Crude Oil and Natural Gas - Formation Link™

con-slot™ ‘FORMATION LINK™’ screens


‘There has been a remarkable development in drilling techniques for oil and gas exploitation over the last 10 to 15 years followed by an even stronger development of the upper completion equipment for production wells, yet there is missing a real sophisticated development of downhole completion equipment for the most important part of a well structure – namely where the hydrocarbons are produced from.’ These are the words of an expert with a long term experience in the field of well completion.

Even considering the recent developments of different screen design, we can see that this industry has not gone a step further for improvements of reservoir exploitation in order to reduce the costs of well completions and to produce more economically.

As long as a representative of a well known Service Company is of the opinion that ‘a screen is a screen – no matter how it looks like’ one can guess that we still have to go a long way before we really can say we are even with the developments mentioned above.

Deviated drilling as well as horizontal drilling has been developed to such an extent that sources which were not exploited in the past because of economical reasons can now be reached and developed. Drilling has been performed to exceed 5000 meters of measured depth and extension. Offshore drilling allows the exploitation of sources in extreme water depth and deep into the sea bed.

It is obvious that the improvement of the exploitation of the sources by increased drainage capacity should be the future challenge emerging from all the above mentioned.

After finding out all the advantageous parameters from the mass research con-slot has always been dedicated to find out all details of performances of profile wire screens in order to enable and improve the determination for optimum application.

The following information is a compressed report of the mass research work performed, the conclusions drawn from it and the experiences of practical installations and completions confirming the results of the research.

Screen Design

Permeability and Flow Characteristics for unconsolidated formations

In 1974 con-slot decided to invest into mass research before it started manufacturing, in order to determine the hydraulic flow pattern of the screen and as such the flow capacity and performance.

con-slot chose the Laboratories of the Technical University Munich, which was undertaking mass research to determine flow pattern on permeable media – many different kind of screen structures from wire mesh over slotted pipes to profile wire screens have been tested, of which the profile wire screen has proven the best performance parameters.

The screen is designed that two adjacent profiles form a V-slot opening with a Venturi Nozzle configuration by having a relief angle between 12° to 15°. This Venturi Nozzle configuration gives the most favourable hydraulic condition as is explained further on.

The screen is designed that two adjacent profiles form a V-slot opening with a Venturi Nozzle configuration by having a relief angle between 12° to 15°. This Venturi Nozzle configuration gives the most favourable hydraulic condition as is explained further on.
Immediately after beginning with the tests to determine the flow pattern: laminar – transition – turbulent, we experienced a very early stage of turbulent flow with uncontrollable pressure losses or high skin. The screens were dismantled from the test unit and we found grains being trapped in the slot opening thus plugging the entrance area and thus destroying the Venturi Nozzle configuration for which a sharp cornered entrance is necessary.

It was found that the round corner radius adjacent to the entrance of the slot opening allowed the grains to wedge-in and partly close the entrance area.

The corner radius of the profiles was reduced from 0,45mm to 0,15-0,10mm which was the solution for the plugging problem:
Round cornered profiles have the tendency to plug – consequently the flow enters early into turbulent conditions with low permeability and high skin effect.

Sharp cornered profiles allow the grains to bridge (arch) across the slot opening. Thus the slot remains open, allows laminar flow conditions with a high permeability and low skin effect.

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Round cornered Profiles: low permeability with high skin

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Sharp cornered Profiles: high permeability with very low skin

The reason why we rest on the topics of the screen design and the achieved advantageous flow conditions is, because you have to understand the hydraulics of the screen to understand also the amount of possibilities of well completion improvements resulting from it, as described further on.

The mass research performed at the Technical University of Munich gave us the possibility to perform a calculation of the screen permeability depending on the screen structure chosen (profile of wrapping wire; profile of inner supports and amount of inner supports on the circumference) and to perform a calculation of the flow capacity of the screen for laminar flow condition.

Using these two equations we were able to see that even with high viscous fluids like heavy crudes the flow capacity was large and the skin built-up was low.

