1. No flaring well testing (Injection Fall-off testing)
Arild Fosså | Expro Norway AS

2. Traditional well testing
Low actual emissions, but aesthetically un-appealing
Important data for development decisions
Are there real alternatives ??

3. Background
Dynamic Well Test data is used as input data for the following;
Reservoir Model
Well performance Model, and
Pipeline and Facilities Models
Key data provided are;
Pressures,
Temperatures
Flow rates
Fluid data (both PVT and large volume)
Well Bore
Pressure transient from a Well Test

4. Example of conseq. from missing data
K Field knowledge
Reservoir evaluation
Development strategy
Business
Miss understand :
heterogeneity org.
K anisotropy
Poor estimate :
Reserves
Production profiles
Wrong decisions for
recovery mechanism
(gas/water injection,
gas cycling/depletion)
Less Profits /
Lose money
Underestimate
heterogeneity size
Overestimate :
sweeping efficiency
reserves
Overestimate Facilities
Lose money
Ref. SPE Applied Technology Workshop – Well Testing – London 2001.
Miss or not quantify
extra permeability
(fract. , dissolution)
Underestimate
Production profiles
Poor estimate
segregation effects
Underestimate :
Facilities
Profitability
Less Profits
Project
abandonment

5. Backdrop on the Barents Sea
The big picture for the Barents Sea area is characterized as follows;
Same formal rules & reg’s as the rest of the NCS.
High focus from environmental groups
High focus on spill prevention from the authorities
High Political focus – (spills, blow-out risk, ice-edge, public opinion, etc. etc.)
Due to this an alternative way of testing wells could be of interest.

6. What is needed for a Well Test?
The “simple” option…..
Gas test - Westhope, North Dakota, January 07
Porosity From cores
Viscosity From Fluid Samples
Flow rates From Surface (normally)
Pressures From Bottom Hole
In addition it is important to minimize the accumulator effect downhole inside the string – Well Bore Storage effects.
Due to this there is great freedom in how you achieve the above.
The elaborate option…….
Bideford Dolphin plant

7. Injection Fall-off testing
One of the parameters we have freedom over is the flow rate. Which is used to induce the pressure transient.
There is nothing stopping us from doing an injection period rather than a flow period to get the pressure transient going.
The equivalent to the normal pressure build-up would be a pressure Fall-off.
Gives the same type of data as a normal well test.
Except Fluid Data - obtained during open hole logging.
Classic Injection Fall-off test.

8. Injection Fall-off application
Traditionally Injection Fall-off tests have been used for;
Water zones and water-flood projects.
Measure pressure increases to accurately predict pressure rises from long-term fluid injection.
Verify that reservoir zones are «non-migration» types – i.e. no communication between reservoir zones.
Changes in permeability and skin over time.
Geothermal wells
Water disposal wells
Traditionally used for Gas Storage wells
There isn’t anything stopping a wider use.

9. Injection Fall-off issues
Issues associated with Injection Fall-off testing vs. normal Well Tests
First, the character of the system changes.
Instead of single-phase flow, we are now faced with two-phase water/oil flow governed by relative permeability's
Second, injection of cold water induces temperature changes in the formation
This complicates the pressure behaviour through temperature effects on the oil and water viscosities.
Third, injection of water may result in the formation accidentally fracturing and in coupling of rock mechanics and fluid flow problems.
It is critically important for successful test interpretation to avoid accidental fracturing and to inject water at below the formation fracturing pressure.
SPE PA “Application of Water Injection/Falloff Tests for Reservoir Appraisal: New Analytical Solution Method for Two-Phase Variable Rate Problems” is a good reference paper to start with.
Note: A large proportion of the world’s Injection Fall-off tests are designed as Fracture Injection Fall-off tests. However, these are specially designed as such. If you don’t plan for this, accidental fracturing of the formation can make the pressure transient analysis of an Injection Fall-off test very difficult, and can lead you to using non-unique type-curve matching instead of Radial Flow analysis. There is no problem conducting a Mini-Frac sequence as part of the Kill procedure after an Injection Fall-off test.

