Van Der Knaap Research Articles

Land Subsidence Above Compacting Oil and Gas Reservoirs

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

Pore-Volume Compressibility of Consolidated, Friable, and Unconsolidated Reservoir Rocks Under Hydrostatic Loading

The pore-volume compressibilities and porosities presented here were derived from 256 samples of sandstone and limestone representing 40 reservoirs. These and previously published data are in poor agreement with compressibility-porosity correlations in the literature. The salient conclusion is that to evaluate rock compressibility for a given reservoir it is necessary to measure compressibility in the laboratory. Introduction The use of pore-volume compressibility-porosity correlations in engineering calculations is well known. The correlations developed by Hall for both sandstones and limestones have been widely distributed. Van der Knaap published a similar correlation using limestone samples from a single well and also correlated the data with net pressure. Such correlations are attractive because of the simple relationship established. However, those correlations were intended only for well consolidated samples; correlations for friable or unconsolidated samples have not been published. This study compares our laboratory data with the published correlations of consolidated samples as published correlations of consolidated samples as well as with values for friable and unconsolidated sandstones. Compressibility values are presented for 256 rock samples from 40 reservoirs - 197 samples from 29 sandstone reservoirs and 59 samples from 11 limestone reservoirs. Porosities ranged from less than 1 percent to 35 percent. Compressibility values from the literature for 79 samples are added, including Hall's and Van der Knaap's. The Experiments Sampling To obtain a representative sample of a formation for testing, one must avoid grain rearrangement. This problem is unlikely to occur with consolidated problem is unlikely to occur with consolidated samples or friable samples containing some cementation, although the effect of removing the overburden is still unknown. Unconsolidated samples, on the other hand, present a much more complex problem, in that grain rearrangement is very likely during either coring or subsequent handling. The advent of the rubber-sleeve core barrel much improved the chances of obtaining, representative samples. We have some evidence that, if carefully handled. rubber-sleeve cores will provide reasonably undisturbed samples. However, even if the sand is captured undisturbed in the rubber sleeve, internal gas can expand the core during the trip to the surface. The history of all the samples used in this study is not complete, but most of the unconsolidated samples were obtained from rubber-sleeve cores. Preparing the Samples Preparing the Samples The consolidated and friable samples used in this study were generally plugs 1 in. in diameter and 3 in. long, and their condition ranged from well preserved to dry and weathered. The core plugs were extracted in solvent to remove water and hydrocarbons, put into a flexible jacket, and saturated with a refined oil. The unconsolidated samples of about the same dimensions were generally cored from rubber-sleeve cores that had been frozen in liquid nitrogen and for which liquid nitrogen had been used as a drilling fluid. The frozen samples were placed in a Teflon sleeve and allowed to thaw. End plates and screens were then placed on the ends of the samples. JPT P. 129