Copyright 2007, Society of Petroleum Engineers This paper was prepared for presentation at the 2007 SPE Hydraulic Fracturing Technology Conference held in College Station, Texas, U.S.A., 29–31 January 2007. This paper was selected for presentation by an SPE Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Papers presented at SPE meetings are subject to publication review by Editorial Committees of the Society of Petroleum Engineers. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, SPE, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-943 5. Abstract It has been long established from well testing concepts that an elliptical flow pattern exists in hydraulically-fractured wells producing from low permeability (<0.01 md) and ultra-low (<0.001 md) formations (often referred to as "tight gas" or "shale gas" sands). Conceptually, the e lliptical flow peri od is a transitional flow regime that occurs between the end of bilinear and/or formation linear flow and the onset of pseudo- radial flow. In a practical sense, the elliptica l flow regime can (and does) dominate the well performance for very low permeability r eservoir systems. Elliptical flow (as with pseudo-radial flow) represents a time period when the reservoir properties begin to dominate the reservoir performance. Moreover, the duration of the elli ptical flow period may last for many months, perhaps even years, depending on the reservoir and hydraulic fracture properties. Consequently, understanding the elliptical flow period and its impact on well performance is critical for the optimal development of a tight gas sand reservoir. This paper presents a series of decline type-curves for a system consisting of a hydraulic fracture at the center of an elliptical reservoir. The curves are generated from the pressure solution obtained using an analytical method. The curves are generated for different values of the fracture conductivity and as a function of the elliptical boundary characteristic parameter (ξ 0 ). For the low permeability cases where the elliptical flow period dominates, the elliptical boundary characteristic parameter (ξ 0 ) is a useful parameter for establishing the optimum well spacing and the optimal design for the drainage aspect ratio. We note that the ξ 0 -parameter essentially reduces the optimization process for tight gas reservoirs to a single parameter that should yield the optimum configuration for drainage and pro duction performance. Obviously the fracture conductivity is also a major factor, as is the reservoir permeability and the fracture half-length — but the ξ 0 -para- meter is the key to recovery and performance in systems dominated by elliptical flow behavior. Introduction Elliptical flow has long been considered as a transitional flow pattern that occurs between linear and pseudoradi al flow in the traditional well test analysis of fractured wells. 1 Elliptical flow also occurs also in circular anisotropic reservoirs. The study of elliptical flow has a considerable history in the petroleum engineering literature 2-10 (as well as other disci- plines — e.g., water resources and physics). Prats 2, 3 studied the effect of vertical fracture on the reservoir behavior for incompressible and compressibl e fluid cases. Prats proposed the representation of a fracture with an equivalent effective well radius (this was a convenience of the times (early 1960's)). Kuchuk6 developed the analytical solution for the transient elliptical flow problem resulting from an infinite conductivity fracture producing from an elliptical, or an anisotropic-radial reservoir. The behavior of composite, elliptically -shaped reservoirs has also been studied by Obut, and Ertekin 5 and Stanislav et al6,7 — assuming a variety of boundary condi- tions. Perhaps the most "analytical" treatment of the problem was provided by Riley 8 who developed an analytical solution for the case of a vertical well with a finite conductivity vertical fracture in an infinite-act ing reservoir system. In addition to attempts to obtain analytical solutions for the elliptical flow problem, there are numerous studies which consider numerical modeling of the elliptical flow problem. Hale 9 applied type curve solutions (dimensionless plots) derived from numerical simulation of the elliptical problem as a mechanism to interpret well tests in tight gas reservoirs. Liao 10 developed a general numerical model for the elliptical flow case which is capable of accounting for the simultaneous effects of wellbore storage and fracture face skin effects on the behavior of pressu re transient te sts. Based on practical experience as well as numerical/analytical models, it is apparent that elliptical flow behavior dominates the performance of fractured wells in low/ultra-low perme abil- ity gas reservoir sy stems. The primary obje ctive of this paper is to develop and validate a series of "type curve" solutions for a system consisting of a hydraulically fractured well in a bounded elliptical shape reservoir. The "type curves" are pre- SPE 106308 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 University
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This paper was prepared for presentation at the 2007 SPE Hydraulic Fracturing TechnologyConference held in College Station, Texas, U.S.A., 29–31 January 2007.
