Chapter 6_Gas Well Performance
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Transcript of Chapter 6_Gas Well Performance
1
GAS FIELD ENGINEERING
Gas Well Performance
2
CONTENTS
6.1 Gas Well Performance
6.2 Static Bottom-hole Pressure(static BHP)
6.3 Flowing Bottom-hole Pressure(flowing BHP)
3
LESSON LEARNING OUTCOME
At the end of the session, students should be able to:
• Determine static bottom-hole pressure(static BHP) using
different methods
• Determine flowing bottom-hole pressure(flowing BHP) using
different methods
4
Gas Well Performance
Figure (6.1) Gas Production Schematic
5
Gas Well Performance
• Referring to Fig.(6.1), ability of a gas reservoir to produce for a given set of reservoir conditions depends directly on the flowing bottom-hole pressure, Pwf.
• The ability of reservoir to deliver a certain quantity of gas depends on
• the inflow performance relationship
• flowing bottom-hole pressure
• Flowing bottom-hole pressure depends on
• Separator pressure
• Configuration of the piping system
6
Gas Well Performance
• These conditions can be expressed as:
(8.1)
(8.2)
7 Figure (6.2) Deliverability test plot
•The static or flowing pressure at the formation must be
known in order to predict the productivity or absolute open flow
potential of gas wells.
•Preferred method is a bottom-hole pressure gauge (down-
hole pressure gauge).
•However, Static BHP or Flowing BHP can be estimated from
wellhead data (gas specific gravity, well head pressure, well head
temperature, formation temperature, and well depth.)
Static and Flowing Bottom-Hole Pressures
8
Basic Energy Equation
In the case of steady-state flow, energy balance can be
expressed as follows:
OR
(8.3)
(8.4)
9
Basic Energy Equation
Figure (6.3) Flow in pipe (After Aziz.)
10
Basic Energy Equation
• Second term ( ) kinetic energy is neglected in pipeline flow
calculations.
• If no mechanical work is done on the gas (compression) or by
the gas (expansion through a turbine), the term ws is zero.
• Reduced form of the mechanical energy equation may be
written as:
OR
cg
udu
2
(8.5)
(8.6)
11
Basic Energy Equation
•All equations now in use for gas flow and static head
calculations are various forms of this Equation.
•The density of a gas( )at a point in a vertical pipe at pressure
p and temperature T may be written as:
(8.7)
g
12
Fig.(6.4) Compressibility
factor for natural gases
for Ppr (0 to 10)
13
Fig.(6.5) Compressibility
factor for natural gases
for Ppr (9 to 20)
14 Fig.(6.6) Moody Friction Factor Chart
15
Basic Energy Equation
• The velocity of gas flow ug at a cross section of a vertical pipe
is
(8.8)
16
Basic Energy Equation
• General vertical flow equation assuming a constant average
temperature in the interval of interest is
(8.10)
17
Basic Energy Equation
•Sukker & Cornell and Poettmann assumed gas deviation
factor varies with pressure. But accurate in relatively shallow
wells.
•A more realistic approach is that of Cullender & Smith.
•They treated gas deviation factor as a function of both
temperature and pressure.
18
Static Bottom-Hole Pressure
Average Temperature and Deviation Factor Method
The Equation is:
(8.20)
19
Example (1)
Calculate the static bottom-hole pressure of a gas well having a
depth of 5790 ft. The gas gravity is 0.60 and the pressure at the
wellhead is 2300 psia. The average temperature of the flow
string is 117oF.
Solution
20
First trial
Second trial
21
QUIZZ 4
1. Calculate the static bottom-hole pressure of a gas
well having a depth of 8570 ft. The gas gravity is 0.63
and the pressure at the wellhead is 2800 psia. The
average temperature of the flow string is 124oF.Use
average Temperature and Deviation factor method.
Pc=672,Tc=358
2. Calculate the static bottom-hole pressure of a gas
well having a depth of 9230 ft. The gas gravity is 0.66
and the pressure at the wellhead is 3100 psia. The
average temperature of the flow string is 119oF. .Use
average Temperature and Deviation factor method.
Pc=672, Tc=358
22
Cullender and Smith Method
This is a more realistic approach that gas deviation factor is a
function of both temperature and pressure.
Define
(8.25)
(8.26)
23
Cullender and Smith Method
Which, for the static case, reduces to
For the upper half,
For the lower half,
Static bottom-hole pressure at depth Z in the well is finally given by
Where Its is evaluated at H = 0, Ims at Z/2 and Iws at Z.
