UNCONVENTIONAL RESERVOIR ENGINEERING
PROJECT
PHASE I
WORK STATEMENT
Erdal Ozkan
Marathon Center of Excellence for Reservoir Studies (MCERS)
Colorado School of Mines
Golden, Colorado, USA
August 1, 2012
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1. PREAMBLE
In the last two decades, tight unconventional gas and oil reservoirs have gained an impeccable
standing among our energy resources. This is not only because of the current share of these
unconventional reservoirs in our hydrocarbon production capacity, but also because of the
common conviction in their growing future potential. The emergence of the unconventional
reservoirs as a prevalent energy resource has been mainly due to the technological advances in
horizontal-well drilling, completion, and fracturing practices. Despite the impressive
technological advances, however, the understanding of physical mechanisms governing fluid
production from these tight unconventional resources has been limited. As the long-term
reservoir-management concerns have started offsetting the initial hype about unconventional
shale-gas and liquids-rich reservoirs, interest in genuinely-unconventional reservoir-engineeringresearch has also started growing lately.
2. SIGNIFICANCE
The significance of the proposed research is in the pressing need of the industry to improve
characterization and modeling of flow in nano-pore, nano-Darcy, highly fractured and layered
unconventional reservoirs, such as shale-gas and liquids-rich formations. This task, however,
cannot be achieved only by modifying the conventional reservoir engineering theory and
practices and revising the available tools and techniques. What is currently available was
developed for fluid-flow problems in conventional rocks, which possess categorically different
properties from the unconventional reservoirs under scrutiny today. Consequently, the early
approach of the industry to respond to this need by an adjustment of scale of the porous medium
properties and incorporation of multiple fractured horizontal wells into conventional flow models
has not yielded satisfactory results.
3. GENERAL OBJECTIVE
The general objective of this project is to discover tight unconventional oil and natural gas
reservoirs with a view to attain a more complete reservoir engineering understanding and develop
more appropriate reservoir engineering tools and practices for these reservoirs. This objective
covers the entire spectrum of reservoir engineering research of nano-pore, nano-permeability,
microfractured, unconventional formations. Under scrutiny are the discerning physical
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characteristics, non-Darcy flow mechanisms, overlooked capillary and surface-forces
relationships, unaccustomed multi-phase flow concepts, and new fluid exchange mechanisms
between fractures and the rock matrix. Development of reservoir models, analysis techniques, and
prediction tools are also part of the research spectrum.
4. RESEARCH TOPICS
The following is a list of the potential research topics for Phase I. These topics have been selected
as a result of our communications with some of the potential members. They also reflect our areas
of expertise based on our past and present research programs. The final selection of the research
topics and the allocation of funds will be determined based on the level of membership and the
interests of the paid members.
I.
Unconventional Flow Mechanisms in Shale-Gas Reservoirs
Objective: Discover, document, and formulate unconventional, flow mechanisms
contributing to gas production from the heterogeneous pore systems of shale
formations.
Description: Darcy-flow assumption used for conventional reservoirs breaks down in
nano-pore systems of unconventional shale-gas reservoirs where surface forces and
molecular level interactions become significant. These forces and interactions give
rise to flow physics unaccounted for in conventional reservoirs. In this study, mixed
flow models incorporating different flow mechanisms as a function of pore sizes,
reservoir PVT conditions, pore-surface interactions, and desorption characteristics
will be developed. Unlike the current literature, which defines unconventional flow
mechanisms in uniform pore networks and assumes linearly additive fluxes due to
different flow mechanisms, this research will examine the effect of heterogeneous
pore-size distributions and the linearity of flux addition.
Tasks: (i) Document potential flow mechanisms in nano-pores, (ii) describe and
formulate Knudsen diffusion in nano-pores, (iii) examine surface forces and
molecular level interactions, (iv) explore desorption characteristics at pore-level, (v)
delineate the practical conditions for different mechanisms to dominate or coexist,
and (vi) investigate the effect of heterogeneous pore-size distributions on total flux
formulations.
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Deliverables: (i) Documentation of unconventional flow mechanisms in shale-gas
reservoirs, (ii) formulations of flux relations for each mechanism as a function of
pore-size distributions, and (iii) a total flux definition for mixed flow in
unconventional reservoirs.
