Engineering Aspects of Unconventional Oil and Gas Reservoirs ...

StrataGen Engineering — 2011 Workshop — 27 April 2011 

Stimulation of Unconventional Oil and Natural Gas Liquid Reservoirs 

Engineering Aspects of 

Unconventional Oil and Gas Reservoirs 

Background 

(Production Analysis) 

[TAB] 

Tom BLASINGAME 

Department of Petroleum Engineering 

Texas A&M University 

College Station, TX 77843-3116 (USA) 

Dilhan ILK 

DeGolyer and MacNaughton 

5001 Spring Valley Road 

Suite 800 East 

Dallas, TX 75244 (USA) 

+1.979.845.2292 +1.214.891.7381 

t-blasingame@tamu.edu 

dilk@demac.com 

StrataGen Engineering — 2011 Workshop 

Stimulation of Unconventional Oil and Natural Gas Liquid Reservoirs — 27 April 2011 

Engineering Aspects of Unconventional Oil and Gas Reservoirs 

Dilhan Ilk (DeGolyer & MacNaughton)/T.A. Blasingame (Texas A&M U.) 

Slide — 1/37

Starting Points: Engineering Aspects of UR 

●What we REALLY know… 

■ Tight Gas is Relatively Easy (Vertical Wells, HP/HT, PVT) 

■ Gas Shales are Technically Viable as a Resource (Economics?) 

■ We Can Consistency Deploy Horizontal Multi-Fracture Wells… 

●What we THINK know… 

■ The fracture geometry is… 

■ The phase behavior… 

■ The p tf to p wf conversion(s) 

●What we may NEVER know… 

■ Distribution of natural fractures… 

■ Transport mechanism for gas/liquids in shales 

(planar? complex?) 

(Can be extremely complex…) 

(what about the heavy water load?) 

●What we should know in the near future… 

■ Minimal/average duration of data required for EUR? 

■ Much better understanding of phase behavior. 

■ Optimal well spacing/orientation/placement. 

(impossible) 

(organic) 

[TAB] 





Slide — 2/37

Overview: Engineering Aspects of UR 

●Where we are: (history lessons) 

■ Background 

■ Recent work 

●Where we want to be: (or so we think) 

(Historical Perspective) 

(Blasingame/team) 

■ Fit for purpose stimulation. (oil/gas/condensate/geology) 

■ More effective reservoir monitoring 

(this is important!) 

■ Early EUR 

(prediction/correlation) 

■ Well Spacing 

(geology + PVT + modeling) 

●How do we get there… 

■ Better understanding of flowback/dewatering (optimization) 

■ Pressure-dependent properties… 

(k, F cD , desorption?) 

■ Understanding of the pore-scale (what flows when/how) 

■ Petrophysics (conventional petrophysics not adequate) 

■ PVT 

(oil/gas/condensate/water — HP/HT) 

[TAB] 





Slide — 3/37

Orientation: Engineering Aspects of UR 

●Is "shale gas" here to stay? 

■ We know where it is… 

■ We know how to get it (more/less…) 

■ We know how to produce it (sort of…) 

■ It all boils down to: Price/Timing/Technology 

●What could be "disruptive" 

■ Being able to deliver significantly more stimulation energy. 

■ "Green energy" initiatives (unlikely in the short term). 

■ Supply pressure on oil (this is more likely than we think). 

■ Environmental constraints (e.g., limiters to drilling/stimulation). 

[TAB] 





Slide — 4/37





Background 

(Production Analysis) 

[TAB] 





Dilhan ILK 





+1.979.845.2292 +1.214.891.7381 







Slide — 5/37

Decline Analysis — Arps Relations: Base Relations 

Loss Ratio: 

1 qg 

 

D dq / dt 

Loss Ratio Derivative: 

b 

d 

 

dt 

g 

q 

 

 

dq 

 

 

q 

g 

 

q 

gi 

exp[ D t] 

g 

gi 

qg 

 

g / dt (1 

bD ) 

(1/ b) 

 

it 

q 

i 

Trans., AIME (1945) 160, 228-247. 

Analysis of Decline Curves 

J.J. Arps 

Question(s): 

●How were the Arps' rate relations derived? 

