Bestemmelse av Fluidinnstr°mming og utstr°mming

摘要

When drilling a borehole for an oil well it is desirable to know when fluid is passing through the walls of the borehole between the borehole itself and the formation through which the borehole is passing. It has now been discovered that useful information about the influx or efflux of fluid can be gained by observing the pressure within the borehole in the region of the drilling bit, this observation being carried out not when the drill is actually operating but when it is still (and the pipe string to which it is mounted is also still) and the drilling fluid is not being pumped; the basis for this is that the drilling fluid is thixotropic, and when not disturbed will form a gel which is capable of quite accurately transmitting the force created by moving fluids at the interface between the borehole and the earth formation being drilled through - that is inflowing or outflowing fluids - displacing the drilling fluid along the borehole, and with an efficiency far greater than previously recognized. It is this which is the present invention - a method of, and apparatus for, determining fluid inflow or outflow during drilling, by using a gelling drilling fluid whose characteristics - yield stress τy and gelation period tg - are known, and then, while all drilling and pumping is ceased, measuring downhole differential pressure ΔP and using the observed changes therein to allow a determination of the fluid flow.