It was also confirmed by the tests that as long as the flow remains in laminar flow condition, the performance is stabilized: optimizing the entrance capacity, no abrasion and no erosion of the screen will occur and sand control is never a problem.

It was also confirmed by the tests that turbulent flow condition will create high skin effect with reduced entrance capacity, abrasion and erosion can occur with the consequence of continuous sand production.

The range of laminar flow on profile wire screens is enormous allowing the combination of high entrance capacity with high mechanical strength. Easy handling during installation and improved performance after downhole completion have been the findings from this research.

We also found out that sand control is not a problem because sooner or later the grains build an arch across the slot opening due to the Venturi performance and retain fines from being produced – but to avoid building up a skin by the retained layers of fines around the screen is the challenge.

It must be clearly understood that a perforated base pipe, as being used on all other screen designs creates an obstacle to flow with the consequence of turbulent flow through the screen structure and subsequently the problems of high skin effect, abrasion and erosion.


Permeability And Flow Characteristics

A screen is an open media to allow retension of solids and free flow of liquids. The direct comparison between different screen structures can only be made by comparing the Permeability of each screen. The open area alone is not a comparison criteria because it does not include the hydraulic radius of the inflow area. The friction losses due to the mechanical structure of the screen and the viscosity of the liquid passing through the screen give different flow gradients (Forchheimer-Schnebelli Law).

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Often enough you hear the comment that the perforations in the base pipe is enough flow area for the production rate which can be expected. You also hear that the screen on top of the blind pipe area between the perforations is contributing to the entrance flow capacity.

This comment is without any technical fundament and is not true.

Experience has proven that, in most cases, the entire screen structure has not been considered and very little – if not none – information exists about flow characteristics of screen structures.

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The hydraulic properties of screen structure as medias separating solids from liquid/gas are directly related to physical flow laws determining the flow gradient (pressure loss) through the wall of screen structures as a function of the velocity of the flowing media.

The experiments performed in at the Institute of Radiohydrometry of the Technical University of Munich on Rod Based Wire Wrapped Screens have proven that with the increasing flow velocity ‘ν’ through the screen wall the gradient ‘I’ also increases.

Therefore the tests have clearly determined the difference between laminar and turbulent flow and, what is most important, the transition between laminar and turbulent flow does not occur instantaneously but gradually through Rod Based Wire Wrapped Screens. This is due to the favourable shape of the slot opening forming a ‘V’ together with the sharp corner radii at the slot entrance like a Venturi Nozzle.

Due to this long range laminar flow pattern is most significant that erosion and/or corrosion are avoided with is evidently a consequence of turbulent entrance flow condition through the screen wall.

There is no mathematical equation available to formulate the resistance of a screen structure either for laminar or for turbulent flow. The best approach is to distinguish between ‘viscous’ and ‘inert’ flow through the screen wall structure.

These basic laws are very important to determine the physical size of the RBWW-Screen needed for a specific application, to determine the very low pressure losses due to the flow characteristics through the screen and also to enable stabilized sand control in laminar flow for a better solid/liquid and solid/gas separation.

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One can easily observe in the nature that flow follows always that easiest, non-obstructed way.

Therefore the flow from the reservoir formation intends to hit the area of the perforations. The cross-sectional area of the perforation is obstructed by the overlaying screen structure thus reducing the entrance area even more.

Natural flow by reservoir pressure will be limited by the hydraulic law when the flow leaves laminar conditions and enters into turbulent conditions. If the flow is forced by differential pressure it will certainly enter turbulent flow with all the negative consequences like low productivity, abrasion, erosion and early water coning thus increasing water/oil ratio.

The same is also valid for flow by artificial lift.

The target is to clean-out the mud system if possible 100%, to loosen the compaction of the borehole wall due to the drilling process and improve the permeability of the formation around the borehole in order to achieve a constant higher productivity of the hydrocarbons.


