Method for steam quality measurement

Steam quality in flow lines equipped with orifice meters may be determined by injecting a predetermined quantity of water into the flow line and measuring the orifice pressure differential with and without water injection. An approximation of the James correlation, which relates the orifice differential pressure, the total fluid mass flow rate and steam quality raised to the 1.5 power, may be used for conditions both before and during water injection together with an energy balance for conditions both before and during water injection to determine steam quality before water injection. A portable water injection unit may be temporarily connected to each flow line at a tap installed in the flow line upstream of the orifice. The effect of water injection for each flow condition may be prior confirmed by injecting a predetermined quantity of water downstream of the orifice based on an initial estimate of dry steam flow in the line.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention pertains to a method of measuring steam quality, 
particularly in flow lines to heavy oil production wells and the like, 
using a sharp-edged orifice and injection of a measured quantity of water 
into the flowstream passing through the orifice. 
2. Background 
So-called heavy oil production from subterranean reservoirs often involves 
the use of steam to stimulate movement of the viscous oil into the 
production wells. The steam is typically supplied to individual injection 
wells through a network of distribution pipes or "lines" from a central 
steam generation facility such as an electric cogeneration plant. 
Accordingly, the steam quality may vary somewhat in the complex 
distribution system and the individual supply lines leading to the 
respective injection wells. 
Various techniques have been proposed for measuring the quality of the 
steam flow to each of the injection wells. Control over steam quality is 
important to balance the amount of energy being injected into a particular 
formation in order to control oil production and the efficiency of the 
stimulation process. Techniques which have been developed for measuring 
steam quality include, for example, that disclosed in U.S. Pat. No. 
4,193,290 to Sustek, Jr., et al which describes an acoustic transducer 
associated with an orifice. Publications entitled "Kern River Field Test 
of a Steam Quality Measurement Technique" by C. L. Redus, et al (Society 
of Petroleum Engineers Publication No. SPE 17445, 1988) and "Steam Quality 
and Metering" by Thomas M. Wilson, The Journal of Canadian Petroleum 
Technology, April-June, 1976 also discuss methods for measuring steam 
quality utilizing a sharp-edged orifice or so-called orifice meter. 
One objective in improving methods for measuring steam quality is to 
utilize existing facilities, where possible. Many steam distribution 
systems for stimulating the production of heavy oil include, in the 
individual, relatively small-diameter steam flow lines to each well, an 
existing sharp-edged orifice together with means for measuring the 
pressure drop across such an orifice. With these existing systems in mind, 
it has been determined that it would be desirable to develop a relatively 
uncomplicated and inexpensive method and system for surveying relatively 
small-diameter steam flow lines to measure steam quality in these lines 
from time to time and utilize such information for better control of the 
stimulation of the particular heavy oil reservoir. 
The system and method described in the Sustek, Jr. patent requires the use 
of an acoustic transducer and an installation of an orifice whose acoustic 
signature is known from previous tests. Moreover, the technique described 
in the SPE publication requires installation of an orifice plate in series 
with a critical flow choke in each of the lines. This technique would 
require retrofitting of a substantial number of existing lines which 
incorporate only an orifice plate. Still further, the method described in 
the above-referenced article in the Journal of Canadian Petroleum 
Technology requires the use of flow meters, again, a technique which 
requires installation of flow meters and nozzles into already existing 
facilities. The disadvantages of these approaches are thus significant in 
existing, large and complex steam distribution networks. However, the 
method of the present invention substantially overcomes the disadvantages 
of prior art methods and systems. 
SUMMARY OF THE INVENTION 
The present invention provides a unique method for measuring steam quality, 
particularly in relatively small diameter flow lines which include 
sharp-edge orifice plates or similar orifice meters which determine fluid 
flow by measuring a pressure drop thereacross. The present invention, in 
particular, provides an improved a method for determining steam quality in 
the individual distribution lines of a steam distribution network for 
stimulating the production of heavy oil from subterranean reservoirs. 
