The elimination of redundant computations and the moving of invariant computations out of loops are often done separately, with invariants moved outward loop by loop. We propose to do both at once and to move each expression directly to the entrance of the outermost loop in which it is invariant. This is done by solving a more general problem, i.e. the elimination of computations performed twice on a given execution path. Such computations are termed partially redundant. Moreover, the algorithm does not require any graphical information or restrictions on the shape of the program graph. Testing this algorithm has shown that its execution cost is nearly linear with the size of the program, and that it leads to a smaller optimizer that requires less execution time.

Summary A non-linear groundwater model has been developed as an aid in the management of the Chalk aquifer in the Upper Thames Basin. The area of investigation includes Stage I of the Thames Groundwater Scheme, and the model has been used to assess the performance of the aquifer during the severe drought of 1975-76 when river flow was augmented by pumping groundwater into the river. Results indicate that the main loss of water during this period was caused by stream-bed leakage and evaporation from the riparian zone, but that these losses were relatively small. The aquifer is shown to have considerable potential for further development as a major source of water, provided that use is made of denser networks of production boreholes located in the middle and lower slopes of the valleys in the area of Chalk outcrop.

A method is proposed for the simulation of groundwater flow in regional aquifers dissected by dykes of low permeability or by geological faults, which act as barriers to flow and which can have a considerable influence on the available groundwater resources. The method has been verified by a comparison of numerical results with the analytical solution to a test problem. The method has been applied to an area of the Cave Sandstone aquifer in Botswana which has been affected by dyke intrusions which significantly affect its potential for large-scale groundwater development. Etablissement de modèles numériques pour l'écoulement souterrain dans des aquifères régionaux cloisonnes par des dykes RESUMEOn propose une méthode pour la simulation de l'écoulement souterrain dans des aquifères régionaux cloisonnés par des dykes de faible perméabilité ou par des failles géologiques qui se comportent comme des barrières, vis à vis de l'écoulement et qui peuvent avoir une influence considérable sur le volume des ressources en eaux souterraines disponsibles. La méthode a été verifée par la comparaison des résultats numériques avec ceux de la solution analytique à un problème de test. La méthode a été appliquée à une zone d'aquifères situés dans des grès caverneux dans le Botswana. Cette zone a été affectée par des intrusions de dykes qui ont eu une influence significative sur les possibilités d'y réaliser un aménagement à grande échelle des eaux souterraines.

Automatic methods used for the calibration of regional groundwater models are notoriously sensitive to inadequacies in hydrological data used to determine the groundwater regime. The situation is often aggravated by errors introduced by manual interpolation to give estimates of piezometric head between observation boreholes, particularly in areas where these are sparse or locally nonexistent. In this paper a new technique is proposed for the automatic interpolation of groundwater level data that are scattered thinly over a regional aquifer system and the simultaneous determination of transmissivity and storage coefficient distributions. The method makes full use of all available information, and it is consistent with the two‐dimensional groundwater flow equations. Results are given to illustrate the success achieved by the method in a practical application.

Heather field, currently producing about 5,000 bopd, is one of the most mature North Sea oilfields being produced from a large fixed platform. Production from Heather field over much of its life has been marginally economic as revenue and operating costs are finely balanced. Over the last five years a change of Operator of the field, and simplified ownership, has energised the management of the asset. The new Operator has brought new investment in the field, the platform and exploration of neighbouring satellites. The extra production from infield drilling and well intervention activity has prolonged field life and increased oil reserves. It has also deferred the eventual decommissioning of the Heather facilities. This paper describes the Operator's approach to managing the asset, and its rationale for the investment in four new platform wells, an active well intervention campaign, and exploration of the greater Heather area. The acquisition of new seismic data has improved the reservoir description of the field and satellites, leading to appraisal drilling of the West Heather area. It reviews the successes and failures of these projects, and describes the main lessons learned, which may be applicable to other fields in a similar economic situation. The principal factors affecting the company's willingness to invest have been its view of the overall economic outlook, the performance of existing wells and the ability to maximise production from them, the potential of new infield drilling targets to achieve rapid payback, and most importantly, the maintenance of a safe low-cost offshore and onshore operation. Contrary to popular belief the uncertainty in predicting future production from mature fields such as Heather is as great, or greater than predicting future production in younger assets. The gains to be had from mature assets such as Heather are significant enough to attract the interest of specialist producers. Given favourable tax regimes further investment in other mature fields could yield significant quantities of oil and gas which would otherwise be lost because of premature decommissioning. Introduction Heather field has been on production since 1978, and has produced only 127 MMbo out of an original oil in place of 488 MMbo. The field is situated in the Brent province the British sector of the Northern North Sea. Current production is about 5,000 bopd (Figure 1), and the maximum ever produced was about 36,500 bopd yearly average when on plateau. The low recovery factor, 26 percent to date, has been due to a combination of compartmentalisation of the reservoir by faulting and diagenesis, heterogeneous reservoir quality, and by low porosity and permeability leading to low well productivity.