Slot Size Selection    

C.J. Coberly was the pioneer in research of sand control. He published his results in 1937 under the title of “Selection of Screen Openings for unconsolidated Sands” – Drilling and Production Practice – API.

He defined stable arching across the slots as arches that would re-form more or less immediately after they were disturbed. He found out that stable arches would form across the slots when the ratio between slot width and particle size was equal or less than two (2).

(slot size 200 micron – 100 micron grain still arch across the slot)
If the ration was larger than two (2) unstable arches would form, which would only be re-formed after a large number of particles had passed through the slots. For uniform sands (UC less than 2) no arches at all would form if the ratio was larger than three (3).

The properties of mixtures with varying particle sizes were dominated by the largest particles, as long as the fraction of large particles was sufficiently high: above 10% retained!

For actual reservoir sands, Coberly found that stable arches would form, if D10 (the theoretical sieve size that would retain 10% by volume of the sand grains) of the formation sand was equal or larger than half the screen slot size.

Some of his work was later repeated by McCormack, who generally confirmed Coberly’s conclusions. He also reported that arches consisting of 3 particles across the slot tended to be stable while arches consisting of 4 to 5 particles across the slot were unstable.

What does this mean practically?

If the sieve analysis of the reservoir sand contains a minimum of 10% of 100 micron grain size retained in volume, the slot of the screen should be selected with 200 micron minimum.

This gives completely new prospective for the clean-up of the wellbore and the extraction of the fines from the formation.

Working with a conditioned Fluid System (Max solids size =1/3rd of the slot opening) the mud can be washed-out from the annulus space of the wellbore through the screen into the wash pipe up to the surface. There is no other method available to clean-up the wellbore 100%!

Another advantage is that the fines can be produced out of the formation before an arch is built across the slot opening of the screen to retain further sand material flow. The permeability transmissibility will be increased to stabilize laminar flow with higher productivity! 

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The con-slot Guidelines for the Slot Sizes Design are:

very uniform unconsolidated sand (Uniformity Coefficient UC < 2.00)
slot size at d50 or 50 % retained

uniform unconsolidated sand (Uniformity Coefficient UC ~ 2.00)
slot size at d40 or 40% retained

non-uniform unconsolidated sand (Uniformity Coefficient UC > 2.00)
slot size at d30 or 30% retained


Mechanical Strength

Already in the late 70ties we experienced the obstructive behaviour of a perforated base pipe during the work-over of the Emlichheim-field of the German Oil Company WINTERSHALL and the Schoonebeek-field of the Dutch Oil Company NAM (affiliate of SHELL). Both fields were changed from Water Flooding to Steam Injection which made it necessary to work-over all wells in the fields and to install new completion concepts.

Wintershall used a ‘Stand-Alone’ non gravel pack screen design completion, however with base pipe, while NAM used a gravel packed screen completion.

NAM experienced plugging of the gravel packs in a relatively short time ( many times already after a week), which made an expensive work-over program necessary after three months operational time of the well.

WINTERSHALL experienced plugging only by the too early arching effect across the slots after slot sizes were selected too conservative.

Screens design was discussed in mutual sessions between con-slot, WINTERSHALL and NAM. WINTERSHALL wanted to use the screens also in their injection wells and were questioning the base pipe structure of the screen.

con-slot proposed already in those days a ‘Stand-Alone-Completion with a heavy duty screen without a base pipe.

We experimented with flows of High Viscous Crudes through Heavy Duty Screen Structures and could see that the permeability of the screen was high to keep the skin or differential flow pressure low. We changed the structure of the screen until we achieved the combination of a really extra heavy screen structure with a permeability of the screen high enough to keep the skin very low and operational.

This was the moment when the con-slot Formation LinkÔ screen was born: High mechanical strength combined with high permeability – no base pipe obstructing flow and reducing operational procedures to improve the productivity of the well.

What is the mechanical strength of a con-slot Formation LinkÔ screen?