In accordance with one important aspect of the present invention a method 
for measuring steam quality in a flow line is provided wherein a 
correlation between steam quality, a pressure drop across an orifice, and 
the mass flow of fluid through the orifice is used for two flow 
conditions. In particular, the use of an approximation of a mathematical 
correlation developed by R. James ("Metering of Steam-Water Two-Phase Flow 
by Sharp Edged Orifices", Proc. Inst. Mech. Engrs., Vol. 180, No. 23, pp. 
549-566 (1965)) together with the injection of a measured quantity of 
water into the steam flow line and the measurement of the pressure drop 
across the existing orifice plate, both with and without the measured 
water injection, is utilized to determine steam quality. 
The method of the present invention also provides for determining the 
quality of steam flow utilizing an approximation of the James correlation 
for two measured pressure differentials across an orifice, one with a 
measured quantity of water injection and one without the introduction of a 
measured quantity of water into the flow line, together with the 
calculation of an energy balance from known conditions of steam 
temperature, injected water temperature, specific heat of injected water 
and the latent heat of vaporization of steam. Accordingly, the quality of 
steam flowing through a flow line may be determined by measurements taken 
both before and after injection of a known quantity of water upstream of 
an orifice plate, utilizing the James correlation and an energy balance 
equation, whereby plural equations may be solved simultaneously to 
determine the steam quality. 
In accordance with the method of the present invention, steam distribution 
lines which include existing orifice meters may be measured to determine 
the quality of steam flow by introduction of a relatively small amount of 
water using equipment which is portable and inexpensive and does not 
require interruption of steam flow through the line being tested. 
Moreover, existing steam flow lines may be easily modified by merely 
adding small branch conduits or "taps" to the lines upstream and 
downstream of the existing orifice plate without disassembly of the line 
or without interrupting the steam flow within the line. 
Those skilled in the art will further appreciate the advantages and 
superior features of the invention together with other important aspects 
thereof upon reading the detailed description which follows in conjunction 
with the drawing.

DESCRIPTION OF PREFERRED EMBODIMENTS 
In the drawing, FIG. 1 is a schematic diagram in which conventional 
components are shown in schematic or diagrammatic form and in certain ones 
of the equations unit conversion constants may be omitted, all in the 
interest of clarity and conciseness. 
Referring to FIG. 1, there is illustrated an exemplary arrangement 
according to the present invention for injecting steam into a steam 
injection well for stimulating the production of heavy oil from a 
subterranean reservoir. The injection well is indicated by the numeral 10 
and is connected to a steam injection flow line or conduit 12 which, in 
turn, is operably connected to a source of steam such as a generator 14. 
The generator 14 may also distribute steam to a network which includes 
other flow lines 15, 16 and 17 leading to similar injection wells, not 
shown. Each of the relatively small-diameter steam injection lines or 
conduits 12, 15, 16 and 17 are adapted to include a conventional 
sharp-edged orifice plate 18, one shown for the line 12, interposed 
therein, together with suitable means 20 for measuring a fluid pressure 
differential across the orifice plate. The pressure differential 
measurement means 20 may comprise a conventional manometer or pressure 
differential gauge of any suitable configuration. 
In accordance with the present invention, the individual flow lines, such 
as the flow line 12, leading to injection wells are modified to include 
small branch conduits 22 and 24 which may be connected to the flow line 
using conventional "hot tapping" procedures upstream and downstream of the 
orifice plate 18, respectively. The locations of the branch conduits or 
taps 22 and 24 may each be a linear distance on the order of 50 to 100 
times the nominal diameter of the conduit 12 from the position of the 
orifice plate 18, respectively. 