The BASIS for the Arps' relations — i.e., 

the behavior of the D- and b- parameters, 

is derived from OBSERVATIONS. These 

are empirical results. 

[TAB] 

Case Rate-Time Relation Cumulative-Time Relation 

qg 

qgiexp[ Dit] 

qgi 

Gp 

[1 

exp[ Dit]] 

D 

Exponential:(b=0) 

Hyperbolic: (0

Decline Analysis: EUR Plots (Arps' relations) 

Question(s): 

●Graphical extrapolations of EUR? Family of "EUR 

plots" derived from the Arps' exponential and 

hyperbolic relations. Hyperbolic "EUR plot" 

requires a modular computing environment (e.g., a 

spreadsheet), as multiple variables are established 

simultaneously. 

SPE 98042 (2005) 

A Production-Based Method for Direct 

Estimation of Gas-in-Place and Reserves 

T.A. Blasingame, Texas A&M U. and 

J.A. Rushing, Anadarko Petroleum. 

[TAB] 

Case 

Exponential: 

(b=0) 

qgi 

 

q g q gi D i G p G 

 

 

Di 

 

Hyperbolic: (0

Decline Analysis: Blasingame-Rushing EUR Plot 

Question(s): 

●Is there a distinctly unique mechanism for 

establishing the validity of the hyperbolic 

relation? Yes, the "hyperbolic" decline "type 

curve" plot yields straight-line trends. 

Gp 

qg 

qgi 

1 

 

G 

 

 

 

(1 

b) 

 

G 

 

 

qgi 

(1 b) 

Di 

 

 

 

SPE 98042 (2005) 

A Production-Based Method for Direct 

Estimation of Gas-in-Place and Reserves 

T.A. Blasingame, Texas A&M U. and 

J.A. Rushing, Anadarko Petroleum. 

Hyperbolic Decline: (0

Decline Analysis: Fetkovich-Carter Type Curve 

JPT (June 1980) 1065-1077. 

Decline Curve Analysis Using Type Curves 

M.J. Fetkovich, Phillips Petroleum 

SPEJ (October 1985) 719-728. 

Type Curves for Finite Radial and Linear Gas Flow 

Systems: Constant Terminal Pressure Case 

R.D. Carter, Amoco Production 

[TAB] 

Variables for the Carter Decline Type Curve 

0. 

00633 kt 

1 

tDd 

 

gictir 

2 2 

w 1 re 

re 

 

 

 

 

1 

 

 

1 

ln 

 

2 rw 

rwa 

 

 

2 

 

 

q( 

t) 

q 

s 

Dd 

rwa 

rwe 

 

kh ( pi 

pwf 

) 

r 

. 

e 1 

141 2 giBgi 

ln 

 

 

rwa 

2 

a. Original format Fetkovich-Carter type curve — most important 

observation is that 01: transient flow or 

external drive energy. 

•: numerical gas flow 

solutions ( =f(p wf /(p i )). 

Reservoir Properties: 

•k — y-axis match. 

•G — x&y-axis matches. 

•s —r eD match. 





Slide — 9/37

Decline Analysis: "Flowing Material Balance" Plot 

Question(s): 

●What is the "Flowing Material Balance" plot? In 

simple terms, p wf (t) data are "converted" to p avg (t) 

data using the pseudosteady-state flow equation, 

then plotted as a straight-line extrapolation function 

and "solved" for gas-in-place. 

JCPT (June 1997), 52-55. 

The 'Flowing' Gas Material Balance 

L. Mattar and R. McNeil, Fekete Associates 

"Flowing Material Balance" Plot: 

Theory: 

●Palacio and Blasingame [1993] 

●Mattar and McNeil [1997] 

●Agarwal et al [1999] 

[TAB] 

a. The "Flowing Material Balance" (Normalized Rate-Cumulative 

Function Plot) formulation is derived using the solution 

for the diffusivity equation during boundary-dominated flow 

regime. This formulation provides a direct estimate of the 

contacted gas-in-place using time, flowing wellbore pressure, 

and flowrate data. 

Advantages: 

●Straightforward and intuitive. 

●Shut-in pressures NOT required. 