The screen structure consists of two members:

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h2 = width of the wrapping profile                  t   = height of the wrapping profile
h1 = width of slot opening                             te = equivalent wall thickness


Remark : The support rods cannot be considered for the collapse strength!

For the determination of the collapse strength the following equations are normally used:

  1. Thimoshenko
  2. API Formula
  3. AD-Merkblatt B6 (German Standard) Formula


con-slot is using the API Formula and the AD-B6 Formula with the preference of the latter, because it is the only formula considering also the unsupported length of the screen pipe. The smaller result of both equations will be valid.

Collapse tests have proven that the results are very realistic and compatible with API Tubing and Casing.

In order to give the screen body the required strength it is absolutely necessary to achieve a fusion between the wrapping wire and the support rods at all the thousands welds of each crossing of the two members throughout the screen structure.

A fusion is achieved when the metallurgical inner-granular structure does not differ between the wrapping wire and the support rod at the fusion of both.


Enlargement 500 : 1 – The Line is not a crack but a step from surface polishing


Enlargement 1000 : 1 – Here you can clearly see the polishing step

For the tensile and compressive strength the cross-sectional area of the support rods is valid. The large size profile and the large amount of profiles used on the circumference of the inner diameter of the screen secure the requested high tensile/compressive strength of the screen.

Tensile/Compressive tests have proven that the results are very realistic and compatible with API Tubing and Casing.

Testing the strength of the fusion weld between the wrapping wire and the support rods

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Break-off force in kg or lbs

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Acid test of inner granular structure of fusion weld

It is important to understand that the metallurgical inner-granular structure of the two members (wrapping wire and support rod) is the same around the fusion (left and right of the line!)

Imperial Design Data

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The screen OD is the same as the coupling of the corresponding API Tubing or Casing size, the screen ID is the same as the corresponding API Tubing or Casing size.

This makes the screen OUTSIDE and INSIDE flush, thus reducing the friction during the installation and also facilitating all inner screen operations, specifically with coil tubing, because the support rods act like a rail!

Metric Design Data

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The advantages of the findings of pioneers like Coberly and MacCormack, as described under Section 3) are obvious: Led by the fear of not getting the sand flow under control all users of profile wire screens have been much too conservative with the slot design in the past. This is due to the fact the never before has this industry made any attempt to find out by research the influence of the corner radius of the surface (wrapping) profile of the screens. The screens were manufactured in the past with large corner radii profiles leading to turbulent flow and to abrasion and erosion with the result of continuous sand production.

con-slot has been the first company until now to determine the screen structure and make the application sound proof.

After the first application of so-called ‘Stand-Alone-Screens’ (screens without perforated base pipe) offshore Norway / North Sea, where slots were designed much too conservative leading to an early arching of grains across the slot opening and the accumulation of fines layers around the screen periphery, it was decided together with the operator SAGA Petroleum AS to undergo a testing at the Rogaland Research Institute in Stavanger/Norway.

A test unit was assembled to allow the flow test through undisturbed cores from the formation, a section 12.5 degree. In front of the core a rectangular screen-section was mounted with the same structure as provided for the downhole completion (see also diagram page 42). Liquid flow with different viscosities were loaded on the system under laminar to transition conditions to understand the behaviour of the flow of sand particles to the screen and through the screen.

Screen samples with different slot sizes from 75micron to 300micron were tested. An early arching of grains and plugging (accumulation of fines) was experienced with the smaller slot sizes while the larger the slot size got, the more fines were produced through the screen structure into the screen before an arching took place.

This was the moment, when the leader of the test group confirmed the ‘Coberly law’.

This was also the moment to understand the tremendous advantages offered by the ‘Stand-Alone- Screen’:

to increase the porosity of the formation around the borehole by producing the fines from it as long as possible before the arching would occur.

To clean-out the fluid system from the wellbore wall through the screen by conditioning the mud system (maximum solid size 1/3rd of the slot size).

