sonatrach's _well testing
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Development Phase
September October 2005abalt solutions limited - 2005
INTRODUCTION TO HYDROCARBON EXPLOITATION
Introduction to Hydrocarbon Exploitation
2005 Abalt Solutions Limited. All rights reserved
Well Testing
Section
By Pratap Thimaiah
WellTesting
2005 Abalt Solutions Limited. All rights reserved
Well Testing
Well testing procedures are a set of tools which
properly used can provide valuable clues as tothe condition of production or injection wells.
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Development Phase
September October 2005abalt solutions limited - 2005
INTRODUCTION TO HYDROCARBON EXPLOITATION
WellTesting
2005 Abalt Solutions Limited. All rights reserved
Well Testing
Requirements of the following parties should benormally considered:
Senior Management
Explorationists
Petroleum & Reservoir Engineers
Production & Facilities Engineers
Drilling Engineers
Oil & Gas Trading Partners
Regulatory Authorities
WellTesting
2005 Abalt Solutions Limited. All rights reserved
Generally oil or gas well production tests may
be classified as:
1. Periodic Production Tests
2. Deliverability Tests
3. Transient Pressure Tests
Well Testing
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Development Phase
September October 2005abalt solutions limited - 2005
INTRODUCTION TO HYDROCARBON EXPLOITATION
WellTesting
2005 Abalt Solutions Limited. All rights reserved
1. Periodic Production Tests
Determination of relative quantities of oil, gas and water producedunder normal producing conditions.
Aid in well and reservoir operations, meeting legal & regulatoryrequirements.
Productivity or Deliverability Tests
Performed on initial completions or recompletions to determinewell capability under various pressure drawdown.
Aid in selection of well completions & artificial lifts.
1. Transient Pressure Tests
High degree of sophistication and determines formation damage
or stimulation related to individual well or reservoir.
Reservoir parameters such as permeability, pressure, volume and
heterogeneities.
Well Testing
WellTesting
2005 Abalt Solutions Limited. All rights reserved
Periodic Production Tests
Run routinely to physically measure oil, gas &
water produced from a well under normalproducing conditions.
Provide physical evidence of well conditions.
Unexpected changes such as extraneous water
or gas production may signal well or reservoirproblems.
Abnormal production declines may meanartificial lift problems, sand fill up, scale etc.
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Development Phase
September October 2005abalt solutions limited - 2005
INTRODUCTION TO HYDROCARBON EXPLOITATION
WellTesting
2005 Abalt Solutions Limited. All rights reserved
For Oil Wells, results reported as
Oil production rates in bbl/day
Gas-Oil ratio in ft3/bbl
Water-oil ratio in % per system
For Gas wells
Gas production in mscf(1000 of standard cubic feetper day)
Gas production in mmscf
Hydrocarbon liquids or water in bcpmm
Periodic Production Tests
WellTesting
2005 Abalt Solutions Limited. All rights reserved
Productivity or Deliverability Tests
Physical or empirical determination of produced fluid flow versusbottom-hole pressure drawdown.
They predict what the well should produce at other pressuredrawdowns.
Successfully applied to non-Darcy, below bubble point flow conditions.
Do not permit calculation of formation permeability or degree of
abnormal flow restriction near wellbore.
Used as an indicator of well flow conditions.
They involve measurement of BH Static and flowing pressure as well asfluid rates produced to the surface
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Development Phase
September October 2005abalt solutions limited - 2005
INTRODUCTION TO HYDROCARBON EXPLOITATION
WellTesting
2005 Abalt Solutions Limited. All rights reserved
Types
For Oil Wells
Productivity Index
Inflow Performance
Flow after flow
Isochronal or modified isochronal
For Gas Wells
Flow after Flow
Isochronal or modified isochronal
Productivity or Deliverability Tests
WellTesting
2005 Abalt Solutions Limited. All rights reserved
Transient Pressure Tests
Essential to analyse the current behaviour and future performanceof the reservoir.
Designed to provide a quantitative analysis of the reservoirproperties.