The schematic diagram of FIG. 1 indicates that pressure and temperature 
measurements may be taken between the tap 22 and the orifice plate 18 in 
the conduit 12 and between the tap 24 and the well 10, respectively. These 
pressure and temperature measurements may be obtained by conventional 
pressure and temperature measuring means. Moreover, the taps or branch 
conduits 22 and 24 may also be fitted with quick disconnect couplings for 
connection to a water supply conduit 26 which is part of a water supply 
unit generally designated by the numeral 28. The water supply unit 28 may 
include a source of water of known temperature and suitable purity such as 
a tank 30 which is in communication with a pump 32 operably connected to 
the conduit 26. The pump 32 is capable of delivering a measured amount or 
flow rate of water through the conduit 26 into the conduit 12 by way of 
either one of the branch conduits or taps 22 or 24. The temperature of the 
water being injected into the conduit 12 from the unit 28 may also be 
measured as indicated in the diagram of FIG. 1. The amount or flow rate of 
water injected into the conduit 12 may be determined from a suitable flow 
measuring device 33 or by a known displacement of the pump 32. 
One important advantage of the method of the present invention is that only 
one pump and flow meter unit 28 is required to test any number of flow 
lines since the unit may be moved from line to line and connected to each 
line at branch conduits or taps upstream and downstream of the orifice 
plate disposed in each line. As will be understood from the following 
description it may, in many instances, not be necessary to install the 
unit 28 for operation at the downstream tap, such as the tap 24 in FIG. 1. 
However, to improve the accuracy of the measurement technique, it is 
considered desirable to test the conditions in the steam flow line, in 
each case, by pumping water into the line at the downstream connection or 
tap 24 before the method is carried out to determine steam quality. 
The instant method utilizes the above-mentioned correlation established by 
James which may be approximated by the use of certain equations to be 
described below. The method according to the present invention for 
determining quality of steam flow in the conduit 12, for example, uses an 
approximation of the James correlation which is as follows: 
EQU H=W(W/K)X.sup.1.5 (a) 
Wherein 
H=the orifice pressure reading taken, for example, from the pressure 
differential gauge means 20 in inches of water, 
W=the steam flow in pounds per hour, 
K=the saturation temperature of steam at the measured pressure conditions 
upstream of the orifice plate 18 in .degree.F., and 
X=the steam quality. 
Steam quality is defined as the ratio of steam flow to total mass flow. An 
estimation of the flow rate of steam is made assuming the flow is dry 
steam (X=1). The saturation temperature (K) at the pressure and 
temperature conditions upstream of the orifice 18 is obtained from steam 
tables (Keenan, Keyes, et al, "Steam Tables", 1969). Accordingly, an 
estimate of dry steam flow may be obtained from equation (a). 
The unit 28 is then connected at the downstream tap 24 and a measured 
quantity-or flow rate of water is injected into the conduit 12 at about 
one-half or somewhat less of the flow rate of the above-mentioned estimate 
of dry steam flow. After a suitable period of time to reach steady-state 
conditions (about 10 minutes) the orifice differential pressure is noted 
at the gauge means 20. If the pressure differential reading does not vary 
by more than about five percent (5%) from the previous reading at the 
assumed dry steam conditions, then the original pressure differential 
reading is used for the further calculations in accordance with the method 
of the invention. If the pressure differential reading with water 
injection at downstream tap 24 changes by more than about five percent 
(5%) then the pressure differential value read with the downstream 
injection is used for further calculations in accordance with the method 
of the invention. 
After testing the influence, or lack thereof, of the water injection at the 
downstream tap 24, the unit 28 is moved to connect its discharge line 26 
to the upstream tap 22. Water is then injected into the conduit 12 at the 
above-mentioned rate for a like period of time to achieve steady-state 
conditions and an orifice differential reading is taken from the gauge 
means 20 which now reflects the effect of the water flow. The orifice 
pressure differential reading, together with pressure and temperature 
readings of the fluid flow in conduit 12 upstream of the orifice 18 and a 
reading of the temperature of the injection water is then used in a series 
of calculations as set forth hereinbelow. The calculations assume that a 
constant pressure mixing process occurs with little heat loss in the 
conduit 12. Steam quality may be determined from the following set of 
equations wherein equations (b) and (c) are approximations of the James 
correlation from equation (a), for the respective conditions noted. 