II.
Coupling Fracture and Matrix Flows in Tight Unconventional Reservoirs
Objective: Develop a matrix-fracture coupling procedure for tight unconventional
reservoirs by introducing a boundary layer to compensate the different orders of flux
relations in the matrix and fracture media.
Description: When flow in a nano-pore matrix and high-permeability fracture
medium is considered, it is necessary to be concerned about the mismatch of the
orders of the flux equations for the two media. In conventional dual-porosity
formulation of fractured reservoirs, Darcy flow is assumed in both fractures and
matrix leading to first order velocity equations for both media. In shale-gas
formations, for example, slip flow in shale matrix leads to a second-order flux term
while Darcy flow in the fractures is represented by a first order flux relation.
Likewise, consideration of pressure-dependent matrix and fracture properties lead to
non-matching flux relationships at their interface. Under these conditions, coupling
flows in matrix and fracture media requires special considerations. Similar
considerations have been noted in the literature, e.g., by Beaver and Joseph (1967)
and Brinkman (1949), in different context, but the matrix-fracture coupling issues for
unconventional reservoirs have not been reported. In this reserach, analytical
modeling techniques will be used to define the boundary layer. The results will be
applicable to reservoir simulation for the dual-porosity representation of flow in
naturally fractured, tight, unconventional reservoirs.
Tasks: (i) Build an analytical model to represent fluid exchange between matrix and
fracture media under different flux laws, (ii) Introduce a fictitious boundary layer
between the matrix and fracture to absorb the effects of the non-matching flux
conditions at their interface, (iii) Determine the characteristics of the boundary layer,
especially the thickness and mobility, which are needed to create a smooth transition
from the matrix to fracture and thus to lead to a stable solution of the pressure and
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flux, and (iv) Document the practical applications for analytical and numerical
modeling of flow in unconventional tight reservoirs.
Deliverables: (i) Boundary layer formulation to couple matrix and fracture flows in
unconventional tight reservoirs, (ii) a dual-porosity transfer function incorporating
new coupling considerations, and (iii) computational procedure for the non-linear
flow equation and application examples for analytical and numerical modeling.
III.
Dual-Porosity Modeling for Shale-Gas Reservoirs with Macro- and Micro-Fractures
Objective: Extend the classical dual-porosity formulations to shale-gas reservoirs
with macro- and micro-fractures and define the appropriate transfer functions.
Description: Fractures in shale may be categorized as micro-fractures due to
hydrocarbon generation in source rocks and macro-fractures primarily due to tectonic
events, faulting, folding, uplift, etc. Dual-porosity formulations are convenient for
fractured shale-gas reservoirs where discrete fracture models are not the norm.
Micro-fractures in shale cannot be considered as the third porosity because they do
not form a continuum. The contribution of unconnected micro-fractures in shale
matrix is negligible except for those that touch the surface of the matrix block (like
wormholes) and cause a matrix-surface stimulation effect. In this project, a nested
dual-porosity solution will be developed by coupling three flow regions,
homogeneous matrix-core, micro-fractured matrix-surface-layer, and the macrofracture network.
Tasks: (i) Develop a solution for flow in a shale matrix block with a surface-layer of
microfractures, (ii) develop a nested dual-porosity solution incorporating the macrofracture network, and (iii) define the appropriate dual-porosity transfer function, and
(iv) incorporate the transfer function into a fractured horizontal well solution
(trilinear model) to study the effects of macro- and micro-fractures on productivity.
Deliverables: (i) A nested solution to account for the interactions among shale
matrix, micro-fractures in matrix, and the macro-fracture network in shale-gas
reservoirs, (ii) new dual-porosity transfer function incorporating the effects of macroand micro-fractures in shale-gas reservoirs, and (iii) an analytical fractured horizontal
well solution for shale-gas reservoirs with macro- and micro-fractures.
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IV.
Thermodynamics of Multi-Phase Flow in Liquids-Rich Reservoirs
Objective: Investigate the thermodynamics of gas-phase formation and multi-phase
flow behavior in liquids-rich reservoirs with heterogeneous distribution of pore-sizes.