Conducted by creating a pressure disturbance in the reservoir andrecording the pressure response at the wellbore (bottom-holeflowing pressure)
Most common tests are:
Pressure drawdown
Pressure build-up Multi-rate
Interference Pulse
Drill Stem
Fall off Injectivity
Step rate
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Development Phase
September October 2005abalt solutions limited - 2005
INTRODUCTION TO HYDROCARBON EXPLOITATION
WellTesting
2005 Abalt Solutions Limited. All rights reserved
Information available from a well test includes:
Effective permeability
Formation damage or stimulation
Flow barriers and fluid contacts
Volumetric average reservoir pressure
Drainage pore volume
Detection, length, capacity of fractures
Communication between wells
Transient Pressure Tests
WellTesting
2005 Abalt Solutions Limited. All rights reserved
Drawdown Test
Series of bottom-hole pressure measurements during a
period of flow at constant producing rate.
Well is shut-in prior to the flow test for a period of timesufficient to allow the pressure to equalize throughout theformation.
Main objective
To obtain the average permeability k of the reservoir within
the drainage area of the well. Sub-objectives
Assess the degree of damage of stimulation induced in the
vicinity of the wellbore through drilling and completion practices.
To determine the pore volume and to detect reservoir
heterogeneities within the drainage area of the well.
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Development Phase
September October 2005abalt solutions limited - 2005
INTRODUCTION TO HYDROCARBON EXPLOITATION
WellTesting
2005 Abalt Solutions Limited. All rights reserved
Drawdown Test
WellTesting
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During flow at constant rate of Qo, the pressurebehaviour of a well in an infinite-acting reservoir
(unsteady state flow) is given by:
Can be re-written as:
Then:
Semi-log plot of pressure drawdown data
Drawdown Test
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Development Phase
September October 2005abalt solutions limited - 2005
INTRODUCTION TO HYDROCARBON EXPLOITATION
WellTesting
2005 Abalt Solutions Limited. All rights reserved
Drawdown Test
Other parameters that can be obtained from plot
Semi-log plot of pressure drawdown data
If p wf=p1hr hich is found on the extension of the
straight line at log t (1 hr), then:
If pressure data measured at 1 hour do not fall on t hat line, the line must beextrapolated to 1 hour and extrapolated value of p1 hr must be used then.
WellTesting
2005 Abalt Solutions Limited. All rights reserved
Steps involved in drawdown test analysis:
1. Plot (pi - pwf) versus t on a log-log scale.
2. Determine the time at which the unit slope line ends.
3. Determine the corresponding time at 1 log cycle,ahead of the observed time in Step 2. This is thetime that marks the end of the wellbore storageeffect and the start of the semi-log straight line.
4. Estimate the wellbore storage coefficient from:
Drawdown Test
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Development Phase
September October 2005abalt solutions limited - 2005
INTRODUCTION TO HYDROCARBON EXPLOITATION
WellTesting
2005 Abalt Solutions Limited. All rights reserved
Drawdown Test
Steps involved in drawdown test analysis continued
5. Plot pwf versus t on a semi-log scale
6. Determine the start of the straight-line portion assuggested in Step 3 and draw the best line throughthe points.
7. Calculate the slope of the straight line and determinethe permeability k and skin factor s by applying the
equations below:
WellTesting
2005 Abalt Solutions Limited. All rights reserved
8. Plot all the recorded pressure data after teia as afunction of time on a regular Cartesian scale. Thesedata should form a straight-line relationship.
9. Determine the slope of the pseudo steady-state line,i.e., dp/dt (commonly referred to as m) and use theequation below to solve for the drainage area A,
10.Estimate the time to the end of the infinite-acting(transient flow) period, i.e., teia, which marks thebeginning of the pseudo steady-state flow.
Drawdown Test
where m = slope of the semi-steady-state Cartesianstraight-lineQ = fluid flow rate, STB/day
B = formation volume factor, bbl/STB
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Development Phase
September October 2005abalt solutions limited - 2005
INTRODUCTION TO HYDROCARBON EXPLOITATION
WellTesting
2005 Abalt Solutions Limited. All rights reserved
Steps involved in drawdown test analysis continued
11.Calculate the shape factor CA from a expression thathas been developed by Earlougher (1977).Earlougher has shown that the reservoir shape factorcan be estimated from the following relationship:
12.Use Table of shape factor to determine the drainage
configuration of the tested well that has a value ofthe shape factor CA closest to that of the calculatedone, i.e., Step 11.
Drawdown Test
WellTesting
2005 Abalt Solutions Limited. All rights reserved
Pressure Build-up Test
Describes the build up in wellbore pressure with timeafter a well has been shut in.
