Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Earth science
Reexamination Certificate
2003-05-29
2004-10-05
McElheny, Jr., Donald (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Earth science
Reexamination Certificate
active
06801857
ABSTRACT:
The present invention relates to methods and apparatuses for determining the reserve quality of an oil well delivering a given fluid coming from a productive layer, by measuring the response R of the well.
Oil is produced from wells made in oil fields, and such oil fields are to be found in various different types of ground and subsoil which do not all have the same production quality. This quality is represented essentially by the productivity index IP of the well which depends on the radius of the well r
w
, the drainage radius R
e
of the well, the viscosity &mgr; of the recoverable oil, and also the transmissivity of the productive layer which is defined as the product of its permeability k multiplied by its height h, and possibly also on any clogging of the pores in the rock in the vicinity of the wall of the well which is quantified by a dimensionless parameter S commonly referred to by the person skilled in the art under the generic term “skin”. This productivity index is given by the following formula:
IP
=
2
π
k
h
μ
[
ln
(
R
e
r
w
)
-
0.75
+
S
]
where ln represents the natural logarithm.
The productivity index IP is a direct measure of the ease with which oil can flow into the well under the effect of a drop &Dgr;P in the mean pressure of the deposit around the well, since the flow rate Q of the well as measured in downhole conditions is then equal merely to:
Q=IP.&Dgr;
This downhole flow rate is then evacuated to the surface using means that are known in themselves.
In order to optimize production from a well, in particular an oil well, it is therefore useful to know its reserve quality, in particular by determining the values of certain defined parameters. Referring to the expression for the productivity index IP given above, a first important parameter is the permeability k of the productive layer of the subsoil in which the well has been drilled, and another is the “skin” S which quantifies possible damage to the productive layer. It is thus possible to establish two classes of well from which production is low: wells that are maintained under ideal operating conditions (S=0) but which are taking oil from rock that has low permeability; and wells drilled in deposits presenting high permeability, but which have become clogged (S>0) and which could produce more after being restored by using techniques that are themselves known.
It is therefore important to be able to detect the formation of a layer of clogging in order to take action as soon as possible in effective manner to eliminate that layer and to continue working the well.
Various methods have been developed for monitoring the production quality of a well. Most of the old methods are based on using empirical or statistical relationships between various measurements that can be performed on such a well. Another method giving results that are more accurate consists in completely closing the outlet of the well and in studying the rise in the pressure of the oil in the well as a function of closure time, where examination of curves plotting variation in said pressure makes it possible to deduce whether the well is in its ideal state or whether it is clogged.
That method makes it possible to obtain good results, but it presents the major drawback of being lengthy to implement. In order to obtain a curve that is useful, it is necessary to wait for several hours, or even several days with some wells, during which time the well is not in use, thus constituting certain loss of production, to which there needs to be added the cost of restarting when the pressure of the deposit is no longer sufficient for the well to remain eruptive.
To mitigate that drawback, attempts have been made to develop another method which consists in modulating closure of the well at its outlet and in studying the variation in the pressure of the fluid as a function of such modulation. That method eliminates the above-mentioned drawback of total closure of the well, but presents the drawback of leading to measurements that are not sufficiently accurate.
For example, another method is described in U.S. Pat. No. 3,559,476 and FR-A-2 678 679. It consists in modulating the flow rate of the fluid in the well by means of a sinewave function and in measuring the variations in the flow rate and the pressure of the fluid, from which the response R of the well is deduced in certain special cases.
That method gives results that are relatively satisfactory when the damage to the well consists in its wall being clogged so as to give a positive “skin” value, and providing the “skin” has thickness that can be assumed to be zero. Clearly that type of infinitely thin “skin” is merely a convenient mathematical abstraction which is often satisfactory, but other types of damage can exist which correspond to a positive value for the “skin” but for which the thickness of the “skin” cannot be taken as being zero, or which correspond to a negative value for the “skin”, for example when a well is connected to a network of natural cracks that are open or when a well is being stimulated by hydraulic fracturing, i.e. has an artificially induced fracture passing through it, which fracture is generally symmetrical relative to the axis of the well.
An object of the present invention is thus to mitigate the above-mentioned drawbacks of prior methods of evaluating the reserve quality of an oil well or the like, and to provide a method which, while being easy to implement, nevertheless enables such evaluation to be performed at all levels of the well and regardless of the type of damage to the productive layer, by means of measurements which can be interpreted with a low error percentage of uncertainty.
Another object of the present invention is to provide apparatus enabling the method to be implemented.
More precisely, the present invention provides a method of determining the reserve quality of an oil well or the like delivering a given fluid coming from a productive layer, by measuring the response R of the well, said method consisting in modulating the flow rate of the fluid in the well by means of a sinewave function, and in measuring the variations in the flow rate and the pressures of the fluid, the method being characterized by the fact that:
I) the response Rc of the well when said productive layer includes a damaged zone presenting a positive “skin” value S for “skin” of non-zero thickness is obtained by the equation:
R
c
=
D
R
0
(
β
z
w
)
-
B
-
C
R
0
(
β
z
w
)
+
A
,
and that
II) the response Rf of the well when the productive layer has a fracture presenting a negative “skin” value S is obtained by the equation:
R
f
=
π
F
CD
i
Z
f
2
E
fD
+
2
i
Z
f
F
CD
+
S
wf
in which equations:
A, B, C, and D are functions of the parameters z
w
, &agr;, and &bgr;, as defined below, and are respectively defined by the following four equations:
A
(
z
w
,
α
,
β
)
=
i
z
w
α
ⅇ
ⅈ
π
4
[
kelbe
0
(
β
z
w
α
)
kelke
1
(
z
w
α
)
-
kelbe
1
(
z
w
α
)
kelke
0
(
β
z
w
α
)
]
B
(
z
w
,
α
,
β
)
=
1
α
[
kelbe
0
(
z
w
α
)
kelke
0
(
β
z
w
α
)
-
kelbe
0
(
β
z
w
α
)
kelke
0
(
z
w
α
)
]
C
(
z
w
,
α
,
β
)
=
i
β
z
w
2
[
kelbe
1
(
z
w
α
)
kelbe
1
(
β
z
w
α
)
-
kelbe
1
(
β
z
w
α
)
kelke
1
(
z
w
α
)
]
D
(
z
w
,
α
,
β
)
=
i
β
z
w
α
ⅇ
ⅈ
π
4
[
kelbe
0
(
z
w
α
)
kelke
1
(
β
z
w
α
)
-
kelbe
1
(
β
z
w
α
)
kelke
0
(
z
w
α
)
]
it being specified that in the equations given above:
kelke
n
(
x
)=
ker
Despax Damien
Dovis Robert
Fedele Jean-Michel
Martin Jean-Pierre
Innov-Pro
McElheny Jr. Donald
LandOfFree
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