Description: The flow of undersaturated oil in tight unconventional reservoirs has not
been investigated as much as the gas-flow through the same kind of rocks. In addition
to the unconventional aspects of oil flow in the tight rock matrix, it is essential to
consider the interactions of the capillary and surface forces in nano-pores under
multi-phase flow conditions. This is because most of the pore volume of these
liquids-rich formations is in the form of nano-pores which have much larger surfacearea to volume ratios compared with the micro-pores of conventional reservoirs. In
nano-pores, competing capillary and surface disjoining forces lead to thermodynamic
effects not considered in conventional multi-phase flow models. In this project,
nucleation,
capillary
condensation/evaporation,
and
adsorption/desorption
mechanisms will be considered to explain and model gas-bubble formation and
growth in micro- and nano-pores of liquids-rich reservoirs. The formation of a
continuous gas phase and its flow along with the existing liquids and condensates in a
corrugated pore structure of random sizes will also be investigated. The impact of
ignoring capillary pressure and surface disjoining forces (like Van der Waals,
structural, adsorption) in conventional modeling of phase behavior will be examined
and the range of pore sizes that this approximation is valid will be determined. The
stability of bubbles in pore cavities due to the balance between capillary and
disjoining forces will be modeled and the possibility of nano-pores to serve as
nucleation sites for bubble formation will be demonstrated.
Tasks:
(i)
Investigate
nucleation,
capillary
condensation/evaporation,
and
adsorption/desorption as potential mechanisms of gas-bubble formation in various
sizes of pores, (ii) explore the interactions of capillary and disjoining forces,
including molecular (Van der Waals), adsorptive, electrostatic, and structural surface
forces, in pore spaces, (iii) determine the conditions of formation of free gas in
micro- and nano-pores, (iv) develop a relationship between pore size and
supersaturation in intermediate pore sizes, (v) develop appropriate PVT relationships
incorporating unconventional thermodynamics in nano-pores, (vi) discuss the
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application and significance of the findings for the numerical simulation of liquidsrich reservoirs.
Deliverables: (i) Thermodynamics of multi-phase flow in liquids-rich reservoirs, (ii)
conditions of gas bubble formation in heterogeneous pores, (iii) critical gas saturation
for shale-gas reservoirs, (iv) PVT relationships for liquids-rich reservoirs, and (v)
guidelines for implementation in reservoir simulation.
V.
Pressure-Dependent Natural-Fracture Permeability in Tight, Unconventional
Reservoirs
Objective: Investigate the pressure dependency of fracture networks in shale and
other tight formations and its impact on the overall productivity of the reservoir.
Description: Existing knowledge of permeability alterations due to rock compaction
and fracture closure, and the preliminary experimental data indicate that stressdependent conductivity of poorly propped natural fractures has considerable impact
on long-term production of tight unconventional reservoirs. Closure of poorly
propped natural fractures when pressure drops with production creates an effective
skin around hydraulic fractures and causes significant productivity reduction. In this
project, experimental and analytical techniques will be used to develop practical
correlations to represent natural-fracture permeability as a function of pressure in
tight unconventional reservoirs and the effect of pressure dependency of natural
fracture permeability will be incorporated into a model of fractured horizontal wells.
One of the Two major issues to be addressed in the project will be the experimental
measurement of fracture permeability in a tight core sample and the non-linearity of
the flow equations caused by pressure-dependent permeability.
Tasks: (i) Review the existing models for the pressure sensitivity of permeability, (ii)
define an experimental procedure and interpretation model to determine pressuredependent fracture permeability from core measurements, (iii) collect experimental
data, (iv) develop correlations for pressure-dependent natural-fracture permeability,
(v) incorporate the correlations into an analytical model of a hydraulically fractured
horizontal well in shale to delineate the effect of pressure-dependent natural-fracture
permeability on productivity.
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Deliverables: (i) Stress-dependent permeability correlations applicable for fractures
in unconventional reservoirs, (ii) fractured, horizontal, shale-gas well model with
stress-dependent fracture permeability, and (iii) consequences of near-wellbore
fracture closure and identification from production data.
VI.