This paper was selected for presentation by an SPE Program Committee following review ofinformation contained in an abstract submitted by the author(s). Contents of the paper, aspresented, have not been reviewed by the Society of Petroleum Engineers and are subject tocorrection by the author(s). The material, as presented, does not necessarily reflect anyposition of the Society of Petroleum Engineers, its officers, or members. Papers presented atSPE meetings are subject to publication review by Editorial Committees of the Society ofPetroleum Engineers. Electronic reproduction, distribution, or storage of any part of this paperfor commercial purposes without the written consent of the Society of Petroleum Engineers is
prohibited. Permission to reproduce in print is restricted to an abstract of not more than300 words; illustrations may not be copied. The abstract must contain conspicuousacknowledgment of where and by whom the paper was presented. Write Librarian, SPE, P.O.Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435.
Abstract
It has been long established from well testing concepts that anelliptical flow pattern exists in hydraulically-fractured wells
producing from low permeability (<0.01 md) and ultra-low
(<0.001 md) formations (often referred to as "tight gas" or
"shale gas" sands). Conceptually, the elliptical flow period is
a transitional flow regime that occurs between the end of bilinear and/or formation linear flow and the onset of pseudo-
radial flow. In a practical sense, the elliptical flow regime can
(and does) dominate the well performance for very low permeability reservoir systems.
Elliptical flow (as with pseudo-radial flow) represents a time
period when the reservoir properties begin to dominate the
reservoir performance. Moreover, the duration of the ellipticalflow period may last for many months, perhaps even years,
depending on the reservoir and hydraulic fracture properties.
Consequently, understanding the elliptical flow period and its
impact on well performance is critical for the optimaldevelopment of a tight gas sand reservoir.
This paper presents a series of decline type-curves for a
system consisting of a hydraulic fracture at the center of anelliptical reservoir. The curves are generated from the
pressure solution obtained using an analytical method. The
curves are generated for different values of the fracture
conductivity and as a function of the elliptical boundary
characteristic parameter (ξ 0).
For the low permeability cases where the elliptical flow period
dominates, the elliptical boundary characteristic parameter (ξ 0)is a useful parameter for establishing the optimum well
spacing and the optimal design for the drainage aspect ratio.
We note that the ξ 0-parameter essentially reduces the
optimization process for tight gas reservoirs to a single
parameter that should yield the optimum configuration fordrainage and production performance. Obviously the fracture
conductivity is also a major factor, as is the reservoir
permeability and the fracture half-length — but the ξ 0-parameter is the key to recovery and performance in systems
dominated by elliptical flow behavior.
Introduction
Elliptical flow has long been considered as a transitional flow
pattern that occurs between linear and pseudoradial flow in thetraditional well test analysis of fractured wells.1 Elliptica
flow also occurs also in circular anisotropic reservoirs.
The study of elliptical flow has a considerable history in the
petroleum engineering literature2-10 (as well as other disci-
plines — e.g., water resources and physics). Prats2, 3
studiedthe effect of vertical fracture on the reservoir behavior for
incompressible and compressible fluid cases. Prats proposed
the representation of a fracture with an equivalent effective
well radius (this was a convenience of the times (early
1960's)).
Kuchuk 6 developed the analytical solution for the transien
elliptical flow problem resulting from an infinite conductivity
fracture producing from an elliptical, or an anisotropic-radiareservoir. The behavior of composite, elliptically-shaped
reservoirs has also been studied by Obut, and Ertekin5 and
Stanislav et al 6,7 — assuming a variety of boundary condi-
tions. Perhaps the most "analytical" treatment of the problemwas provided by Riley8 who developed an analytical solution
for the case of a vertical well with a finite conductivity vertica
fracture in an infinite-acting reservoir system.
In addition to attempts to obtain analytical solutions for the
elliptical flow problem, there are numerous studies which
consider numerical modeling of the elliptical flow problem
Hale9 applied type curve solutions (dimensionless plots)
derived from numerical simulation of the elliptical problem as
a mechanism to interpret well tests in tight gas reservoirs
Liao10 developed a general numerical model for the elliptica
flow case which is capable of accounting for the simultaneouseffects of wellbore storage and fracture face skin effects on the
behavior of pressure transient tests.
Based on practical experience as well as numerical/analytica
models, it is apparent that elliptical flow behavior dominates
the performance of fractured wells in low/ultra-low permeabil
ity gas reservoir systems. The primary objective of this pape
is to develop and validate a series of "type curve" solutions fora system consisting of a hydraulically fractured well in a
bounded elliptical shape reservoir. The "type curves" are pre
SPE 106308
Evaluation of the Elliptical Flow Period for Hydraulically-Fractured Wells in Tight GasSands — Theoretical Aspects and Practical ConsiderationsS. Amini, D. Ilk, and T. A. Blasingame, SPE, Texas A&M University
2 S. Amini, D. Ilk, and T. A. Blasingame SPE 106308
sented (and implemented) as a diagnostic tool — which isused to assess the flow behavior based on production data.
