Unit 1
Topics:
❖Principles Of Fluid Flow For
Steady State
❖Semi Steady State & Unsteady
State Conditions
❖Diffusivity Equation Derivation
& Solutions
❖Radius Of Investigation
❖Principle Of Superposition
❖Horner’s Approximation
Introduction
• Well Testing is a technique and method for the evaluation of well
condition and reservoir characteristics.
• It involves producing a well at constant rate or series of rate some of
which may be zero ( well closed in), While simultaneously raking a
continuous recording of the changing pressure in the well bore using
form of pressure recording device
• PTA – Pressure Transient Analysis
• Qualitative analysis of reservoir properties
• Creating a pressure disturbance in the reservoir with changing flow
rate (q) and recording a pressure response in the bottom of the
reservoir.
• E.g – FBHP is recorded as a function of time.
Some parameters estimated are:
–Permeability
–Near-wellbore conditions: damage etc.
–Boundary distance
–Reservoir connected volume
–Average reservoir pressure
–Reservoir temperature
Diagram:
Well Test : How it is done?
Well is flowing
allow to flow
Sensor is lowered
Pwf
Pres
Well Test : How it is done?
Well is flowing
Closed, Q = 0
q
Flow
dropsProduction
to zero.
Constant
t
pwf
Stabilized
Pressure
Pressure Buildup
Starts
Pwf
Pres
t
Why it should be done?
Well test provides information that can be used for reliable panning
of oil and gas field production and development, and reducing the
financial risk in investment in oil and gas projects. Followings are
list of important information that well test can provide:
•
•
•
•
Well test provides information not available in static model of the reservoir.
Increases reliability of geologic interpretation and reserve estimates.
Provides actual and potential productivity of the well tested.
Collects reservoir representative fluid sample for PVT and phase behavior analysis, and flow
assurance studies.
• Helps in identifying well problems such as sand production, asphaltene or paraffin
presence, water or gas coningetc.
When it should be done?
Well test can be done at any stage of reservoir life cycle. The purpose of
carrying out well test at each stage may be different. Some typical
objective of well test at different stages of reservoir life cycle are given
below:
• Exploration: Reservoir and Fluid Properties, Reservoir
Deliverability.
• Appraisal or Early Stage of Field Development: Refine previous
interpretation.
• Production: Reservoir Management: need for well treatment,
interference testing .
Types and purpose of well test:
• Pressure transient tests
• Generate and measure pressure changes with time
• Deliverability tests
• Well controlled production
Production Data Analysis:
• Use of Production Data for Well Test Analysis
➢ Reservoir properties
• (permeability, skin factor, fracture half-length etc).
➢ Reservoir pore volume
• (estimated using long-term production performance).
➢ Estimated Ultimate Recovery (EUR) Movable fluid
volumes.
Objective:
• Characterize reservoir
• Estimate average drainage area pressure
• Define reservoir limits
• Diagnose productivity problems
• Evaluate stimulation treatment effectiveness
Typical Objectives:
➢ Check the presence of hydrocarbons in the reservoir
➢ Measure the reservoir fluid flow rate under typical operating conditions
➢ Measurement of reservoir temperature and pressure
➢ Estimate of the flow capacity (Kh) of the producing interval of thickness h
➢ Identify reservoir heterogeneities
➢ Estimate connected volume of the reservoir and establish connectivity among
wells
➢ Evaluate properties of multi-later reservoirs
•(A multi-layer test should involve running a production-logging tool (PLT) string to
determine flow contributions from each layer.)
Typical objective
➢Check injective capacity of reservoir
➢Determine the level of damage or stimulation near the wellbore
➢Estimate well productivity and absolute open-flow (AOF)
potential
➢Obtain representative fluid samples for PVT and phase
behavior analysis and flow assurance studies
Issued to conducted well test
➢ Type of well test depends on –
• Fluid type: Oil or Gas, Volatile, Possibility of Hydrates, Sour Crude,
Flow : Injection or Production
• Pressure- Temperature Condition: High-Pressure & High-Temperature (HPHT) or normal
Objective of Test: Extended Well Test, Interference, Pulse Test, Multi-rate, Multilayer
tests.
➢ Equipment's : Downhole and Surface tools, equipment's, instrumentsetc.
➢ Technology: Special technology requirement such as HPHT, viscous crude, hydrate conditionsetc.
➢ Cost – rig rates, test duration, services cost
➢ Reservoir Geology: Structure and Stratigraphy
➢ Lead time depends on
•Location: remote locations or easily accessible location, offshore/onshore, Arctic Conditions,
Deepwater, Forest Area, HillyTerrains
• Specialized Equipment Requirement: Sour reservoir fluid, local environment rules,
• Other condition: logistics, planning, manufacturing of equipment, and transportation to location.
➢ Environment: Special conditions: zero emissions, contingency forspills
Types of well test:
Single well test
1. Build up test
2. Drawdown test
3. Injective test
4. Fall off test
Multi well test
1. Interference test
Single well test
Well is flowing
allow to flow
Sensor is lowered
Pwf
Pres
Single well test
Well is flowing
Closed, Q = 0
q
Flow
dropsProduction
to zero.