Serves to determine the static pressure
Also serves to determine:
Effective reservoir permeability
Extent of permeability damage around the wellbore
Presence of faults and to some degree the distance tothe faults
Any interference between producing wells
Limits of the reservoir where there is not a a strongwater drive or where the aquifer is no larger than thehydrocarbon reservoir.
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Development Phase
September October 2005abalt solutions limited - 2005
INTRODUCTION TO HYDROCARBON EXPLOITATION
WellTesting
2005 Abalt Solutions Limited. All rights reserved
Pressure Build-up Test
In pressure build-up and drawdown analyses,the following assumptions are usually made:
With regard to reservoir
Homogeneous
Isotropic
Horizontal of uniform thickness
With regard to fluid
Single phase
Slightly compressible
Constant mo and Bo
With regard to flow behaviour
Laminar flow
No gravity effects
WellTesting
2005 Abalt Solutions Limited. All rights reserved
Pressure Build-up Test
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Development Phase
September October 2005abalt solutions limited - 2005
INTRODUCTION TO HYDROCARBON EXPLOITATION
WellTesting
2005 Abalt Solutions Limited. All rights reserved
Horners Plot
Pressure Build-up Test
WellTesting
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The Back-Pressure Test-Flow after Flow
Conventional deliverability test: gauging the ability of the well to flowagainst various back pressures. Rawlins and Schellhardt (1936)
Shut in the gas well sufficiently long for the formation pressure toequalize at the volumetric average pressure
Place the well on production at a constant flow rate Qg1 for asufficient time to allow the bottom-hole flowing pressure tostabilize at pwf1 to reach the pseudo steady state.
Repeat step 2 for several rates and the stabilised bottom-hole flowpressure is recorded at each corresponding flow rate. If three orfour rates are used, the test may be referred to as three-point orfour-point flow test.
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Development Phase
September October 2005abalt solutions limited - 2005
INTRODUCTION TO HYDROCARBON EXPLOITATION
WellTesting
2005 Abalt Solutions Limited. All rights reserved
The Back-Pressure Test
WellTesting
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Conventional back-pressure test
Length of the flow periods is the most importantfactor to be considered in performing theconventional deliverability test.
Each rate must be maintained sufficiently long for thewell to stabilise to reach the pseudosteady state.
The stabilisation time for a well in the centre of acircular or square drainage area may be estimatedfrom:
The Back-Pressure Test
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Development Phase
September October 2005abalt solutions limited - 2005
INTRODUCTION TO HYDROCARBON EXPLOITATION
WellTesting
2005 Abalt Solutions Limited. All rights reserved
Isochronal Tests
Developed to shorten test times in wells that take long time to stabilize
Series of single point tests developed to estimate stabilizeddeliverability characteristics without actually flowing the well for timerequired to achieve stabilized conditions.
Conducted by alternatively producing the well, shutting in the well andallowing it to build-up average reservoir pressure before beginning ofthe next production period.
Pressures are measure several time increments during each flowperiod.
Ex- BHFP could be measured at 0.5,1.0,1.5 etc hrs after beginning ofeach flow period.
More practical for low-permeability formations.
Although not required, a final stabilized flow point usually is obtained atthe end of the test.
WellTesting
2005 Abalt Solutions Limited. All rights reserved
Isochronal Tests
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Development Phase
September October 2005abalt solutions limited - 2005
INTRODUCTION TO HYDROCARBON EXPLOITATION
WellTesting
2005 Abalt Solutions Limited. All rights reserved
Modified Isochronal Tests
Time to build up to average reservoir pressure before flowing for acertain period of time may be impractical.
Isochronal test is modified to shorten test times further and obtainsame data without using the sometimes lengthy shut-in periods
required to reach average reservoir pressure in the wells drainage
area.
The shut-in periods are of equal duration & should equal or exceed
length of flow periods.
Shut-in sand face pressures are recorded immediately before each flow
period and used instead of average reservoir pressures.
This test is less accurate than the isochronal test.
Note-As the duration of the shut-in periods increases, the accuracy ofthis test also increases.
Although not required, a final stabilized flow point usually is obtained at
the end of the test.
