Land Subsidence Above Compacting Oil and Gas Reservoirs

Abstract

Notable subsidence above producing oil and gas fields is the exception rather than the rule. A simple procedure is outlined to single out the exceptional but real problem areas. This exercise in potential-problem analysis shows that the huge Groningen gas field in The Netherlands is a candidate for causing subsidence troubles in a lowland area. Introduction During the last 20 years, the Royal Dutch/Shell Group has conducted extensive investigations into the phenomenon of reservoir compaction and subsidence. These have included research projects to study subsidence above Bolivar Coast oil reservoirs in Venezuela and to examine the huge Groningen gas reservoir in The Netherlands. The latter investigation was conducted by a team of specialists from both the Koninklijke/Shell Exploratie en Produktie Laboratorium (KSEPL) and BV Nederlandse Aardolie Maatschappij (NAM), the latter being the producing company owned jointly by Shell and Esso. Details of the Groningen investigation are published elsewhere but as it may have consequences for other operating companies working in lowland and other subsidence-prone areas, we shall consider here the causes of subsidence above hydrocarbon-producing reservoirs in a more general way, and review the state of the art of its prediction. A simple method will be presented for estimating the order of magnitude of both compaction and the accompanying subsidence. Application of this method, which can be used to explore the need for an investigation in depth, requires hardly any specialist knowledge. The objective is twofold: to demonstrate that land subsidence due to hydrocarbon production seldom leads to serious subsidence, and production seldom leads to serious subsidence, and to pinpoint the few potential problem areas. Earlier Field Observations The literature on subsidence deals mainly with a few notable examples, such as the Goose Creek oil and gas field in Harris County, Tex., where dramatic subsidence occurred between 1918 and 1925, and the Wilmington field below Long Beach, Calif., where almost 10 m of subsidence was experienced in 1960. Further subsidence could be avoided in this latter case after unitization and pressure maintenance as a result of water injection. More recently, a search for additional, documented surface depressions over oil and gas fields in the U. S. was reported by Yerkes and Castle. This search revealed only a few other significant cases, mainly fields close to Wilmington, such as those at Buena Vista, Huntington Beach, and Inglewood. From this concentration of subsidence bowls, it may be inferred that such events are somehow related to a similarity in reservoir conditions. Shell has been confronted only once with a major land-subsidence problem. It is related to the production of oil and gas in Venezuela, where subsidence above a number of important oil reservoirs bordering Lake Maracaibo is a constant phenomenon, and huge dykes have been built to protect the coastal area from flooding. Its cause is discussed by Van der Knaap and Van der Vlis. Subsidence data for oil and gas fields outside the Americas are very scarce indeed. Okumara and Hirono describe a case from the Niigata district of Japan that results from the production of methane dissolved in water. In Italy, AGIP has been accused of contributing to subsidence in the Po Delta by producing from a number of gas fields. JPT P. 734