Recovery Efficiency
Authors: van Everdingen A.F. , Kriss Hyla Swesnik 
Published: December 2007

Abstract
For the past 50 years, data on recovery efficiency have been collected and organized on the basis of discovery year; in reviewing these data, we note that the efficiency has declined during the last three decades. The modern petroleum industry was born in the U.S.A. In this country, all tools are readily available; many ideas on recovery efficiency have originated here and have been put into practice with results reported. Therefore, we fail to understand why the average efficiency is 30 percent when the best of the fields has a recovery of close to 90 percent. We could argue that the data for the last 10 years are still incomplete; this argument might explain the low figures for recent years. However, no such argument can be advanced for the reservoirs found 10 years ago or earlier. Moreover, the decrease in efficiency occurred during a period in which the usefulness of Schlumberger logs came to be fully recognized, in which the flow of reservoir fluids was better understood, and in which powerful computers became available and were used extensively for a rapid analysis of any situation.
Possible Explanations Possible Explanations The decrease in efficiency might be explained by:We were apt to consider formations as homogeneous when in fact they were not.We have been unable to properly assess the effect of layering overwide distances.We had an incomplete understanding of and explanation for the skin effect.We have failed to use pressure and production data effectively.We have not kept reservoir pressures at the desired level, giving the Jamin effect an opportunity to develop.We have, in many instances, used well spacing that is too wide.

Introduction
1937 saw the first report on the reserves in the U.S.A., "Proved Reserves of Crude Oil, Natural Gas Liquids and Natural Gas," prepared under the auspices of the API and AGA; the report subsequently was issued yearly. Throughout World War II and most of the postwar years, the reviews contained only two categories: "Changes in Proved Reserves Due to Extensions and Revisions" and "Proved Reserves in New Pools." These categories were unchanged until 1966 when the API's work with respect to proved reserves was expanded to include the development of estimates for proved reserves was expanded to include the development of estimates for crude oil as follows:original oil in place and ultimate recovery categorized bygeologic age of reservoir rockb. reservoir lithologyc. type of entrapment;indicated additional reserves from cased-off reservoirs and from future installation of fluid injection projects in known fields;allocations back to year of discovery ofcurrent estimates of ultimate recoverycurrent estimates of original oil in place;reserves and production data by subdivision for the states of California, Louisiana, New Mexico, and Texas;crude oil productive capacity in the United States.
The report format has not been revised since 1966.
During each year, the existing data on any state or portion thereof are reviewed by a small group of engineers and geologists. This group also prepares the first estimates for new discoveries and extensions of oil prepares the first estimates for new discoveries and extensions of oil fields. The findings of all the groups (totaling some 120 people) are discussed in a special meeting of the reserves committee each spring. The data agreed upon are assembled by the API office in Washington, D.C.