Analytical Modeling of Interference between Wells in Shale-Gas Reservoirs
Objective: Develop an analytical model to predict, interpret, and analyze interference
effects between wells in shale-gas reservoirs.
Description: This project will develop a two-well model in a shale-gas reservoir to
study the interference between wells. The model will be versatile to consider
different combinations of fractured vertical and horizontal wells. Intended uses of the
model will include (i) investigation of the conditions of interference between wells,
(ii) deciphering the extend of the stimulated (fractured) reservoir volume (SRV)
around wells, (iii) studies of optimum well spacing, (iv) new interference-test
applications to estimate reservoir characteristics. Initially, uniform distribution of
identical hydraulic fractures along the horizontal well will be considered with the
option to remove these assumptions in the later phases. The SRVs around the wells
will be modeled as a dual-porosity zone and the model will be capable of considering
a homogeneous or dual-porosity reservoir beyond the SRVs. Pseudopressure
linearization of gas-flow equations will be used.
Tasks: (i) Develop the algorithm and the computational code, (ii) verify the model,
(iii) run a sensitivity analysis for the key parameters, (iv) document the interference
characteristics for some common well configurations, and (v) demonstrate and
document the practical consequences.
Deliverables: (i) The algorithm and the computational code, (ii) procedures to
interpret interference effects, (iii) interference-test analysis, and (iv) general
guidelines for the use of the model and example applications.
VII.
An Efficient Production-Data Analysis Algorithm for Layered Tight-Gas Reservoirs
Objective: Develop an algorithm and the computational code to analyze production
data from fractured vertical wells in layered, tight-gas reservoirs.
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Description: Because of the increased emphasis on tight- and unconventional-gas
reservoirs in the last decade, new gas-production-data-analysis techniques have
emerged. However, despite that layering is a key characteristic affecting production
and ultimate recovery in most gas reservoirs, the existing production-data-analysis
techniques fundamentally assume a single producing layer. Bundling multiple layers
with contrasting properties into a single zone of average properties for analysis
purposes causes the loss of essential information about layer production and
depletion characteristics. This project will develop a robust algorithm for practical
interpretation of continuous production and pressure data of fractured vertical wells
in layered, tight-gas reservoirs. Because practical applications of production-data
analysis require fast and iterative evaluations, the algorithm will be based on an
analytical model of gas flow in a multi-layer reservoir. The model will accommodate
different layer skin factors and unequal initial-pressures in layers. As a viable
condition for long-periods of production, shut-in periods for all or selected layers will
also be permitted. The analysis procedure will require the availability of production
logs indicating the break-down of the well production into layers.
Tasks: (i) Develop an analytical model for flow in multi-layer reservoirs with
contrasting layer characteristics, including natural fractures, and non-identical
hydraulic fractures in layers, (ii) test the sensitivity of the well responses to layer
properties and assess the possibility of estimating layer properties from production
data, (iii) set up the procedure to process the field data to be analyzed by the
analytical model, and (iv) demonstrate the analysis procedure.
Deliverables: (i) The analytical model and the algorithm, (ii) procedure to process
and prepare the field data for analysis, (iii) analysis procedure, and (iv) application
examples.
5. PAST AND CURRENT RESEARCH
From the emerge of the unconventional reservoirs as a significant resource, we have been
involved in research to improve our understanding of reservoir flow mechanisms, develop
analysis tools and interpretation models, and increase the production efficiency by optimizing
well completion and stimulation treatments. Our research has introduced some key concepts of
current unconventional reservoir engineering, contributed to the basis of the development of tight
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unconventional reservoirs, and developed several models used by the industry for the analysis and
interpretation of pressure and production data. We have published and presented over a dozen
technical papers and, currently, seven graduate students in our group are conducting research on
topics directly related to unconventional reservoir engineering. A partial list of our publications
and ongoing research work on unconventional reservoir engineering topics is provided below.
i) Papers and Presentations (Last 5 Years):
* Medeiros, F., Ozkan, E., and Kazemi, H.: “Productivity and Drainage Area of Fractured
Horizontal Wells in Tight Gas Reservoirs,” SPE Reservoir Evaluation & Engineering (Oct. 2008)
902-911
* Medeiros, F., Ozkan, E., and Kazemi, H.: “A Semi-Analytical Approach to Model PressureTransients in Heterogeneous Reservoirs,” SPE Reservoir Evaluation & Engineering (April 2010)
341-358.