Type curves are presented for different values of the fracture
conductivity ( F E = 1, 10, 100, and 1000 (very low to very high
fracture conductivity cases)), as well as a function of theelliptical boundary characteristic parameter (ξ 0).
Development of the Model
Elliptical flow is considered to be the governing flow regimefor low permeability gas reservoirs — and, as such, weassume that the reservoir has some sort of an elliptical outer
(or perhaps, inner) boundary. Cases such as production from
an elliptical wellbore, an elliptical fracture, or a circular well- bore in an anisotropic reservoir system can be considered to be
examples of an elliptical inner boundary. An elliptic reservoir
surrounded by an (elliptic) aquifer is a relevant example of an
elliptical outer boundary — as is also the case of a very low permeability gas reservoir that achieves only a limited drain-
age pattern (usually modeled by an ellipse).
Our model analytical consists of a hydraulically fractured wellin the center of a reservoir with a closed elliptical boundary.
The schematic illustration for an elliptical reservoir with ahydraulic fracture is shown in Fig. 1.
Figure 1 — Schematic of the elliptical reservoir model.
The following assumptions are made in order to develop an
analytical solution for this case:
The reservoir is assumed to be a single-layer system that isisotropic, horizontal, and of uniform thickness with con-
stant reservoir characteristics. Production is obtained from a hydraulic fracture which
intersects the wellbore. The fracture is assumed to haveelliptical shape — but we can also assume that the fractureis very narrow compared to the length (and area) of the
fracture, and in such cases, we assume a zero-width frac-ture and the fracture transforms into a line whose length isequal to the focal length of the elliptic system.
The elliptical outer boundary is assumed to have a focal
length equal to the fracture length Fracture properties are assumed constant — and we have
considered the cases of both infinite and finite conductivityvertical fractures.
Wellbore storage and skin effects are neglected for the sakeof simplicity (as we are focusing on production data analy-
sis, this issue not significant) The flowing fluid is assumed to be a single-phase, slightly
compressible fluid (although rigorous transformations can be used for gas flow cases). Fluid flow in the formation and in fracture is laminar, and
Darcy's law is presumed valid.
The analytical solutions for the case of an elliptical fractureand outer boundary have been discussed in prior literature2-4,8
However, we provide an appropriate discussion of the compu
tational issues encountered in the solution of the finite and
infinite fracture conductivity cases in Appendix A.
We also introduce a parameter called the elliptical boundary
characteristic parameter (ξ 0) — which is a variable that cor-
relates all the aspects of the drainage area (e.g., the drainage
aspect ratio (a/b) and penetration ratio based on the fracture
half-length ( x f /a). The ξ 0-parameter helps us to characterize
the drainage area using a single dimensionless parametersimilar in concept and application to the dimensionless radius
for the circular bounded reservoir (r eD = r e/ x f ) as introduced by
Pratikno and Blasingame.13 In Appendix B we provide usefu
orientation and detail regarding the correlation between theelliptical boundary characteristic parameter (ξ 0) and the
drainage area size and shape.
Elliptical Flow Model — Type Curves
Given a volumetric reservoir acting under pseudosteady-state
flow conditions (any well/reservoir configuration), we can
write the following material balance/flow relation for thiscase:
DA Dpss pss D t b p π 2, += ...................................................... (1
Where b Dpss is the "pseudosteady-state constant" for a particu-
lar well/reservoir configuration. We can calculate a series o
pressure solutions as functions of dimensionless time for
different values of the fracture conductivity and the elliptica
boundary characteristic parameter (ξ 0) in order to estimate theb Dpss-parameter for different configurations. Cartesian-style
plots of p D,pss versus t DA are used to establish the b Dpss
parameter values. and a sampling of b Dpss-values is presentedin Table 1 for the cases of F E =1, 10, 100, and 1000. The pro
cess of estimating the b Dpss-parameter is discussed in detail inref. 13
Table 1 — Values of bDpss from elliptical flow solution.