●Direct estimation of contacted 

gas-in-place. 

Limitations: 

●Boundary-dominated flow 

regime must exist. 





Slide — 10/37

Decline Analysis: Palacio Material Balance Time 

Question(s): 

●Can the well-reservoir model be inferred 

from such data? Yes. 

●Is diagnosis sufficient? No, we must also 

be able to model/history match data with 

a model (complete process). 

SPE 25909 (1993) 

Decline Curve Analysis Using Type Curves — 

Analysis of Gas Well Production Data 

J.C. Palacio and T. Blasingame, Texas A&M U. 

q 

g 

 

q 

gi 

(1 bD t) 

i 

( 1/ b) 

? 

Transient 

Flow 

m( 

p) 

 

Gp 

mˆ g,pss 

 

qg 

 

 

trans 

qg 

(1/4) 

 

 

 

Boundary-Dominated 

Flow 

m( 

p) 

 

Gp 

mˆ g,pss 

 

qg 

 

 

bdf 

qg 

(1) 

 

 

 

[TAB] 

a. Raw (daily) rate and pressure data — bottomhole 

pressures are calculated, note the effect of liquid 

loading. 

b. "Transformed" data shows fractured well 

response at early times, very strong evidence 

of closed system at late times. 





Slide — 11/37





Historical Work 

(Blasingame Team) 

[TAB] 





Dilhan ILK 





+1.979.845.2292 +1.214.891.7381 







Slide — 12/37

Decline Analysis: Tight Gas Systems 

x 

X 

Pressure 

Monitoring 

Point No. 2 

SPE 109625 (2007) 

Estimating Reserves in Tight Gas Sands at HP/HT 

Reservoir Conditions: Use and Misuse of an Arps 

Decline Curve Methodology 

J.A. Rushing, A.D. Perego, R.B. Sullivan, 

Anadarko Petroleum, and T.A. Blasingame, Texas 

A&M U. 

y 

Wellbore 

Pressure 

Monitoring 

Point No. 1 

X 

Hydraulic 

Fracture 

[TAB] 

Numerical Model Considers: 

●Reservoir Layering. 

●k v /k h ratio. 

●Fracture Length, x f . 

●Fracture Conductivity, F cD . 

Analysis/Validation Approach: 

●Fit q(t) with Arps' hyperbolic relation. 

●Compare reserves to model at 30 years. 





Slide — 13/37

Vertical TG/SG Wells: Elliptical Flow Domination 

SPE 106308 (2007) 

Evaluation of the Elliptical Flow Period for 

Hydraulically-Fractured Wells in Tight Gas 

Sands — Theoretical Aspects and Practical 

Considerations 

S. Amini, D. Ilk, and T. A. Blasingame, 

SPE, Texas A&M U. 

a. Elliptical flow type curve solution — low fracture 

conductivity case. 

[TAB] 

b. Elliptical flow type curve solution — high fracture 

conductivity case. 

c. Elliptical boundary configurations (finite conductivity 

fracture case [Amini, et al (2007)]. 





Slide — 14/37

Horizontal TG/SG Wells: Compound Linear Flow 

Presented at the 2nd European Conference 

on the Mathematics of Oil Recovery, 

Cambridge, England (1989). 

A Boundary Element Solution of the Transient 

Pressure Response of Multiply Fractured 

Horizontal Wells 

C.P.J.W. van Kruysdijk and G.M. Dullaert, Shell 

a. Rate performance behavior for a horizontal well with 4 transverse 

fractures — infinite-acting reservoir (analog to van 

Kruysdijk and Dullaert work). Fine-scale numerical model . 

[TAB] 

b. Specialized derivative plot (ref: van Kruysdijk and Dullaert) for 

a horizontal well with 4 transverse fractures — infinite- and 

finite-acting reservoir cases. Fine-scale numerical model . 

c. Schematic diagram for the "compound linear flow" 

concept [van Kruysdijk and Dullaert (1989)]. 





Slide — 15/37

Low k Tight Gas Sands: Petrophysics/Permeability 

Gas Flow 

v y 

v x 

SPE 107954 

Improved Permeability-Prediction Relations 

for in Low Permeability Sands 

F.A. Florence, Occidental Petroleum Corp., 

J.A. Rushing, Anadarko Petroleum Corp., K.E. 

