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Guide For Fault Analysis
Flamenco Anylsis
Common Pitfalls of Mini-frac Analysis
Robert Hawkes, Director of Completion Technologies Technologies
Pure Energy Services
What They Didn’t Tell You About G -function -function and log-log plots
What They Didn’t Tell You About G -function -function and log-log plots
Reappraisal of the G Time Concept in Mini-Frac Analysis (SPE 160169, Bachman et al)
G-function
Bachman
Thank you Google Images
Pre-Frac Diagnostics…. …….are we wasting our time?
Most of our Pitfalls are coming from conventional thinking. Closure Pressure
Still needs a holistic approach ........and more work!
Leak-off Behaviour
Pressure diagnostics is interpretive and heavily by your discipline. influenced by
After Closure Analysis
Be careful how you impose a flow regime on the data.
Diagnostic Approach o
Step-up or Step-down (neither)
o
Low Rate or Treatment Rate?
o
o
What's the the objective
Interpretation Pitfalls
ISIP
Closure Identification Think again
Flow Regime Identification as it seems Not as easy as
Fracture Flow Regimes from Cinco-Ley (1978) (Static )
Carter Leak-off Model (Dynamic )
The idea behind Carter's 1D leakoff coefficient
Fluid Loss Velocity
• if a filter-cake wall is building up it will allow less fluid to pass through a unit area in unit time
√-0
h
• the reservoir itself can take less and less fluid if it has been exposed to inflow • Both of these phenomena can be roughly approximated as "square-root time behavior "
Who Has Seen This Shape of G-Function Plot??
Who Has Seen This Shape of G-Function Plot??
Who Has Seen This Shape of G-Function Plot??
Who Has Seen This Shape of G-Function Plot??
Who Has Seen This Behaviour on the Log-log Plot??
SPE 140136
(2011)
Paper identifies 3/2 slope feature and gives some theory
_______.
So What Does the Welltesting Community Have to Offer to Mini-frac analysis?
15
SPE Calgary, Sept 6, 2012
First Mini-frac Derivative Plot, ~1990, Dr. Ted. Leshchyshyn
Well Test Analysis Log-Log Derivative Term (Agarwal Equivalent Time)
Uses radial equivalent time ter for derivative
This is NOT the same as Dt t er
d (D P ) dt er
Log _ Derivative
t er
t p Dt t p Dt
Versatile even when radial flow not present
This is why it is a universal approach to all well test problems
Nolte G -Function Analysis
Using the Nolte G Function construct the various plots Assume
no closure and the G Function Solution runs on forever……..
What
do things look like?
Fluid Loss Velocity
√-0
h
Nolte G Function
Nolte G Function P vs G
Classic Nolte behavior with no closure
GdP/dG vs G
dP/dG vs G
Nolte G Function Square root-t plot shows a character change from early time to late time with a constant flow regime.
Nolte G Function
Early Time Slope = 1 Late Time Slope = 0.5 Derivative is the logarithmic derivative of ∆t…… …conventional approach.
Late Time slope = 0.5 is Carter leak-off, not the conventional linear flow.
m = 1.5
Nolte G Function Early Time Slope = 1 Late Time Slope = 1.5*
*as identified in paper 140136
Derivative is the logarithmic derivative of Agarwal equivalent (radial) time or…… m = 1.0
…standard well test derivative.
NO linear flow slope, not picked up in paper 140136.
Classic Linear Flow Solution
Infinite acting linear flow Injection period - classic linear flow Shut-in period - classic linear flow
Use superposition to compute fall-off response
Linear Flow
Linear Flow
No closure - infinite acting linear flow
The G-function plot does not give meaningful results as the character changes from early time to late time with the same flow regime.
Linear Flow
No closure - infinite acting linear flow
Square root-t plot also has a character change from early time to late time with the same flow regime.
Linear Flow No closure - infinite acting linear flow
Early Slope = 0.5 Late Slope = -0.5
Intercept is not closure for the special case 0.5 to -0.5 slope but is instead an artifact of the plot.