It was decided to complete all further wells (TORDIS FIELD) in the first field where ‘Stand-Alone- Screens’ were installed, with a larger slot size to condition the mud system and wash-out from the annulus space of the wellbore through the screen into the wash pipe stinger installed inside the screen, and up to the surface while retrieving the wash pipe along the screen interior.

The decision was made to use only one mud system: for drilling and completion – a tremendous cost saving up to several Mio. US Dollars for the field completion.

The mud was withdrawn over 90 percent from the wellbore – what was never achieved before with another screen type – thus improving the inflow conditions from the reservoir formation into the wellbore and increasing the productivity.

Further to this, producing sand fines out from the formation around the wellbore thus increasing the permeability under laminar flow condition was another target to achieve.

On the first completions we created an impact by closing and opening the top valve, thus creating an impact wave into the formation, which was travelling at a speed of approx. 1000 meters per second. This was the leading time of how long the valve should remain closed and then opened again.

Sand was produced during several hours and we experienced the moment when the arching over the slot openings occurred and the sand flow ceased.

The impact created was on a self-producer, however the same can be achieved on a pumped well by shutting-off the pump and starting it again.

Today, con-slot is working together with a German Group on an impact system producer, which should be used directly after the mud clean-up procedure. The system will certainly increase the production of fines from the formation prior to the forming of the arch across the slot opening.

Higher productivity under laminar flow conditions throughout the life time of the well will be the improved achievement!

In case Zone Isolation will be necessary for non producing formations:

With the reliable External Casing Packer systems being available on the market today zone isolation is not a problem any more. This introduces a tremendous advantage of the open hole completion against cased hole completions with expensive cementing and perforating. The latter will never allow the high productivity achieved by an open hole completion.

Sample Flow Calculation     

Flow Calculation to determine Pressure Losses

- due to the flow entrance of the produced media through a‘STAND-ALONE’ Wire Wrapped on Rod Base Screen Structure and
- due to the flow of the media inside the screen to the surface

Calculation for:
Oyong Oil and Gas Field

Rogaland Research Institute Stavanger - Erosion Test

Comparison between

Please read in between the lines the commentary concerning the con-slot Single Wire Wrap Screen:

Even after 50 days the screen no erosion was detected on the screen, but the performance of the other screens was already reduced by showing erosion within hours! Whether and when a con-slot single wire wrap screen will erode could not be determined by this research.

For further information please follow this link.

Summary of Advantages of the Formation Link Type of Well Completion

Case Histories

SHELL Venezuela                            1996 / `97

    • Completion of the Icotea Field under the Lake Maracaibo

BSP (SHELL Brunei)                        1995 /`96 / `97

    • Completion of two Multilaterals and a horizontal well repair

SAGA Petroleum                              1993 / `95

    • Horizontal well repair in the Tordis Field

NORSK HYDRO                                  1995

    • Completion of one well in the Brage Field

WINTERSHALL                                 2002

    • Completion of two wells in Emlichheim

Gas Wells NAM Offshore North Sea

 SHELL Venezuela – 1996 / `97

Completion of the SHELL Venezuela Icotea Field under the Lake Maracaibo (Isle du Levant): Intent was to drill 8-1/2 in. hole and complete it with 6-5/8 in. screens.

con-slot® convinced SHELL with the flow calculation to reduce the borehole to 6-1/4 in. and to install 4-1/2 in. formation link screens.

Contact person during the design phase was Sr. Einstein A. Millan Arcia (fax 0058-61-924440). Contact person during the procurement phase was Ronald de Bruin - Head Procurement OPL/1 (fax 924440). Person responsible for running the screens and completing the wells was Willi Quakernak - same fax-number.

In ten wells with 395 m horizontal section 4-1/2 in. screens were installed in 6-1/4 in. borehole with one mud system (oil based), conditioned, solids max 80 micron size for 250 micron slot size.

Mud had been circulated out of the hole through the screen by using a 3-1/2 in. Hydril PH-6 wash pipe. Only one mud system was used. Running procedures are available and can be submitted upon request. 

Average productivity of each well is 3000 bpd - please take oil data from below. 