Constant
t
pwf
Stabilized
Pressure
Pressure Buildup
Starts
Pwf
Pres
t
Multi-Well Tests
Multi Well Test
Well is flowing
flowing
Well is shut
in.
Pressure and flow are
recorded at offset well.
q
Sensor is lowered
in the offset well.
t
pwf
Pwf
Pres
Pwf
Pres
t
Multi-Well Tests
Multi Well Test
Well is Shut
in.
flowing.
Well is flowing.
shut in.
Pressure and flow is
recorded at offset well.
q
Sensor is lowered
in the offset well.
t
pwf
Pwf
Pres
Pwf
Pres
t
Types: Build up
Well is flowing.
shut in.
Pressure and flow are
recorded at well.
q
Pws
Sensor is lowered
in the offset well.
t
t
pwf
Pwf
Pres
t
Types: Draw Down
Well is Shut
in.
flowing.
Pressure and flow is
recorded at offset well.
q
Sensor is lowered
in the offset well.
Pwf
t
pwf
t
Pwf
Pres
t
Types: Injection fall off test
Pressure and flow
recording at the well.
injected.
Well is shut
in.
q
Q =0
Pws
t
Sensor is lowered
in the offset well.
p1hr
pwf
pi
Injection Time, t
Semi Log Plot
(Horner Plot)
Pwf
Pres
t
Types: Injection Test
Pressure and flow
recording at the well.
Well is Shut
in.
injected.
q
t
Q =0
p1hr
Pwf
Sensor is lowered
in the offset well.
pwf
ps
Injection Time, t
Semi Log Plot
Pwf
Pres
t
Types: Interference test
Well is Shut
shut in.
Well is flowing.
shut in.
Pressure and flow is
recorded at offset well.
q
Q =0
Qinj
Sensor is lowered
in the offset well.
t
pwf
pi
Pwf
Pres
Pwf
Pres
t
Types of Reservoir flow:
• Transient flow - Pressure transient migrates outwards from the well
without encountering boundaries
• Steady state flow – Pressure transient reached all out the boundaries
but the static pressure at the boundary doesn't decline. This is often
called as constant pressure boundary
• Pseudo steady state flow – Pressure transient reached all out the
boundaries and the static pressure declining at the boundary and
uniformly through out the reservoir.
• Boundary dominated flow – Pressure transient reached all out the
boundaries and the static pressure declining at the boundary but not
uniformly flow rate is not constant. This is also called as tank type
flow.
Effect of depletion:
Well bore storage:
• Well bore storage effect defined as immediately after the production
starts up or shut in, when the pressure behavior is in early times is
dominated by compressibility and volume of wellbore fluid.
• At the start of flow period production at the surface is due to the
expansion of fluid in the wellbore and not in the reservoir. There is
a time lag when the surface production rate become consistent with the
sand face rate and define the wellbore storage period.
• At the shut-in well fluid gets compressed in the wellbore and the rate of
the sand face gradually goes to zero
• Wellbore storage is a function of wellbore fluid and wellbore volume.
Note: compressibility of fluid in the wellbore is greater than the oil in
the reservoir condition because oil release gas
Graph
Surface Flow rate
Buildup
Sand face Flow rate
Skin:
• It is a zone of altered permeability in the vicinity of wellbore
Effect of wellbore radius:
• The value of wellbore radius that produce equivalent result to those obtained using
skin factor is zero. It is possible to represent departures from ideal behavior by
using a skin effect or alternative changing value in the effective wellbore radius.
• For positive skin effect the effect of wellbore radius is smaller than the actual
wellbore radius.
• For a negative skin effect such as obtained after well stimulation effective
wellbore radius is large than the actual well radius.
Radius of Investigation:
• It represents the how far into the reservoir the transient effects have
travelled
• A pressure transient is created when a disturbance such as a change
in rate occurs at a well. As time progresses, the pressure transient
advances further and further into the reservoir. Theoretically when a
pressure disturbance is initiated at the well it will have an immediate
effect however minimal at all points in the reservoir. At a certain
distance from the well however the effect of the disturbance will be
so small as to be un measurable. The furthers distance at which the
effect is detected is called radius of investigation
Principle o super position:
• The superposition principle asserts that the total pressure change at a
point in the reservoir is a liner sum of the changes in pressure due to
each well in the reservoir
• The superposition principle implies that a pressure disturbance will
propagate through the reservoir even if the source of disturbance
change or disappears.
1. Effect of rate change
2. Effect of multiple wells
3. Effect of boundary
Horner Plot:
• Applying the superposition Principe to a shut in well the total
pressure change i.e (Pi- Pws) which occurs at the wellbore during the
shut- in time Δ𝑡, is essentially the sum of the pressure change caused
by the constant flow rate (Q-0) and that (0-Q)
• Assumptions:
1. Reservoir is homogeneous, isotropic slightly compressible single
phase fluid
2. Infinite acting reservoir
3. Ei or log approximation is applied