Delta Time
Linear Flow
Early Time Slope = 0.5 Late Time Slope = 0.5
Agarwal Equivalent Time
Well Test Analysis Approach The welltest log-log pressure derivative is the best flow regime indicator in our arsenal
Theory is very well established in traditional PTA
Cinco-ley and Samaniego, 1981
You need to dust off those old and forgotten welltesters in your closet and exploit them in the mini-frac world.
Welltest Log-log derivative:
the time function is NOT dt of the flow period the time function is with respect to Agarwal equivalent time – te = tp*dt/(tp+dt)
The Log-log derivatives in Barree’s paper SPE 107877 are all with respect to dt as far as we can tell.
Observations and Comments: 1.
We need to do flow regime identification before picking closure pressures
Standard well test log-log derivative plot is key
Flow regime dependent plots
2.
Do we need replacements for the Combination G Function plot?
3.
We need to calculate the correct derivatives
4.
Do we even need ACA plots ?
Why not use standard well test plots to calculate properties ?
Classic Barree Case Jean Marie oil well example
Conventional Approach
Conventional Approach
Barree log-log dt plot
Pfoc = 8300 kPa
Conventional Approach
PDL ?? Pfoc = 8400 kPa
Conventional Approach
Pfoc = 8400 kPa
Proposed Approach
Pfoc = 8400 kPa
Agarwal Equivalent Time
1.
Early Time Linear Flow (0.5)
2.
Middle Time Carter Flow (1.5)
3.
Late Time Linear Flow (after closure)
Carter Equivalent Time
1.
Early Time Linear Flow (0.5)
2.
Middle Time Carter Flow (1.0)
3.
Late Time Linear Flow (0*) *zero slope SPE 160169
Over the time range for which Carter leak-off has been identified, the slope does not need to go through the origin. Is a Positive Y-intercept an indictor of PDL, …..not sure.
PPD Curve
1.
Late Time Linear Flow (after closure: +0.5)
2.
PPD: Late Time Linear Flow (-1.5)
Agarwal Equivalent Time Function and their Slopes
Derivative
Early Time Carter
Late Time Carter
te
1.0
1.5
PPD
0
-0.5
Early Time Linear
Late Time Linear
te
0.5
0.5
PPD
0
-1.5
Early Time Radial
Late Time Radial
te
0
0
PPD
-1
-2 SPE 160169
CRDM Example
CRDM Example
Recorders set @ 1780m KB MD
Establish breakdown and pump 5.0 m3 of fresh water @ 0.45 m3/min
CRDM Example
Establish breakdown and pump 5.0 m3 of fresh water @ 0.45 m3/min
See Log-log plot for ISIP determination.
CRDM Example
CRDM Example
CRDM Example
∆ p = EOJ - Pw
Conventional Delta Time
Welltest Log-log
CRDM Example
m = 1.5
Adjusted Agarwal Time
CRDM Example Overlay Log-log Diagnostic Plot
m = 1/2
m = 3/2
m = -1/2
Adjusted Agarwal Time Delta-Time
CRDM Example PPD
Adjusted Agarwal Time
Observations and Conclusions: 1.
The starting point for ANY mini-frac analysis should be the standard welltest Agarwal equivalent time log-log plot. You
must do flow regime identification before picking closure pressure.
The PPD curve has been shown to contain flow regime identification properties – a bonus diagnostic curve to the welltest community.
The industry should discontinue using the delta-time log-log derivative plot.
2.
The G-Function and square root-time plots have been shown to be poor flow regime (and closure) identification plots?
3.
PDL (pressure dependent leakoff) diagnostics using the G-Function plot needs a holistic approach with the Agarwal equivalent time and PPD log-log plot.
Needs more study
Observations and Conclusions: 4.
Traditional welltest after-closure analysis techniques can be used for reservoir property determination.
5.
Mini-frac analysis should no longer be viewed as an independent discipline.
6.
Talk to your software vendor.
7.
and finally……..say Flow Regime at least once day when in the office.
Banff SPE Workshop, April 2012
Fracture Closure
Flowback
DFIT/MiniFrac/MDT/XLOT d o h t e M t s e T
d s n o o e i t c h m n u t i e T F M n o i t a t e r p r e t n I