WellTesting
2005 Abalt Solutions Limited. All rights reserved
Modified Isochronal Tests
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Development Phase
September October 2005abalt solutions limited - 2005
INTRODUCTION TO HYDROCARBON EXPLOITATION
WellTesting
2005 Abalt Solutions Limited. All rights reserved
Drill Stem Testing
Provides a method of temporarily completing a well and determining itsproductive characteristics of the specific zones
DST can provide
Good data to evaluate productivity zone
Completions practises
Extent of formation damage
Need for stimulation
Reservoir characteristics that can be estimated from analysis are:
Average effective permeability
Reservoir pressure
Wellbore damage Barriers, permeability changes & fluid contacts
Radius of investigation
depletion
WellTesting
2005 Abalt Solutions Limited. All rights reserved
DST Operations
Running in hole on drill pipe, bottom hole assemblyconsisting of a packer and a surface operated valve.
The DST valve is closed while running the drill string,thus pressure inside the drill pipe is very lowcompared to hydrostatic mud column pressure.
On bottom, the packer is set to isolate desiredformation zone from mud column and control valveis opened to allow formation fluids to enter the drillpipe.
After a suitable time valve is closed and pressurebuilds-up.
Control valve is opened again, and the flowing andshut-in periods are repeated to obtain additionaldata
Drill Stem Testing
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Development Phase
September October 2005abalt solutions limited - 2005
INTRODUCTION TO HYDROCARBON EXPLOITATION
WellTesting
2005 Abalt Solutions Limited. All rights reserved
Sequence of Operation:
Tool is lowered on the drill pipe to a position opposite to theformation to be tested.
Packer expands against the hole to segregate the mud filledannular section from the interval of interest.
The test valve allows formation fluids to enter the drill pipeduring the test.
The equalizing valve allows pressure equalization after the testso the packer can be retrieved.
By opening the test valve a decline in pressure is obtained.
During the test, pressures and flow rates are measured as afunction of time.
Drill Stem Testing
WellTesting
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Drill Stem Testing
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Development Phase
September October 2005abalt solutions limited - 2005
INTRODUCTION TO HYDROCARBON EXPLOITATION
WellTesting
2005 Abalt Solutions Limited. All rights reserved
Pressure Vs. Time Plot Point A: As the tool is run in hole the Initial hydrostatic mud pressure is
recorded.
Point B: Initial shut in pressure, which is measured by closing the controlvalve and allowing pressure build-up towards static reservoir pressure.
Point C: Second flowing and shutting in periods permits the calculation ofreservoir parameters. As the control valve is opened, pressure falls to initialflowing pressure.
Point D: As fluids move up the drill pipe, pressure increases due to flowrestrictions in tools. After a suitable time, the tool is shut-in and termed finalflowing pressure.
Point E: Pressure then increases to final shut-in pressure
Drill Stem Testing
WellTesting
2005 Abalt Solutions Limited. All rights reserved
Recommendations We have to consider that the main objective is to record good data.
Therefore the DST must be planned to accommodate any specificsituation.
FIRST FLOW: (5-15 minutes), the objective is to remove any excesspressure which may have resulted from setting the packers.
FIRST BUILD UP: (30-60 minutes)
SECOND FLOW: (60 minutes)
SECOND SHUT IN: ( from 30 minutes to several hours) finding out thetransmissibility and other characteristics of the reservoir.
Drill Stem Testing
Pressure
Time
1 2
3
4
56
1. Going into the hole2. Initial flow period
3. Initial shut-in period4. Final flow period5. Final shut-in period6. Coming out of the hole
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Development Phase
September October 2005abalt solutions limited - 2005
INTRODUCTION TO HYDROCARBON EXPLOITATION
WellTesting
2005 Abalt Solutions Limited. All rights reserved
Drill Stem Testing
Analysis methods:
Horners Plot method to determine permeability
Type curve matching method to account for wellbore storage.
Kohlaas type curve matching to determine
Correa & Ramey for multiphase environments
Computer matching DST
/kh
WellTesting
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Injection Well Tests
Pressure transient testing during injection into
a well.
Important for efficient planning & operation ofboth secondary and tertiary recovery projects.
For analysis, the mobility ratio of injected fluid
and in situ fluids must be near unity.
Mobility Ratio
M= (k/)w(k/)o
Ratio of mobility of the displacing (injected) fluid to the fluidbeing displaced.
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Development Phase
September October 2005abalt solutions limited - 2005
INTRODUCTION TO HYDROCARBON EXPLOITATION
WellTesting
2005 Abalt Solutions Limited. All rights reserved
Procedure Well Shut-in to allow pressure to
stabilize at initial reservoir pressure
pi.
