depart their plateau phase

Temperature at the bottom of the well while flowing

The temperature at the bottom of a borehole occurring at the time of the activity being performed on the well.

The temperature at the wellhead occurring at the time of the activity being performed on the well

Temperature at the Wellhead with well flowing.

Conventional oil fields refer to reservoirs that dominantly allow oil to be recovered as a free-flowing dark to light-coloured liquid. Heavier crude oils that require special production methods are excluded.

Giant oil fields are the world’s largest. There are two ways to define a giant oil field. One is based on ultimately recoverable resources (URR), and the second is based on maximum oil production level. The URR definition considers giants to have more than 0.5 Gb of ultimately recoverable resources. The production definition assumes a production of more than 100,000 barrels per day (b/d) for more than one year. IEA classify super-giants as fields with more than 5 Gb of initial 2P reserves. Giants contain more than 500 million barrels of initial 2P reserves, and large fields contain initial reserves of more than 100 million barrels. The most important contributors to the world’s total oil production are the giant oil fields. Their contribution to world oil production was over 60% in 2005, with the 20 largest fields alone responsible for nearly 25% . Giant fields represent roughly 65% of the global ultimate recoverable conventional oil resources. The overall production from giant fields is declining, because a majority of the largest giant fields are over 50 years old, and fewer new giants have been discovered since the decade of the 1960s. About 500 or about 1% of the total number of world oil fields are classified as giants. Thus, with few exceptions, e.g., Ghawar, the contribution from a single field is generally small compared to the total.