ξ 0 F E = 1 F E = 10 F E = 100 F E = 1000
0.25 0.8481 0.2150 0.1306 0.1220
0.50 0.9902 0.3337 0.2396 0.2298
0.75 1.1671 0.4609 0.3540 0.3426
1.00 1.3627 0.6109 0.4936 0.4812
1.25 1.5733 0.7880 0.6634 0.6501
1.50 1.7963 0.9884 0.8591 0.8453
1.75 2.0293 1.2067 1.0743 1.0602
2.00 2.2682 1.4363 1.3021 1.2877
3.00 3.2529 2.4084 2.2716 2.2570
4.00 4.2503 3.4040 3.2669 3.2522
5.00 5.2486 4.4021 4.2649 4.2502
Using the data in Table 1 and a non-linear regression methodwe can estimate the b Dpss-parameter as a function of the
fracture conductivity and the elliptical boundary characteristic
SPE 106308 Evaluation of the Elliptical Flow Period for Hydraulically-Fractured Wells in Tight Gas Sands — 3
Theoretical Aspects and Practical Considerations
Where the auxiliary functions are:
)ln( E F u = ........................................................................ (4)
45
34
2321 uauauauaa A ++++= .................................... (5)
45
34
2321 ububububb B ++++= ..................................... (6)
Where the coefficients are given as:
a1 = -4.7468 b1 = -2.4941
a2 = 36.2492 b2 = 21.6755
a3 = 55.0998 b3 = 41.0303
a4 = -3.98311 b4 = -10.4793
a5 = 6.07102 b5 = 5.6108
The b Dpss results for our work and Eq. 3 (the correlation
equation) are presented graphically in Fig. 2.
Figure 2 — bDpss results and the correlating relation for thecase of a well with a finite conductivity verticalfracture in an elliptically bounded reservoir.
This work pertaining to the b Dpss-parameter is directed
primarily at establishing a "correlation" for those interested in
developing "Fetkovich-style" type curves which are based on
the so-called decline variables. For our work, we intend todemonstrate the elliptical boundary model as a diagnostic to
establish the elliptical flow regime using production data — as
such, for convenience (and simplicity) we utilize type curvesolutions in terms of the equivalent constant rate case in
"decline" form (i.e., q D and auxiliary functions q Di and q Did
versus t DA).
Application of the Elliptical Flow Model — IllustrativeExamples
In this section we provide diagnostic examples to demon-strate/illustrate value of the elliptical boundary model. The
purpose of this exercise is not to obtain "answers" (although
several analyses are performed), but rather, to illustrate the
quality matches obtained using the elliptical boundary model.
Example 1: East Texas (US ) — Tight Gas ( good production)
This case is taken from ref. 13, and all of the relevant data for
this case can be found in that reference. The diagnostic plots
for this case are shown in Figs. 3-5. The operator used
"better-than-average" data acquisition practices, and theresults confirm the validity of the elliptical flow model.
Figure 3 — Example 1: Time-Pressure-Rate (TPR) history plot
Shows very good correlation of rate and pressuredata, indicates likelihood of good analysis.
Figure 4 — Example 1: Log-log (rate function) diagnostic plot.Excellent data functions — strong indication offractured well regimes tending to boundary flow.
Figure 5 — Example 1: "Type curve" plot. Extraordinarymatch of data and elliptical boundary model,match confirms concept.
4 S. Amini, D. Ilk, and T. A. Blasingame SPE 106308
Example 2: North Texas (US ) — Very Tight Gas This case has reasonably good correlation of time-rate-
pressure data as shown in Fig. 6. All analyses for this case
yield a permeability of < 0.001 md. The log-log diagnostic
and type curve matches are shown in Figs. 7 and 8 (respect-ively) — elliptical reservoir match is reasonable (Fig. 8).
Figure 6 — Example 2: Time-Pressure-Rate (TPR) history plot.Only limited well history was provided, but thedata do appear reasonably correlated.
Figure 7 — Example 2: Log-log (rate function) diagnostic plot.Data functions suggest a high to very high con-ductivity vertical fracture.
Figure 8 — Example 2: "Type curve" plot. Good to possiblyexcellent match of data functions with ellipticalboundary model. Transition flow regime is ap-parent at late times.
Example 3: North Texas (US ) — Tight Gas
This is a "sister" case to Example 2 — but appears to be lesswell-stimulated ( F E ). The data are reasonably well-correlated
(Fig. 9) and the log-log plot (Fig. 10) clearly suggests a low
conductivity vertical fracture. The data match (Fig. 11) isvery good, indicating validity in the elliptical reservoir model.
Figure 9 — Example 3: Time-Pressure-Rate (TPR) history plotData correlation is fair — most significant featuresdo correlate, but not all features are related.
