Overview of Reservoir Simulation by Prem Dayal Saini Reservoir simulation is the study of how fluids flow in a hydrocarbon reservoir when put under production conditions. The purpose is usually to predict the behavior of a reservoir to different production scenarios, or to increase the understanding of its geological properties by comparing known behavior to a simulation using different geological representations.

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Overview of Reservoir Simulation
By:
Prem Dayal Saini

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What is a Petroleum Reservoir?
• A petroleum reservoir or an oil and gas reservoir (or system), is a subsurface
pool of hydrocarbons contained in porous rock formations. The naturally
occurring hydrocarbons are trapped by overlying rock formations with lower
permeability.

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What is Reservoir Simulation?
• Reservoir simulation is the study of how fluids flow in a hydrocarbon reservoir
when put under production conditions. The purpose is usually to predict the
behavior of a reservoir to different production scenarios, or to increase the
understanding of its geological properties by comparing known behavior to a
simulation using different geological representations.
• Reservoir simulator is a tool for predicting hydrocarbon reservoir performance
under various operating strategies developed by combining physics,
mathematics, reservoir engineering, and computer programming.
• Simulator + Simulation Engineer + Reservoir description

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Why Do We Need Reservoir Simulation?
• What is the most efficient well spacing?
• What are the optimum production strategies?
• Where are the external boundaries located?
• What are the intrinsic reservoir properties?
• What is the predominant recovery mechanism?
• What and how should we employ infill drilling?
• When and which improved recovery technique should we implement?

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Simulation Approaches
• The analytical approach:
Involves a great deal of assumptions-in essence, it renders an exact
solution to an approximate problem (not used much).
• The numerical approach:
Attempts to solve the more realistic problem with less stringent
assumptions-in other words, it provides an approximate solution to an
exact problem.
• The domain of interest :
Focus on a single well, entire field or a section of the reservoir

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Reservoir Simulators
Simulators are the combination of :
1. Flow equations i.e. Mathematical model
2. Algorithms for solving the flow equations (rock and fluid properties)
3. Computer program commands

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Steps of a Simulation Study
• Setting objectives
-Fact-finding, Optimization
• Selecting the model and approach
-Reservoir complexity
-fluid type
-Scope of the study
• Gathering, collecting and preparing the input data
• Planning the computer runs, in terms of history matching and/or
performance prediction
• Analyzing, interpreting and reporting the results

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Data Collection
• Data collection
• Geological Information

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Steps of a Simulation Study…
• Setting objectives:
-Fact-finding - History matching (eg. Well test data to find the damaged zone)
-Optimization

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Overview of how to set up a Simulation case in
Petrel using Eclipse Simulators
1. Build a grid and populate it with properties.
2. (Optionally) scale up the structure and properties onto a coarser grid.
3. Define or import well paths.
4. Define or import well completion events.
5. (Optionally) import historical production rates.
6. (Optionally) define a well segmentation set.
7. Define a fluid model, describing the properties of the reservoir fluids at varying
pressures, volumes and temperatures (PVT) and the initial conditions (pressures and
contacts) in the reservoir.
8. Define a saturation function, describing the relative permeability and capillary pressure
of the fluids as a function of saturation.
9. Define a rock compaction function, describing how the rock expands and compresses
with changing pressure.
10. Define aquifers, describing the type, size and connections of the acting
11. Define a development strategy to control how the wells will produce and inject.
12. Define a simulation case, putting all the above data objects together.
13. Analyze and view the results using the Function window and the Results pane.

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Physical principles
• Conservation of Mass
The first principle is modeled by a so called partial differential equation that says
that if there is a difference between flow into and out of a tiny volume of space,
this will either cause a build-up or a draw down of mass in this tiny volume.
• Conservation of Momentum
The second principle is approximated by an experimental law, called Darcy's law,
that relate the pressure difference (force) across a porous rock containing a fluid
and the resulting velocity (momentum) of this fluid. This experimental law is also
represented by a partial differential equation that is combined with the equation
based on the first principle to form a set of partial differential equations that is the
mathematical description of the flow of fluids through a porous media.
• Conservation of Energy
The third principle is approximated by a relationship between Pressure, Volume
and Temperature (PVT) for the rock and the fluids. The PVT data is represented
by a set of tables for each reservoir.

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Upscaling
• Build a grid and populate it with properties.
• Scale up the structure and properties onto a coarser grid.
-Upscaling is the process of creating a coarser (lower resolution) grid based
on the geological grid which is more appropriate for simulation

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Grid Systems
• Body-centered grids

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Grid Systems continued …
• Mesh-centered grids

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Well Design
Define or import well paths.
-The Well path design process is a tool which enables users to generate well
trajectories based on reservoir properties, seismic attributes or any other
data

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Well Completion Design
• Define or import well completion events
-Well completion consists of sealing off a drilled well in preparation for production.
1. Casing
2. Liner
3. Tubing
4. Packer
5. Perforation

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Well Segmentation
• Define a well segmentation set
-Conventional well models treat the entire wellbore as a single entity, averaging
all the fluid properties in the well bore.
-Well segmentation divides the wellbore into segments, much like the reservoir is
divided up into grid-cells.