Newsham, Apache Corp., and T.A. 

Blasingame, Texas A&M U. 

a. Gas Slippage — Kundt, A. and Warburg, E.: "Über Reibung 

und Wärmeleitung verdünnter Gase, " Poggendorfs Annalen 

der Physik und Che-mie (1875), 155, 337. 

[TAB] 

b. Knudsen "microflow" model (Modified from Karniadakis and 

Beskok, 2002). 

k 

k 

a 

 

 

128 

1 

 

 

2 

15 

 

 

-1 

tan 4a0 

 

 

1 

k 

p 

m 

0.4 

a 

1 a2 

 

a1 

a2 

 

 

 

 

1 

 

11/ 

a 

 

 

c. Microflow model and correlation, "fully implicit" formulation. 





 

 

 

a 

 

 

0 

1 

k 

p 

m 

 

 

 

 

 

 

0 

4 

1 

k 

p 

m 

a1 

a2 

 

 

 

 

 

 

 

Slide — 16/37

Low k Reservoirs: Characteristic Behavior 

SPE 114168 

The Characteristic Flow Behavior of Low- 

Permeability Reservoir Systems 

T.A. Blasingame, Texas A&M U. 

A 

B 

a. Systematic "mapping" of the inter-relation of petro-physical 

properties. Note that Archie observed that permeability was 

"connected" to saturation, porosity, and electrical properties 

— but the relationship was vague, as it remains today. 

[TAB] 

(Formation Resistivity 

Factor) 

(Permeability, md) 

b. Crossplot of formation (resistivity) factor versus permeability 

(F = A/k B ). 

C 

Legend: SEM Micrographs 

A. (240X) Grains with clay overgrowths. 

B. (2000X) Microporosity formed by illite clay filaments. 

C. (600X) Microporosity and clay filling. 

D. (1400X) Rosettes of chlorite (note illite deposition). 

c. Severe influence of clay minerals in this reservoir system — 

production shown to be uniquely tied to reservoir quality and 

effectiveness of well stimulation. 





D 

Slide — 17/37

Permeability: Characteristic Behavior 

k a( c) 

b 

SPE 118026 

Towards a Characteristic Equation for Permeability 

A.A. Siddiqui, D. Ilk, T.A. Blasingame, Texas A&M U. 

k c exp[ ] k xa ( 1 

x) 

exp[ ] 

b 

[TAB] 





Slide — 18/37

Production Analysis: Elliptical Flow 

SPE 110187 

Evaluating the Impact of Waterfrac 

Technologies on Gas Recovery Efficiency: 

Case Studies Using Elliptical Flow Production 

Data Analysis 

D. Ilk, Texas A&M U., J.A. Rushing and R.B. 

Sullivan, Anadarko Petroleum Corp., and T.A. 


a. Elliptical boundary decline type curve match (very high 

conductivity, "thin" elliptical drainage geometry). 

[DI] 

b. Production history plot with model match (very good flowrate 

match, acceptable pressure match). 

c. Results correlation plot — G versus k. 





Slide — 19/37

Production Analysis: Power-law Exponential Decline 

SPE 116731 

Exponential vs. Hyperbolic Decline in Tight 

Gas Sands — Understanding the Origin and 

Implications for Reserve Estimates Using 

Arps' Decline Curves 

D. Ilk, Texas A&M U., J.A. Rushing and A.D. 

Perego, Anadarko Petroleum Corp., and T.A. 


a. Production forecast of a tight gas well using Arps' hyperbolic 

decline relation. 

[DI] 

b. Derivation of the power-law exponential relation. 

c. "qDb" plot — Data matched using power-law exponential rate 

decline relation. 





Slide — 20/37

Production Analysis: Continuous EUR 

SPE 132352 

q qˆ 

i 

exp[ D 

t Dˆ 

i 

t 

n 

] 

Continuous Estimation of Ultimate Recovery 

S.M. Currie., D. Ilk, and T.A. Blasingame, 

Texas A&M U. 

a. Base data plot ("qDb-plot"), data matched using 

Power Law Exponential and Hyperbolic models. 