Three wells have been installed with Baker Excluder screens in the same field formation, producing only between 1/3 to 1/2 = 1000 to 1500 bpd.

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BSP (SHELL Brunei) – 1995 /`96 / `97

Horizontal Well Repair : 1995

400 m of 3-1/2” screens with 50 mm slots (0.002") were installed inside 5” slotted liner producing sand [recommendation was for 100 mm slots]

Test to find out how much productivity would be reduced by retention of fines produced through slotted liner

Production dropped 50% and skin raised to three times it was before fines accumulated in annulus around screen.

Production of 7.6 cp viscosity oil dropped from 590 m³/day to 280 m³/day with a skin 115 kPa

Person responsible for running the screens and completing the wells was Stuart Clayton – BSP-DRO/26 (fax 00673-3-37-4216).

Two Multilaterals : 1996/1997

4-1/2" screen installed in upper lateral formation and 5-1/2" screen installed in the back bone lower lateral formation

Slot size was designed based on sieve analysis, mud was not conditioned and displacement fluid was used with an acid treatment to clean-up laterals

Production rate of 900 cp oil was more than two times more than anticipated:
upper layer » 650 m³/day & lower layer » 820 m³/day

Productivity could have likely been even better if mud would have been conditioned
Person responsible for running the screens and completing the wells was Stuart Clayton – BSP-DRO/26 (fax 00673-3-37-4216).


SAGA Petroleum – 1993 / `95

195 m of 7" WWSORB 316Ti special designed Rod Based Screens for nongravel installation
in the Offshore Tordis Field Well No.34/7-I-3H, deviated 60 degrees.

Slot Size design: 150µ . Complete Pilot Hole. Drilled with Mud NaBr2

Current Production : 3500 m³/day       
610 m of 7" WWSORB 316Ti special designed Rod Based Screens for nongravel installation
in the Offshore Tordis Field Well No.34/7-I-5H, horizontal 1000 m 90 degrees.

Slot Size design:   75µ for Rannoch     Drilled with Mud NaBr2         
Slot Size design: 150µ for Etive          Drilled with Mud NaBr2         


ECP between formations to isolate Etive from Rannoch.

Inner 5" Collecting Pipe with Sliding Port in upper vertical location to allow commingling of production from Rannoch and Etive.

Current Production: 5000 m³/day  Oil

Person responsible for running the screens and completing the wells was Lars Endre Hestenes – Well Completion (fax 47-4570261).



800 m of 7" WWSORB 316Ti special designed Rod Based Screens for nongravel installation in the Offshore Brage Field Well No.31/4-A-21, horizontal 90 degrees 1600 m long.

Slot Size design: 200µ to allow also clean-up of mudcake through screen!

7 ECP's to isolate 5 different formations from each other.

Completed with 5" inner collecting pipe incorporating 5 sliding sleeves to allow alternate combination of production from different formations.    

Drilled with Mud Ancogreen. Friction factor during installation reported 0.20 - easy installation.

No clean-up, Production started cleaning simultaneously mud cake.                           

Current Production : 5500 m³/day Oil

Person responsible for running the screens and completing the wells was Steinar Bringsli – Avdelingsjef (fax 05-995000).



250 m of 4.1/2" formation linkTM RBWWS installed in the Emlichheim Field – Germany, well No.300. Slot Size design: 200µ, conditioned mud system.

250 m of 4.1/2" formation linkTM RBWWS installed in the Emlichheim Field – Germany, well No.302. Slot Size design: 200µ, conditioned mud system.

Persons responsible for running the screens and completing the wells were Otto Wilhelm and Thomas R. Gieles /Betrieb Westemsland (fax 05943-9339-163).  

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Teamleader Production Optimisation Offshore North
NAM Offshore, Beverwijk – The Netherlands
Mr. Arjen Kooijman

Offshore Production Facilities:

Solling reservoir data:

Well data Solling Reservoir

Formation conditions

No sand production observed!

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