3. 3
• Formations assumed to
get stronger deeper
• Measured by formation
tests
– leak off test
– formation integrity test
• Usually measured at the
shoe….Why?
• because it’s weakest
point in the wellbore
because it carries the
weight of CSG
What is fracture pressure?
Fractures and MAASP
Fractures and MAASP
4. 4
• Leak Off Tests
Formation Integrity
Tests
PSI PSI
Fractures and MAASP
6. 6
ACCURATE FORMATION STRENGTH
(LEAK OFF TEST)
1-DRILL OUT THE CASING SHOE AND APPROXIMATELY
15 FEET IN NEW FORMATION (RAT HOLE).
3-CIRCULATE TO GET UNIFORM MUD IN THE HOLE
MUT IN = MUD OUT.
2-TEST SURFACE LINES.
4-EXACT VERTICAL DEPTH (TVD) OF CASING SHOE.
6-ACCURATE PRESSURE GAUGE.
5-USE CEMENT PUMP.
7-RECORD MUD VOLUME PUMPED TILL LEAK-OFF
STARTS.
8. 8
HOW IS FORMATION STRENGTH
MEASURED?
1/ LEAK OFF TEST (L.O.T)
Volume Pumped
Pump
Pressure
FRACTURE PRESSURE
= MUD psi + L.O.T. psi
9. 9
HOW IS FORMATION STRENGTH
MEASURED?
1/ LEAK OFF TEST (L.O.T)
Volume Pumped
Pump
Pressure
FRACTURE PRESSURE
= MUD psi + L.O.T. psi
10. 10
HOW IS FORMATION STRENGTH
MEASURED?
1/ LEAK OFF TEST (L.O.T)
Volume Pumped
Pump
Pressure
FRACTURE PRESSURE
= MUD psi + L.O.T. psi
11. 11
HOW IS FORMATION STRENGTH
MEASURED?
1/ LEAK OFF TEST (L.O.T)
Volume Pumped
Pump
Pressure TREND LINE
FRACTURE PRESSURE
= MUD psi + L.O.T. psi
12. 12
HOW IS FORMATION STRENGTH
MEASURED?
1/ LEAK OFF TEST (L.O.T)
Volume Pumped
Pump
Pressure TREND LINE
FRACTURE PRESSURE
= MUD psi + L.O.T. psi
13. 13
HOW IS FORMATION STRENGTH
MEASURED?
1/ LEAK OFF TEST (L.O.T)
Volume Pumped
Pump
Pressure TREND LINE
FRACTURE PRESSURE
= MUD psi + L.O.T. psi
14. 14
HOW IS FORMATION STRENGTH
MEASURED?
1/ LEAK OFF TEST (L.O.T)
Volume Pumped
Pump
Pressure TREND LINE
FRACTURE PRESSURE
= MUD psi + L.O.T. psi
15. 15
HOW IS FORMATION STRENGTH
MEASURED?
1/ LEAK OFF TEST (L.O.T)
Volume Pumped
Pump
Pressure TREND LINE
FRACTURE PRESSURE
= MUD psi + L.O.T. psi
FRACTURE PRESSURE
= MUD psi + L.O.T. psi
16. 16
HOW IS FORMATION STRENGTH
MEASURED?
1/ LEAK OFF TEST (L.O.T)
Volume Pumped
Pump
Pressure Point of Leak Off TREND LINE
FRACTURE PRESSURE
= MUD psi + L.O.T. psi
17. 17
HOW IS FORMATION STRENGTH
MEASURED?
1/ LEAK OFF TEST (L.O.T)
Volume Pumped
Pump
Pressure Point of Leak Off
Leak Off
Pressure
TREND LINE
FRACTURE PRESSURE
= MUD psi + L.O.T. psi
18. 18
HOW IS FORMATION STRENGTH
MEASURED?
1/ LEAK OFF TEST (L.O.T)
Volume Pumped
Pump
Pressure Point of Leak Off
Leak Off
Pressure
FRACTURE PRESSURE
= MUD psi + L.O.T. psi
TREND LINE
19. 19
PFRAC = (MWHOLE x TVDSHOE x0.052) + PLOT
MWMAX = (PLOT TVDSHOE 0.052) + MWHOLE
Maximum mud weight
Fractures and MAASP
20. 20
Example:
• HP=2500psi….P(LOT)=500 psi
• So Fracture Pressure=3000 psi
Max Mwt.