[DI] 

b. Continuous EUR workflow including Hyperbolic, Power-Law 

Exponential, and straight-line extrapolation models. 

c. Summary "Continuous EUR" plot — note all model 

results are plotted in time. 





Slide — 21/37

Production Correlation: Flowback Analysis 

SPE 135607 

A Comprehensive Workflow for Early Analysis and 

Interpretation of Flowback Data from Wells in Tight 

Gas/Shale Reservoir Systems 

D. Ilk and S.M. Currie, Texas A&M U., D. Symmons, 

Consultant, J.A. Rushing, Anadarko Petroleum, N.J. 

Broussard, El Paso, and T.A. Blasingame, Texas A&M U. 

a. Crossplot — All wells: q g versus q w . 

[DI] 

b. Crossplot — All wells: p cf versus t. 

c. Crossplot — All wells: p 2 /q g versus t. 





Slide — 22/37

Production Analysis: Time-Rate Diagnostics 

SPE 135616 

 

q, 

cp 

d ln( q) 

( t) 

 

d ln( t) 

t dq 

q dt 

Hybrid Rate-Decline Models for the Analysis of 

Production Performance in Unconventional Reservoirs 

D. Ilk and S.M. Currie, Texas A&M U., D. Symmons, 

Consultant, J.A. Rushing, Apache, and T.A. Blasingame, 

Texas A&M U. 

 

q, 

cp 

( t) 

 

d ln( q) 

d ln( t) 

 

 

t 

q 

dq 

dt 

a. -derivative — Holly Branch Wells. 

 

q, 

cp 

d ln( q) 

( t) 

 

d ln( t) 

t dq 

q dt 

[DI] 

b. -derivative — "Shale Gas Field C" Wells. c. -derivative — All Models (Holly Branch Well). 





Slide — 23/37

Production Analysis: Integration of Results 

SPE 140556 

Integration of Production Analysis and Rate-time 

Analysis via Parametric Correlations — Theoretical 

Considerations and Practical Applications 

D. Ilk, DeGolyer and MacNaughton, J.A. Rushing, 

Apache, and T.A. Blasingame, Texas A&M U. 

k a n 

b 

Dˆ 

c 

i 

qˆ 

d 

i 

"Shale Gas Field C" 

Horizontal well with multiple vertical fractures: 

a. Correlation plot — k cal versus k meas . 

[DI] 

EUR ˆ n 

ˆ 

"Shale Gas Field C" 

b. Correlation plot — EUR cal versus EUR meas . 

Power-Law Exponential Relations: 

1 dq ˆ (1 n) 

D( 

t) 

D nD i 

q dt 

t 

 

( ) ˆ exp[ ˆ n 

q t qi 

D 

t Di 

t ] 





Slide — 24/37

Production Analysis: Workflow/Unconv. Reservoirs 

SPE 144376 

Production Analysis in Unconventional Reservoirs — 

Diagnostics, Challenges, and Methodologies 

D. Ilk, C.D. Jenkins, DeGolyer and MacNaughton, and 

T.A. Blasingame, Texas A&M U. 

a. Diagnostic plots (selected) — q/p vs. t and 

D-parameter vs. t. 

[DI] 

b. History match plot (selected well) — q and p wf vs. t. 

c. q/p data (all wells) with constant rate solution and G p vs. t 

for sensitivity analysis for various parameters. 





Slide — 25/37





Well Performance Analysis for 

Tight/Shale Oil Reservoir Systems 

[DI] 





Dilhan ILK 





+1.979.845.2292 +1.214.891.7381 







Slide — 26/37

Field Example: Bakken Oil Well 

[DI] 

●Discussion: Bakken Oil Well 

■ Change in production trend after almost 120 days. 

■ Off-trend data are removed prior to analysis. 





Slide — 27/37


[DI] 

●Discussion: Continuous EUR using Arps' Hyperbolic Decline 

■ b-parameter value ranges between 3.7 (earliest interval) and 1.5 (all 

data) for the subsets of data. 

■ No strong evidence of boundary-dominated flow is observed. 





