Abstract

One of the critical properties for a SAGD project is the steam quality at the injection point in the wellbore. Steam quality is critical because, in order for a SAGD process to work, a steam vapour chamber must be formed in the reservoir so that oil has a volume to drain into that is less dense than itself. Latent heat of condensation at the vapour chamber walls is the prime mechanism of heat delivery to the reservoir.

Unfortunately. there is no practical way to directly measure steam quality in a downhole environment. The problem is thatfor a pure-water system, at a given pressure, steam condenses at a fixed temperature as heat is lost. So if we monitor downhole temperatures, we cannot tell how much of our injection steam has condensed. This paper describes a simple. indirect technique that allows a calculation of downhole steam quality with the single downhole monitor of a thermocouple.

Introduction

Suppose we have a pipe that is conveying high pressure steam. Suppose also. for simplicity. that the pipe is frictionless so that the exit and entrance pressures are equal. At the pipe inlet, the temperature is T1 which is the saturated steam temperature at the pressure of the system.

As the steam moves down the pipe, heat is lost through the pipe walls to the environment surrounding the pipe. These heat losses are reflected in some of the steam condensing to water so that steam quality is reduced continuously as steam moves down the pipe. But at the outlet of the pipe the exit temperature. T2. is exactly the same as the inlet because at a 'fixed pressure steam condenses at a single temperature.

In the real world there will be pressure drops down a pipe due to frictional losses. But this doesn't change the conclusion.

As long as there is some vapour steam at the pipe exit, measurement of outlet T2 and P2 conveys no information about exit steam quality. This is the problem with downhole steam quality measurements.

A SOLUTION

Suppose we have our same pipe as above but now we introduce a small amount of non-condensible gas (at steam conditions) .- ego C1, C2, C3, N2......etc. At the pipe inlet, assuming we have the same pressure as above, the inlet temperature will be reduced because steam saturation temperature is now determined by steam partial pressure lower than total vapour pressure. The outlet temperature will not be the same as the inlet temperature because steam condensation will now follow a dew point curve when mixed with non-eondensible gas.

In this case (and in the case of pressure drops down the pipe), it turns out that measurement of outlet temperature is sufficient to determine outlet steam quality if we know (or can calculate) inlet steam quality, inlet T1 and P1 and outlet P2. This determination is independent of the flow rate or concentration of the noncondensible gas, for small amounts.

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