=(2500/(0.052x5000))+(500/(0.052x5000))
=( 9.615ppg )+( 1.923ppg )
= 11.53 ppg
Or
=3000/(0.052x5000)=11.53 ppg
Maximum mud weight
21. 21
• Maximum Allowable Annular Surface Pressure
• Definition
– the surface pressure, above mud hydrostatic which will cause the
formation at the shoe to take mud (losses)
Example: Drill 10.000 ft. with 10ppg Mwt
HP=5200psi …….Fracture Pressure=6000psi
So the MAASP is 800 psi
Mwt equivalent=(800/(0.052x10000))=1.538ppg
Or 1.538 ppg as Equivalent MASSP
MAASP
22. 22
• MAASP versus LOT
– just after the LOT, the LOT pressure is the MAASP
• As the mud weight increases, the MAASP will
decrease
• Ex. Well TVD is 5000ft.
F.P=3000psi ……Equivalent M.wt. =11.5ppg.
1-HP=2600psi …….Equivalent M.wt.=10ppg.
So MAASP = 400 psi or 1.5 ppg
After while…
2-HP=2700psi………......Equivalent Mw.t=10.38 ppg.
So MAASP= 300psi….Equivalent Mw.t=1.2 ppg.
MAASP
23. 23
Calculation:
MAASP= Fracture Pressure – Hydrostatic Pressure
MAASP=(0.052 X MW max X TVD shoe) - (0.052 x Mw hole x TVD shoe)
MAASP = (MWMAX - MWHOLE) x TVDSHOE x 0.052
MAASP
New MAASP = (MWMAX – Kill Mud Weight) x TVDSHOE x 0.052
OR
MAASPAfter Kill
Hinweis der Redaktion
Primary Well Control is the correct use of mud hydrostatic i.e. a mud weight high enough to balance formation pressure, but not so high as to cause losses.
Fracture pressure is the pressure at which whole mud will be forced into the formation causing losses.
It is generally assumed that formations become stronger as we drill deeper, but as fracture pressure depends very much on lithology, this is certainly not always the case.
Formation Tests
In order to calculate the fracture pressure of the formation at the shoe, the maximum mud weight for the next hole section and the maximum allowable annular surface pressure (MAASP), a formation test must be carried out.
Two types of test are commonly used:
Leak-off test
- test the formation until it takes fluid and allows calculation of the
above.
Formation Integrity Test (Limit test)
- tests the formation only to a predetermined mud weight.
Leak-Off Tests
Purpose of Leak-Off Test
To measure the fracture pressure of the formation at the shoe and consequently:
Maximum allowable annular surface pressure (MAASP)
Maximum mud weight
Kick Tolerance
To test cement job
Procedure
Leak-off tests are normally carried out prior to drilling each new hole section.
The following procedures should be used:
1. Drill out float shoe, rathole and a further 5 - 15 feet of new hole.
2. Circulate and condition hole until the mud weight is uniform throughout.
3. Position bit just above casing shoe.
4. Rig up high pressure, low volume pump such as cement pump. Rig pumps are not suitable for performing leak-off tests and are not recommended.
5. Close B.O.P. (hang off string on floating rigs.) Where practicable, open annulus between last casing and previous casing strings to void pressure build -up.
Fracture Pressures and Maximum Mud Weights
From the L.O.T., both the fracture pressure of the formation (formation intake pressure), and the maximum mud weight for the next hole section can be calculated.
Fracture Pressure
The fracture pressure is the sum of the mud hydrostatic pressure at the shoe, plus the surface L.O.T. pressure.
PFRAC = (MWHOLE x TVDSHOE x 0.052) + PLOT
Where
PFRAC = formation fracture [psi]
MWHOLE = mud weight in use [ppg]
TVDSHOE = shoe vertical depth [ft]
PLOT = surface leak-off pressure [psi]
The fracture pressure can also be expressed as a gradient in psi/ft.
Maximum Allowable Annular Surface Pressure (MAASP)
Definition: the pressure at surface, which, on top of the mud hydrostatic, will cause the formation at the shoe to breakdown.
Calculation
The maximum allowable annular surface pressure can be calculated as follows:
MAASP = [(MWMAX - MWHOLE) x TVDSHOE x 0.052]
MAASP changes with each change in mud weight, decreasing as mud weight increases. Thus MAASP should be recalculated each time the mud weight changes.