EQU H.sub.1 =W.sub.1 (W.sub.1 /K)X.sub.1.sup.1.5 (b) 
Wherein H.sub.1 is the orifice differential reading prior to injection of 
water into the tap 22, W.sub.1 is the mass flow rate in pounds per hour 
prior to injection of water into the conduit 12, K is the saturation 
temperature of steam at the conditions in the conduit 12 upstream of the 
orifice 18, and X is the steam quality. 
EQU H.sub.2 =W.sub.2 (W.sub.2 /K)X.sub.2.sup.1.5 (c) 
Equation (c) is for the conditions with water injection into the line 12. 
EQU W.sub.2 =W.sub.1 +Y (d) 
wherein Y is the water injection rate in pounds per hour. 
Moreover, an energy balance may be computed from the following equation: 
EQU Z=W.sub.1 X.sub.1 -W.sub.2 X.sub.2 (e) 
wherein Z is the amount of steam which condenses to raise the injected 
water to the fluid flow temperature in line 12. This quantity may be 
computed from the following equation: 
EQU Z=c.sub.p (T.sub.s -T.sub.y)Y/.lambda..sub.s (f) 
Wherein c.sub.p equals the specific heat or "heat capacity" of water, which 
is taken at the temperature and pressure of the flow in the conduit 12 
upstream of the orifice 18, T.sub.s is the steam temperature upstream of 
the orifice 18 prior to water injection, T.sub.y is the temperature of the 
injected water, Y is the injection water flow rate in pounds per hour and 
.lambda..sub.s is the latent heat of vaporization of steam at the pressure 
in the conduit 12 upstream of the orifice 18 prior to water injection, 
which may be determined from steam tables. 
Accordingly, the amount of steam which condenses (Z) may be determined 
based on the water injection conditions from equation (f) and substituted 
in Equation (e). 
The above set of equations (b) through (e) represent four equations and 
four unknowns, the unknowns being the steam quality under the initial 
conditions and during water injection (X.sub.1, X.sub.2), respectively, 
and the total fluid mass flow rates under both conditions (W.sub.1, 
W.sub.2). The steam quality and mass flow rate under one condition can be 
expressed in terms of the steam quality and mass flow rate under the other 
condition and Equation (b) may be solved for the steam quality, X.sub.1. 
The computations may, of course, be carried out by machine processes 
including the computation to determine Z. 
From the foregoing, it will be appreciated that a relatively uncomplicated 
procedure may be carried out for determining the quality of steam flow in 
lines containing orifice meters wherein the quality measurement may be 
taken over a relatively short period of time without interrupting steam 
flow to the end use such as a steam injection well, without adversely 
affecting the steam flow to the end use during the measurement period and 
without the use of expensive equipment or processes which comprise 
permanent installations in each of the lines. The unit 28 may be, as 
mentioned previously, be moved from one steam flow line to another to 
carry out the measurement process and the steam flow through each line may 
then be adjusted to balance the energy being input to a particular 
reservoir based on the quality of steam flow. The size of the unit 28 may 
require a pump capacity no greater than about 10 gallons per minute and 
the pump connection taps 22 and 24 may be relatively small diameter, 
one-half inch nominal pipe size, for example, and may be installed in 
steam flow lines without interrupting steam flow using conventional "hot 
tapping" techniques for installing branch conduits in flow lines operating 
under flow conditions. 
Referring briefly to FIG. 2, there is illustrated a diagram which confirms 
the efficacy of the method of the invention wherein steam quality versus 
the orifice differential pressure, in inches of water, is plotted for a 
typical field condition wherein the initial differential pressure across 
the orifice 18 was assumed to be 100 inches of water, the steam rate was 
424 barrels (42 gallons) per standard day of cold water equivalent and the 
water injection rate was carried out at a rate of 200 barrels per standard 
day. Nominal steam pressure in the line 12 under the measurement 
conditions was assumed to be 400 psig. 
Although a preferred embodiment of the present invention has been described 
in detail herein, those skilled in the art will recognize that certain 
substitutions and modifications may be made to the method without 
departing from the scope and spirit of the invention recited in the 
appended claims.