Test begins at t=0 with constant
injection rate q (taken ve).
For the constant rate, the injection
bottom hole pressure Pwf is
modelled.
Plot of BHP Pwf vs. logarithmic
injection time t is plotted and
forms a straight line with slope m.
Allows the estimate of formation
permeability k and skin factor s.
Injection Well Tests
WellTesting
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Step Rate Injectivity Testing
Normally used to estimate formation or fracture
parting pressure, which is the injection pressurethat will initiate a fracture in an un-fractured well orextend an existing fracture.
Parting pressure determined from the test is
equivalent to fracture extension/propagationpressure in hydraulic fracturing.
Helps determine that any injection above parting
pressure will can result in pre-mature breakthrough, poor sweep, reduced oil recovery etc.
Procedure
Well shut-in before test begins .BHP nears
static reservoir pressure.
Injection rate stabilized at reduced andconstant rate,stabalization period should belong to achieve steady state or pseudo-steady
state flow.
Injection rates are increased step wise.
In low permeability formations (k10 mD)injection rates of 30 mins is adequate.
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Development Phase
September October 2005abalt solutions limited - 2005
INTRODUCTION TO HYDROCARBON EXPLOITATION
WellTesting
2005 Abalt Solutions Limited. All rights reserved
Interference Tests
Involves creating pressure disturbances in one well and
measuring the pressure response in the other wells.
Done by either producing from or injecting into at least onewell, called active well, and observing the pressure response inanother well called observation well.
Characteristics of pressure behaviour as function of timereflects the reservoir properties.
Tests can be conducted with more than one active and more
than one observation well.
These tests are ideal for establishing continuity of pay zonewithin the reservoir ,estimating volume of hydrocarbons withindrainage area of wells and quantifying reservoir anisotropies.
WellTesting
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Note that a time lag exists between the time when a rate
change is made at active well and time when pressure transientis seen in the observation well.
Interference Tests
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Development Phase
September October 2005abalt solutions limited - 2005
INTRODUCTION TO HYDROCARBON EXPLOITATION
WellTesting
2005 Abalt Solutions Limited. All rights reserved
A coded signal is sent from anactive well to a shut-inobservation well.
Signal is generated by producingfrom or injecting into the activewell, shutting in the well,producing again, shutting inagain, and repeating in a regularpattern.
The well is pulsed so that we canidentify effective of active well onpressure response at observationwell amid noise of reservoirpressure trend.
Highly sensitive pressure gaugesare used.
These tests can quantifydirectional reservoir properties,such as permeability.
Advantages
Fewer problems withreservoir noise & pressuretrends that cause errors.
Pulse Tests
WellTesting
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Surface Test Equipments
Surface Test Tree
Used in DST operations and are four-way cross with four valves.
Surface test tree should be able to support the full string weight
and all four valves should provide a seal with differential pressure.
Surface test trees with have either a master or production wingvalve or both, hydraulically operated.
Most trees include a swivel to allow pipe rotation to set packers
during tests.
Coflexip Hoses
High pressure hose used between surface trees & choke manifoldson tests conducted from floating rigs.
Allows good flexibility & better corrosion resistance.
Rig permanent Pipe works Information regarding line size, wall thickness, sour service, i nspection
reports must be checked.
High and low pressure pipe works need to be present.
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Development Phase
September October 2005abalt solutions limited 2005
INTRODUCTION TO HYDROCARBON EXPLOITATION
WellTesting
2005 Abalt Solutions Limited. All rights reserved
Hydraulic surface safety valve Free standing valve incorporated for additional safety. Used where long high pressure flow lines are used.
Sand trap Functions as sand separator during testing and used when
excessive sand production is anticipated.
Consists of 2 vertical cylinders with screen meshes of varying s izes Normally installed between flow head and choke manifold.
Sand detection equipment Has an intrusive probe set into the flow line. Used to detect sand grains across multiphase flow conditions.
Data reader Used to monitor wellhead pressure and temperatures and chemical
injection upstream of choke.
Surface Test Equipments
WellTesting
2005 Abalt Solutions Limited. All rights reserved
Other equipments Gauge tanks
Surge tanks
Transfer pumps
Gas diverter manifold
Oil diverter manifold
Burners
Flare lines
Vent lines
Emergency shut down systems
Relief lines
Surface Test Equipments
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