Alireza Agharazi Microseismic

1. A Geomechanical Study of Refracturing
based on Microseismic Observations
Alireza Agharazi, Geomechanics Engineer

2. Microseismic Response to Refracturing
 Significantly different from microseismic
response to an initial fracturing treatment
 Why?
 Practical implications?
 How to improve refracturing efficiency?

3. Microseismic response: refracturing vs fracturing
 Concentration of events towards heel
Eagle Ford
Initial Treatment
~ 30% Lateral coverage
~ 25% Lateral coverage
~ 20% Lateral coverage
- Pumping Stages: 18
- Pumping Hours: 40 hrs
- Pumping Stages: 16
- Pumping Hours: 32 hrs
- Pumping Stages: 25
- Pumping Hours: 60 hrs

4.  Delayed Microseismic Response
Microseismic response: refracturing vs fracturing
0
10
20
30
40
50
60
70
80
90
100
0 20 40 60 80 100 120 140
MSEventCount(%)
Injected slurry volume (1000 bbl)
Well A
Well B
Well C
47,000 bbl
37,000 bbl
21,000 bbl

5.  Increase of event frequency with pump time
Microseismic response: refracturing vs fracturing
Refracturing
Fracturing

6. Refracturing numerical model
 6,400 ft lateral with 30 old perfs
 12-stage pumping
 Injection rate: 90 bbl/min
 90 min pumping / 30 min break

7. Heel
Well pressure profile - simulated refracturing
Microseismic event distribution: Well pressure profile
 Pressure contrast between the heel and toe
ΔP=3000 psi
 Depends on:
 Fracking fluid viscosity
 Pumping rate
 Casing ID
 Lateral length

8. Microseismic event distribution: Diverters efficiency
Perforation
discharge profiles
(with Diverter)
Toe
Toe
Perforation
discharge profiles
(without Diverter)
Heel Toe
With Diverter
Synthetic
Microseismic
Heel Toe
No Diverter
Synthetic
Microseismic

9. Delayed microseismic response
 New perforations: intact rock
perf. pressure = FBP
 Old perforations: pre-existing fractures
perf. Pressure ≥ FCP
Fracture Initiation Pressure ≈ FBP
Fracture re-opening pressure = FCP
HF Re-opening
HF initiation
Refrack ISIP
Initial ISIP

10. Pumping time-dependent microseismicity
 Fracture propagation: Perf. discharge rate > leak-off rate
 Fracking Pump rate:
Q=60-90 bpm
Qi=Q/n  Qi<< qLo
 No fracture propagation
 Re-fracking Pump rate:
Q=60-90 bpm
(Qi≈ 1.2 - 5.5 bpm)
 New hydraulic fracture initiation and propagation? Unlikely!

11. Pumping time-dependent microseismic
 Stimulation mechanism: Pressure-driven stimulation of natural
fractures and weakness planes
 Depends on natural reservoir stress anisotropy, as the main driving force
 Requires increase of reservoir pressure by injection
 Consistent with field observations:
 Delayed microseismic response to
pumping
 Increase of event count per stage as
pumping continues

12. Practical Implications
 The microseismic observations and the findings of this
study suggest that:
1. Diverters are not efficient in many refracturing jobs, resulting in
limited re-stimulation of lateral on the heel side
2. New transverse hydraulic fractures are unlikely to develop, hence
new perforations can be skipped in most cases
3. Dominant stimulation mechanism is pressure-driven stimulation of
natural fractures.
 How can these findings help to design a more efficient
refracturing?

13. Efficient Refracturing
Before refracturing:
 Pre-existing network of fractures
 Lost conductivity
Efficient refracturing
1. Add reserve
2. Restore conductivity
3. Maximize lateral coverage
4. Cost efficient
Heel Toe
Heel Toe
Coverage=100%
Inefficient refracturing
1. Accelerate reserve
2. Partial lateral coverageHeel Toe
Coverage<30%
IP
> 50%IP
< 50%IP

14. Efficient Refracturing Design
1. Adding reserve (vs accelerating reserve) by creating new contact area
 New hydraulic fractures from new perforations
 Stimulating natural fractures to enhance effective complexity
2. Restoring pre-existing fracture network conductivity
 Placing proppant in pre-existing fractures,
 Not too early to block access to fresh rock
3. Maximizing stimulation coverage along lateral
 Efficient diverters
 Mechanical isolation (expandable liner, etc)
 Treatment rate/pressure management
4. Must take into account current reservoir conditions
 Depleted pore pressure
 Altered stresses
 Higher permeability and leak-off rate

15. An Alternative Refracturing Method:
Two-step pumping
0
20
40
60
80
100
0
2000
4000
6000
8000
10000
0 60 120 180 240
FlowRate(bbl/min)
BHPressure(psi)
Time (min)
Fracture Closure Pres.
(Linear Gel 20 cP)(slick water 2.5 cP)
STEP 1
Pressurization
STEP 2
Stimulation
Step 1: Pressurization:
 Low pressure injection (<FCP)
 No proppant added
 No microseismic expected
Step 2: Stimulation:
 High pressure injection (>FCP)
 Proppant added
 Microseismic activity expected
4000
5000
6000
7000
8000
9000
10000
0 2000 4000 6000
BHPressure(psi)
Distance from Heel (ft)
Linear Gel
Step 1
FCP
Step 2
Step 1
Well pressure profile
2
3
4
5
6
0 2000 4000 6000q/qtot(%)
Distance From Heel (ft)
Linear Gel
Step 1
Step 2
Step 1
Well discharge profile
Fracture re-opening
pressure = FCP

16. Two-Step pumping method
Heel
Toe
Plan View
Heel
Toe
Side View
 Microseismic Response
 Cost benefits:
 No new perforations required
 Does not rely on diverters – diversion is achieved by active management of
rate/pressure during treatment
 Intervention not required
Refracked (two-step method)
- 55% IP
- 35% EUR increase
Refracked (conventional)
- 35% IP

17. Remarks
 Conventional refracturing practices are not efficient in most cases
 Dominant stimulation mechanism of refracturing is different than
that of an initial fracturing job
 It is probably more efficient and cost effective to aim at adding
effective complexity rather than creating new hydraulic fractures
 The altered state of reservoir characteristics should be considered
when designing a refracturing job
 The two-step method suggest an efficient and cost effective
alternative to the conventional refracturing methods