Reservoir monitoring

P. Herd, W. Phelps, D. 1. Gustafson, S. H. Jackson, R. L. Jones, M. H. Russell, M. J. Schocken, A Comparison of US EPA s Tier 1 and 2 Index Reservoir Model Estimates to Drinking Water Reservoir Monitoring Results in Selected US Systems in 1999/2000 , Poster presented at lU-PAC 2002, Book of Abstracts, Part 2, page 125. 

Nicolle, G., Boibien, C., ten Haven, H. L., Tege-LAAR, E. Ceiavagnac, P. 1997. Geochemistry a powerful tool for reservoir monitoring. Society of Petroleum Engineers, Paper No. 37804. 

If compaction occurs as a result of production careful monitoring is required. The Ekofisk Field in the Norwegian North Sea made headlines when, as a result of hydrocarbon production, the pores of the fine-grained carbonate reservoir collapsed and the platforms on the seabed started to sink. The situation was later remedied by inserting steel sections into the platform legs. Compaction effects are also an issue in the Groningen gas field in Holland where subsidence in the order of one meter is expected at the surface. 

A programme of monitoring the reservoir is carried out, in which me asurements are made and data are gathered. Figure 14.2 indicates some of the tools used to gather data, the information which they yield, and the way In which the information is fed back to update the models and then used to refine the ongoing reservoir development strategy. 

The reservoir model will usually be a computer based simulation model, such as the 3D model described in Section 8. As production continues, the monitoring programme generates a data base containing information on the performance of the field. The reservoir model is used to check whether the initial assumptions and description of the reservoir were correct. Where inconsistencies between the predicted and observed behaviour occur, the model is reviewed and adjusted until a new match (a so-called history match ) is achieved. The updated model is then used to predict future performance of the field, and as such is a very useful tool for generating production forecasts. In addition, the model is used to predict the outcome of alternative future development plans. The criterion used for selection is typically profitability (or any other stated objective of the operating company). 

Some specific examples of the use of data gathered while monitoring the reservoir will now be discussed. 

Monitoring the resenro/rpressure will also indicate whether the desired reservoir depletion policy is being achieved. For example, if the development plan was intended to maintain reservoir pressure at a chosen level by water injection, measurements of the pressure in key wells would show whether all areas are receiving the required pressure support. 

Obtaining of data concerning the chemical composition of water is critical significance for monitoring water reservoirs and forecasting the quality of drinking water from different water supply sources. A dry residue is commonly used with the methods AAS, ICP-AES, ICP-MS (analysis of liquid) widely applied for determination of water composition. So it is vital to create a standard sample of the composition of dry residue of ultra-fresh Lake Baikal water, its development launched since 1992 at the Institute of Geochemistry SB RAS. 

Lube oil level in the reservoir should be monitored by a sensing deviee to indieate low lube oil level. Loeal and panel-mounted pressure gauges are neeessary to monitor operation of the lube oil system and must be ineluded in the manufaeturer s seope of supply. The purehaser must distinguish between eontrol room instrumentation and instruments mounted on a stand-alone (loeal) panel. There is also a tendeney to plaee monitoring instruments on auxiliary equipment and piping. While this may eost less, it often eomplieates tlie operator s surveillanee tasks. 

Oil and Gas Production This sector is a major user of corrosion monitoring equipment, in particular for offshore fields where ramifications of corrosion and consequent maintenance are far more serious and costly compared with onshore production. Carbon steel is used for approximately 70-80 70 of production facilities. The development of a field is assessed on a defined corrosion risk which may not be correct, leading to serious corrosion. In addition, a reservoir may become more corrosive as the field is extracted owing to (a) increased water content, and (b) eventual souring of the field (hydrogen sulphide production). 

The structures used (platforms) require monitoring in addition to sub-sea pipelines, satellite wells and other equipment (e.g. manifolds) on the sea floor. Corrosion inhibitors are widely used in internal-streams (from the reservoir and many of the downstream operations). Corrosion monitoring can provide valuable data for assessing the effectiveness of the inhibitors used and for optimising dosage rates. 

Fig. 1 The tower of the eleventh century Romanesque church breaking the surface of Sau Reservoir (Spain). In 1963 the old town of Sant Roma de Sau and its church were flooded by the construction of the Sau Dam. However, water level never suffices to completely submerge the prominent tower, which became an unofficial monitor of the water supply available for the city of Barcelona. When the doors of the church emerge, drought is there. .. Photo by Joan Armengol...

Stored in reservoirs, and a growing social perception of problems related to water quantity and quality in reservoirs is expected. Just as an example, the recurrent drought episodes suffered by the Barcelona metropolitan area (Spain) in recent years placed reservoirs at the center-stage since then people are regularly informed about water resources stored in nearby reservoirs in newspapers and TV, and an iconic view from a reservoir has become a nonofficial monitor of the water supply volume available for the city (Fig. 1). 

While an exact number of site clusters has not been proposed, the authors consider from approximately 3 to 10 clusters of sites to be appropriate. These clusters should represent different ecoregions with different ecological characteristics as well as different loadings (both in amount and source) of mercury deposition. Care should be taken to monitor different types of water bodies and watersheds (e.g., seepage lakes, drainage lakes, old reservoirs, rivers, and estuaries). Areas that should be considered as potential cluster site locations include lakes in northern New England/the Adirondacks, lakes in the upper Midwest, rivers and streams in the southeastern coastal plain, lakes in south-central and southeastern Canada, western... 

---