Slide — 28/37


[DI] 

●Discussion: Continuous EUR using Power-law Exponential Model 

■ Regression is used to match the data with the model at specified 

intervals. 

■ Variability in the production trend yields different matches. 





Slide — 29/37


[DI] 

●Discussion: N p,max using Straight Line Extrapolation 

■ Straight line extrapolation yields the minimum value for the reserves 

estimate as boundary-dominated flow assumption is made. 

■ N p,max (t) increases with time. 





Slide — 30/37


[DI] 

●Discussion: Conclusions 

■ EUR estimates from "hyperbolic" relation are greater than estimates 

from PLE especially at early times. 

■ EUR (at 30 years) should lie between 0.14 and 0.26 MMSTB. 





Slide — 31/37


■ Log-log plot: Normalized rate functions versus 

material balance time (data). 

■ Log-log plot: Normalized rate functions versus 

material balance time (data and models). 

[DI] 

●Discussion: Model-Based Production Analysis 

■ Good diagnostic character of the data functions. 

■ No strong evidence of linear flow is observed. 

■ Good match of the data with the analytical model solution. 

■ Model: Horizontal well with multiple transverse fractures. 





Slide — 32/37


[DI] 

●Discussion: Model-Based Production Analysis 

■ Very good match of the data with the analytical model. 

■ No volume results — INFINITE-ACTING RESERVOIR. 

■ Production forecast at 30 years = 0.29 MMSTB. 

■ Consistent estimate with rate-time analysis. 





Slide — 33/37





Where we want to be… 

and how do we get there… 

[TAB] 





Dilhan ILK 





+1.979.845.2292 +1.214.891.7381 







Slide — 34/37

Big Questions: Engineering Aspects of UR 

[TAB] 

●EUR? 

■ Early EUR? 

■ EUR = f(t)? 

■ Well Spacing? 

●QUANTIFYING reservoir properties? 

■ Pressure Transient Analysis 

■ Production Analysis 

■ Petrophysical analysis 

●Liquids-Rich Systems? 

■ Fluid-Flow Mechanisms 

■ PVT 

■ Improved Recovery 

■ Fit-for-Purpose Stimulation 

■ Artificial Lift 

●Oil-Prone Systems? 

■ Recovery 

(can this be meaningful?) 

(how do we incorporate this?) 

(is this really the holy grail?) 

(ultra-low k?) 

(p tf may not be sufficient) 

(theory ≠ application) 

(what is really flowing where?) 

(near-critical fluids are not trivial) 

(we all know this is coming…) 

(higher F cD , more complexity) 

(fact of life…) 

(low to extremely low primary recovery) 





Slide — 35/37

Perspectives: Engineering Aspects of UR 

[TAB] 

●Never-Ending Arguments… 

■ SRV 

(What is it, really?) 

■ Desorption 

(Significance? Timing? Relevance?) 

■ Stimulation Fluids (Where does it go? Does it matter?) 

■ Microseismic 

(Crystal ball or roulette wheel?) 

■ Pressure-Dependent Whatever 

(So what?) 

■ Natural Fractures 

(If/when/why/what?) 

■ Dual Porosity/Dual Permeability (What about the physics?) 

■ Well Placement/Effect of Layering (When does it matter?) 

●Things that SHOULD help… 

■ Production Logs 

(But … just a snapshot in time.) 

■ Optimal Proppant Design/Placement 

(Obvious, but…) 

■ Stimulation Stages/Perforation Clusters (Geology + logs) 

●Things that DEFINITELY WOULD help… 

■ Measured p wf 

■ Downhole Fluid Sampling 

■ Horizontal Core 

(Yes, this is my favorite song…) 

(Sooner or later…) 

(Why not?) 





Slide — 36/37





End of Presentation 

[TAB] 





Dilhan ILK 





+1.979.845.2292 +1.214.891.7381 







Slide — 37/37

Point to Ponder: Nelson Pore/Molecule Size Chart 

[TAB] 





Nelson, P.H.: "Pore-throat sizes in sandstones, tight sandstones, and shales," AAPG Bulletin, 

v. 93, no. 3 (March 2009), pp. 329–340. 

Slide — 38/37

Engineering Aspects of Unconventional Oil and Gas Reservoirs ...

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