(8.30)
(8.31)
(8.32)
(8.27)
(8.29)
24
Cullender and Smith Method Calculation procedure
First: to solve for an intermediate temperature and pressure
condition at the mid point of the vertical column;
Second: Repeat the calculations for bottom-hole condition.
-A value of Its is first calculated from Eqn 8.27 at surface
conditions.
-Then, Ims is assumed(Its=Ims at first approximation) and pms is
calculated for the mid point conditions.
-Using this value of Ims , a new value of Ims is computed.
-The new value of Ims is then used to recalculate pms .
-This procedure is repeated until successive calculations of pms
are within the desired accuracy (usually within 1 psi difference).
25
Cullender and Smith Method
-The Cullender and Smith method is the
most accurate method for calculating
bottom-hole pressures.
-This method is generally applicable to
shallow and deep wells, sour gases, and
digital computations.
26
Example (2)
Calculate the static bottom-hole pressure for the gas well of
Example 1 using the Cullender and Smith method.
Solution
(a) Determine the value of z at wellhead conditions and compute
Its.
depth of the well=5790 ft.,
gas gravity = 0.60
pressure at the wellhead = 2300 psia.
Temperature at well head=74oF
Average temperature of flow string=117°F
Ppc =672psia
Tpc=358°R
27
Example (2)
Solution
(a) Determine the value of z at wellhead conditions and compute
Its.
28
(b) Calculate Its for intermediate conditions at a depth of 5790/2 or
2895 ft, assuming a straight line temperature gradient. As a
first approximation, assume
Ims = Its = 178
Then, from Eqn 8.30,
(8.30)
(8.27)
(8.30)
29
(c) Calculate Iws at bottom-hole conditions assuming, for the first
trial, Iws = Ims = 191. Then, from Eqn 8.31,
Since the two values of Pms are not equal, calculations are
repeated with Pms=2477 psia.
This is a check of the pressure at 2895 ft.
30
Repeating the calculation,
(d) Finally, using Eqn 8.32,
31
QUIZZ
E
1. Calculate static bottom-hole pressure
by using the same data given in exercise.
Take tubing head pressure to be 3340
psia.
32
Flowing Bottom-Hole Pressure
Flowing bottom-hole pressure of a gas well is the sum of the
flowing wellhead pressure, the pressure exerted by the weight
of the gas column, the kinetic energy change, and the energy
losses resulting from friction.
As kinetic energy change is very small, it is assumed zero.
For the situation of no heat loss from gas to surroundings and
no work performed by the system.
This equation is the basis for all methods of calculating flowing
bottom-hole pressures from wellhead observations.
The only assumptions made so far are single-phase gas flow
and negligible kinetic energy change.
(8.33)
33
Assumptions in the average temperature and average gas
deviation factor method are:
1. Steady-state flow
2. Single-phase gas flow, although it may be used for condensate
flow if proper adjustments are made in the flow rate, gas
gravity and Z-factor
3. Change in kinetic energy is small and may be neglected
4. Constant temperature at some average value
5. Constant gas deviation factor at some average value
6. Constant friction factor over the length of the conduit
Average Temperature And Average
Gas Deviation Factor Method
34
(8.39)
Equation for Average Temperature and Deviation
Factor method is
35
(8.40)
If Fanning friction factor is used, use the
following equation.
36
Equation 8.39 is to be applied when
Moody Friction factor is used.
Equation 8.40 is to be applied when
Fanning Friction factor is used.
Moody friction factor= 4* Fanning friction factor
37
Example (3)
Calculate the sandface pressure of a flowing gas well from the
following surface measurements: Use Average temperature and
Deviation Factor method.
Solution
Using Eqn 8.39,
38
First trial Guess, Pwf = 2500 psia
At 1.0 atm and 121.5oF.
Viscosity at average pressure:
39
The Reynold’s number is given by
From the Moody friction factor chart,
OR
Pwf = 2543 psia
40
Second trial
There is no appreciable change in z for this trial; so, first trial is
sufficiently accurate.
41
QUIZZ QUIZZ 5
1. Calculate the sand-face pressure of a flowing gas well
from the following surface measurements:
q= 12 MMscfd D=4 in.
γg = 0.62 Depth = 8400 ft. (bottom of casing)
Twf = 160°F Ttf = 83°F
Ptf = 2755 psia e= 0.0006 in
viscosity at average pressure= 0.0167 cp
length of tubing= 8350 ft. Pc= 672 Tc=358
42
Q & A
43
Thank You