DOWNHOLE TOOL

When an oil well has been completed it is necessary to monitor downhole the rate at which the well's products are flowing along the borehole and up to the surface. One type of method and apparatus previously proposed for this purpose in wells with a horizontal borehole section, where the fluid is stratified into distinct component layers, involves the use of tracer or maker materials; employing an injector/detector tool a suitable material is injected into the fluid, and its arrival time at a downstream detector station is noted, giving the flow velocity by a distance-over-time type calculation. The problem, however, is to get the tracer/marker material, in the form of a suitably soluble (or miscible) composition, into the well fluid in such a way that it ends up in the desired component layer; unfortunately, merely introducing the composition randomly into the fluid somewhere across the borehole is not satisfactory, for if injected into the "wrong" layer it is in practice difficult for the tracer composition to migrate to its intended target. The invention seeks to solve this problem by proposing the use of an injection apparatus that comprises a plurality of spaced ejection ports (22lp, 22r) (from which the relevant tracer composition can be ejected so as to be injected directly into the relevant chosen component layer), together with orientation means (15) whereby in use the orientation of the tool can be so adjusted that the ports are so disposed as concurrently to lie each within the appropriate layer. Each port is connectable to a source of the relevant tracer composition; most conveniently the source is the combination of a reservoir (13l, 13r) and a syringe-like device (12l, 12r) which can draw a suitable amount of the composition from the reservoir and then drive it to, and eject it from, the associated port into the chosen layer. Lorsqu'un puits de pétrole est achevé, il est nécessaire de surveiller en fond de trou le débit auquel les produits provenant du puits s'écoulent dans le trou de forage jusqu'à la surface. Un type de procédé et d'appareil précédemment employé à cet effet dans les puits comportant une partie de trou de forage horizontale dans laquelle le fluide est stratifié en couches distinctes de constituants, consiste à utiliser des substances de détection ou de marquage. A l'aide d'un outil injecteur/détecteur, une substance appropriée est injectée dans le fluide et le temps qu'elle met pour arriver à une station de détection en aval