10. Mitigation #1 System character change
Issue - Introduction of two-phase fluids in the reservoir affecting the relative permeability's
Mitigation – Use theoretical «two-bank» and «multi-bank» models.
handles the two-phase aspect of an Injection Fall-off test.
addresses changing saturation gradients, which has a significant impact on the pressure transients.
Schematic saturation profile top view
Horner plot example illustrating the effect of banking on an analytical plot
Schematic saturation profile side view
SPE PA “Application of Water Injection/Falloff Tests for Reservoir Appraisal: New Analytical Solution Method for Two-Phase Variable Rate Problems” is a good reference paper to start with.

11. Mitigation #2 Reservoir cooling
Issue - cold water reduces formation temperature, hence oil & water viscosities
Mitigation – Research show that for pressure transients governed by a moving thermal front, one should use fluid-properties corresponding to the cold injection fluid to analyse the results correctly.
Prior PVT data a plus for job planning
Extensive Fluid Sampling program for Wireline Formation Tests
Bulk sample issue for facilities & pipeline models not solved fully. (Limitation with method)
Pressure transient data for injection of 95°C water into a 250°C reservoir
SPE PA “Application of Water Injection/Falloff Tests for Reservoir Appraisal: New Analytical Solution Method for Two-Phase Variable Rate Problems” is a good reference paper to start with.
Article – SPE PA - Nonisothermal Effects During Injection and Falloff Tests, S. M. Benson (Lawrence Berkley Laboratories), SPE Formation Evaluation, February 1986.

12. Mitigation #3 Formation fracturing
Issue – Risk of accidental formation fracturing, possibly fracturing the cap rock.
Mitigation – Good control of the Fracture Gradient, plus injectivity modelling before a job.
Requires general good area knowledge.
Possibly that Injection-Fall-off method is best suited for appraisal wells.
Generic Pore-pressure & Frac gradient curve
Example simulation of BHP for Injection Fall-off job
SPE PA “Application of Water Injection/Falloff Tests for Reservoir Appraisal: New Analytical Solution Method for Two-Phase Variable Rate Problems” is a good reference paper to start with.
Note: A large proportion of the world’s Injection Fall-off tests are designed as Fracture Injection Fall-off tests. However, these are specially designed as such. If you don’t plan for this, accidental fracturing of the formation can make the pressure transient analysis of an Injection Fall-off test very difficult, and can lead you to using non-unique type-curve matching instead of Radial Flow analysis. There is no problem conducting a Mini-Frac sequence as part of the Kill procedure after an Injection Fall-off test.

13. Mitigation #3.1 Formation fracturing - vacuum
Other issue – In case a water-injection well is fractured, the wells often go on surface vacuum, where the fluid level can fall below surface, making surface pressure data monitoring impossible.
Mitigation – Normally the vacuum would stem from fracture propagation, which limits the issue. However, dealing with this complication requires bottom hole pressure recorders (memory or Surface Read-Out).
SPE PA “Application of Water Injection/Falloff Tests for Reservoir Appraisal: New Analytical Solution Method for Two-Phase Variable Rate Problems” is a good reference paper to start with.

14. Injection Fall-off application
It is not recommended to do Injection Fall-off tests for rank exploration wells.
However, we would recommend it as a possible Appraisal Well technique, provided the following;
Prior fluid data obtained from earlier exploration wells
Fluid properties available for reduced reservoir temperatures
Sufficient fluid sampling planned on actual well during OH logging
Porosity from earlier cores available
Core saturation experimental results available from earlier wells
Fracture gradient is well known for the target formation

15. Other
To plan a successful Injection Fall-off test the following is needed;
More simulations than normal to model pressure transient responses to ensure correct job design.
A plan for bulk samples for pipeline and facility model experiments
Can you live without?
Can it be obtained from the Initial Flow period to surface tanks?
Bottom Hole gauges must be used for data gathering.
Equipment and services required are the same as for a normal DST, with the exception of Fluid Sampling.
IPTC – Water Injection Fall-off Tests in Deepwater Reservoir: What do we actually see into formation? Petroleum Technology Conference, Doha, Qatar, 7-9 December 2009
US Environmental Protection Agency – Region 6 – The Nuts and Bolt of Falloff Testing, March 5th, 2003

16. Has it been done before? Source: NPD + Press release
Well bore: 7122/7-4S Klappmys
Rig: Polar Pioneer
Time frame: November 2006
Formation: Kobbe ( m)

17. Arild Fosså | Expro Norway AS arild.fossa@exprogroup.com
Thank you!
Arild Fosså | Expro Norway AS