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Making a fluid model
• Define a fluid model, describing the properties of the reservoir fluids at varying
pressures, volumes and temperatures (PVT) and the initial conditions (pressures
and contacts) in the reservoir.
• Black oil fluid models are defined by specifying several properties such as
viscosity, density and volume formation factors for each of the fluid phases

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Reservoir Rock/Fluid Interactions
• Wettability and interfacial tension .

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Reservoir Flow Geometries
• Rectangular flow geometry

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Reservoir Flow Geometries continued …
• Radial-cylindrical flow geometry

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Reservoir Flow Geometries continued …
• Elliptical-cylindrical flow geometry

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Reservoir Flow Geometries continued …
• Curvilinear flow geometry

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Making rock physics functions
• Define a saturation function, describing the relative permeability and
capillary pressure of the fluids as a function of saturation.
• Saturation functions
– are tables showing relative permeability and capillary pressure versus
saturation.
-gas-oil and water-oil capillary pressure versus saturation
Define a rock compaction function, describing how the rock expands and
compresses with changing pressure
Rock compaction functions are tables showing pore volume multipliers versus
pressure, or a single rock compressibility value used by the simulator to calculate
the pore volume change.

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Reservoir Rock Properties
• Porosity
• Permeability

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Reservoir Rock Properties continued ….
• Homogeneous vs. heterogeneous systems

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Make aquifer
• Define aquifers, describing the type, size and connections of the acting aquifer.
• Aquifer modeling is a method of simulating large amounts of water (or gas)
connected to the reservoir whereby it is not essential to know how the fluid
moves in it, but rather how it affects our reservoir.

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Development Strategies
• Define a development strategy to control how the wells will produce and inject
• Development Strategies are used to describe to the simulator how a field will be
developed - that is, which wells will produce or inject, what rates and pressures
they will flow at, what operations will be carried out on the wells over time, and so
forth.

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Numerical Models: Time step selection
• Time is the "fourth dimension" in mathematical representations of flow dynamics
in porous media
• A typical simulation study may cover a number of years
• require subdividing this period into smaller time segments
• As time step size progressively increases, it is common for material balance
errors to appear
• Material balance Saturation and Pressure calculations flow rates
• Material balance checks help the engineer to determine the maximum time step
size that is admissable by the particular problem, and that can be tolerated by
the model we are using.

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Simulation case and Results
• Define a simulation case, putting all the above data objects together
• Analyze and view the results using the Function window and the Results
pane :
1. Summary Vectors
2. Properties
3. Streamlines
4. Simulation logs

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Streamline simulation
• For reservoirs where the movement of the fluids is mostly driven by
the potential field induced by producing wells:
1. Water flooding.
2. Highly heterogeneous reservoirs
• Streamline Calculation
• Streamline representation
Streamline method is the possibility it offers to do relatively quick simulations
on large geologically and architecturally complex models

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Streamline Simulation
• Built for speed (simplified model) - can handle millions of cells

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Reservoir Simulation and the Computational Environment
• Simple material balance calculations are now routinely performed on
desktop personal computers, while running a field-scale three-
dimensional compositional simulator may call for the use of a
supercomputer
• In designing a simulation study, we must always be aware of the
capabilities and limitations of our computing resources.

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History Matching, Prediction and Re-Simulating
• Manual history matching
• Automatic history matching
• Forecasting

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History Matching, Prediction and Re-Simulating
• Analysis of results
• Updating or re-simulating
• Progressive evolution of the reservoir model
• Progressive assessment of the simulation approach

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Types of Eclipse Simulators
• ECLIPSE Blackoil
• It is a fully implicit, three-phase,three-dimensional, general-purpose black oil
• simulator.
• The Blackoil model assumes the reservoir fluids consist of reservoir oil, solvent
gas and water.
• ECLIPSE Compositional
• It is useful when the behavior of the hydrocarbons is complex—condensate or
volatile crude oil, or gas injection developments fall in this category.

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Types of Eclipse Simulators
• ECLIPSE Thermal
• Thermal recovery methods are typically used in heavy oil reservoirs where the oil
viscosity is high at reservoir temperatures, but reduces as the temperature
increases.
• ECLIPSE FrontSim
• It is a three-dimensional, three-phase streamline simulator.

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Special Purpose Reservoir Simulators
• Water Coning Simulators
• Dual Porosity/Permeability Simulators
• Thermal Recovery Simulators
• Compositional Simulators
• Miscible Displacement Simulators
• Chemical and Polymer Flooding Simulators
• Coalbed Reservoir Simulators

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Thanks!