* Medeiros, F., Kurtoglu, B., Ozkan, E., and Kazemi, H.: “Analysis of Production Data from
Hydraulically Fractured Horizontal Wells in Shale Reservoirs,” SPE Reservoir Evaluation &
Engineering (June 2010) 559-568.
* Ozkan, E., Brown, M., Raghavan, R., and Kazemi, H.: “Comparison of Fractured HorizontalWell Performance in Conventional and Unconventional Reservoirs,” SPE Reservoir Evaluation &
Engineering (April 2011) 248-259.
* Raghavan, R., and Ozkan, E.: “Flow in Composite Slabs,” SPE Journal (June 2011) 374-387.
* Medeiros, F., Kurtoglu, B., Ozkan, E., and Kazemi, H.: “Pressure-Transient Performances of
Hydraulically Fractured Horizontal Wells in Locally and Globally Naturally Fractured
Formations,” paper SPE-IPTC 11781, presented at the International Petroleum Technology
Conference, Dubai, U.A.E., 4–6 December 2007.
* Ozkan, E., Raghavan, R., and Apaydin, O. G.: “Modeling of Fluid Transfer from Shale Matrix
to Fracture Network,” paper SPE 134830, to be presented at the 2010 SPE Annual Technical
Conference and Exhibition, Florence, Italy, Sept. 19–22, 2010.
* Ozkan, E.: “On Non-Darcy Flow in Porous Media – Modeling Gas Slippage in Nano Pores,”
paper presented at the SIAM Conference on the Mathematical and Computational Issues in the
Geosciences, Long Beach, California, March 21–24, 2011.
* Apaydin, O. G., Ozkan, E., and Raghavan, R.: “Effect of Discontinuous Microfractures on
Ultratight Matrix Permeability of a Dual-Porosity Medium,” paper CSUG/SPE 147391 presented
at the Canadian Unconventional Resources Conference, Calgary, Alberta, Canada, Nov. 15-17,
2011.
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ii) Current Students’ Thesis and Dissertation Topics
* Apaydin, O. G., PhD Study: “New Coupling Considerations between Matrix and Fracture in
Unconventional Resource Reservoirs.”
* Firincioglu, T., PhD Study: “Phase Behavior in Unconventional Tight Reservoirs.”
* Eker, I, PhD Study: “Analysis of Production Data from Layered Tight-Gas Reservoirs,”
* Cho, Y., MSc Study: “Effects of Pressure-Dependent Natural-Fracture Permeability on ShaleGas Well Production.”
* Carratu, J. C, MSc Study: “Optimization of Fractured Horizontal Well Performance in ShaleGas Reservoirs.”
* Greewood, J., MSc Study: “A Comprehensive Analytical Model of Fractured Horizontal Wells
in Shale-Gas Reservoirs.”
* Torcuk, M. A., MSc Study: “Well Interference Effects in Unconventional Tight Reservoirs.”
6. MEMBERSHIP AND THE ADVISORY BOARD
The membership fee for the budget is $90,000 for two years according to the following fee
schedule:
(i) $45,000 on October 1, 2012
(ii) $45,000 on October 1, 2013.
If on or before January 31, 2013, four participating members are not secured, the Project may be
canceled at the sole option of CSM, but after consultation with the paid members, and the
research fees paid will be returned.
Each Sponsor will designate a technical project representative to consult with CSM personnel
from time to time and represent the Sponsor at the Advisory Board of the Project. The Advisory
Board will serve to make recommendations and provide directions to the Project Director,
representing CSM, who will have the formal authority to make the final decisions about the
selection of the research topics and allocation of the budget items. The Advisory Board will meet
twice a year unless called on special occasions by the members and the Project Director.
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7. TIMELINE
Phase I of the Project is planned to start on October 1, 2012 and end on September 30, 2014.
Specific timelines for the tasks and deliverables will be decided after securing the critical
membership and determining the totals funds available.
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