Figure 10 — Example 3: Log-log (rate function) diagnostic plot.Data functions confirm low conductivity verticalfracture, reasonably good correlation of data.
Figure 11 — Example 3: "Type curve" plot. As noted above,good correlation with low conductivity verticalfracture case. Minor concern regarding dataartifacts at late times (good data interpretation).
SPE 106308 Evaluation of the Elliptical Flow Period for Hydraulically-Fractured Wells in Tight Gas Sands — 5
Theoretical Aspects and Practical Considerations
Example 4: Mexico — Very Tight Gas (long production)
First and foremost, this reservoir has a permeability of < 0.001
md (multiple analyses), and the well is the only well in the
field. The long production history and high quality data yield
"near textbook" quality diagnostic plots (Figs. 12-15). Fig. 15 confirms validity of the elliptical boundary model.
Figure 12 — Example 4: Time-Pressure-Rate (TPR) history plot— excellent rate performance. Constant ptf → pwf is reasonable considering location/production.
Figure 13 — Example 4: Log-log (rate function) diagnostic plot.Extraordinary character in data (1/2 slope trend),very good diagnostic trends.
Figure 14 — Example 4: "Type curve" plot. Excellent to out-standing match of the data functions — strongconfirmation of the elliptical boundary model.
As a closure in this section we present the matching results fo
this work, as well as the "average" analysis results for these
examples (Tables 2a and 2b). Perhaps the most importan
outcome of this work is not any specific result, model, etc. —
but rather, the proof-of-concept for application of the elliptica boundary model. This observation of the usefulness of the
elliptical boundary model as a diagnostic can (and should)
lead to better production data analysis — including better"model-based" estimates of fluids in-place.
Table 2a — Matching results from interpretation/analysis.
Example
t mba /t DA (days)
(q g /Δ p p)/q D (MSCFD/psi) F E ξ 0
1 228 1.35 1 0.50
2 7200 0.052 100 0.25
3 4000 0.145 10 0.75
4 190,000 0.098 100 0.25
Table 2b — "Average" analysis results.
Example
k(md)
x f (ft)
G
(BSCF)
1 0.006 300 1.62 0.0007 250 0.5
3 0.003 400 0.9
4 0.001 825 23.0
Summary and Conclusions
Summary: Simply put, the development of the elliptica
boundary model for production data diagnostics and analysiis long overdue. It is often argued that the same concept can
be applied using a rectangular or circular reservoir model as a
surrogate for the elliptical boundary model. Given theexamples reviewed in this work, it is difficult to justify a
"surrogate" for the elliptical boundary model which appears
(by far) to be the best model for evaluating fractured wells in
low to ultra-low permeability gas reservoirs.
Conclusions:1. The elliptical boundary model is an essential tool for the
diagnosis and analysis of production data from hydrauli-cally fractured wells in low to ultra-low permeability gas
reservoirs.
2. The diagnostic matches of the production data presented inthis work are excellent — despite the fact that the quality ofthese data, while reasonable, is certainly not comparable to
the quality of data that can be acquired in the market ( e.g.,continuous downhole pressure and surface rate measure-ments). In practice, the elliptical boundary model should
be appropriate for the analysis/diagnosis of production data
for virtually all cases of hydraulically fractured wells intight gas reservoirs.
Recommendations/Comment : We note that the solution for the
elliptical boundary model (Appendix A) is extremely tedious
— at present, the solution is not-suited for dynamic analysishistory matching (i.e., computational times for this solution
can take days on a personal computer). The only practica
mechanisms for application of this solution are the preparation
of data tables (for interpolation) and the generation of an ap propriate suite of "type curves" (see Appendix C for example
12 S. Amini, D. Ilk, and T. A. Blasingame SPE 106308
Figure C.1 — Type curve for a fractured well centered in a closed (homogeneous) elliptical reservoir —FE=1, various 0-values;qD functions versus tDA format.
Figure C.2 — Type curve for a fractured well centered in a closed (homogeneous) elliptical reservoir —FE=10, various 0-values; qD functions versus tDA format.
SPE 106308 Evaluation of the Elliptical Flow Period for Hydraulically-Fractured Wells in Tight Gas Sands — 13
Theoretical Aspects and Practical Considerations
Figure C.3 — Type curve for a fractured well centered in a closed (homogeneous) elliptical reservoir —FE=100, various 0-values; qD functions versus tDA format.
Figure C.4 — Type curve for a fractured well centered in a closed (homogeneous) elliptical reservoir —FE=1000, various 0-values; qD functions versus tDA format.