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Return Permeability

In the selection of drilling and completion fluids and procedures basic return permeabilities are routinely used. A basic test involves a permeability measurement followed by test fluid application and a final "return" permeability.

As well as performing basic return permeability tests COREX has had a great deal of experience in interpreting the data produced, which has led to the development of more pertinent tests.

COREX return permeability tests are conducted at reservoir conditions. There is a permeability base case and a return permeability but the COREX test involves a full simulation of all drilling/clean-up/completion operations and a simulated drawdown or return to production.

To clearly identify favourable non-damaging fluid candidates, remedial treatments and / or incorporate mechanisms or design operations to improve results for field use the permeability results, cryogenic SEM analysis and dry SEM analysis are required. If the fluid/fluids applications are considered based solely on permeability results then this may carry an associated risk because of fines migration and / or other flow reducing barriers. If drilling and/ or completion options are considered for any one sample type based on performing only single SEM analysis then this may carry an associated risk. However, if both dry and cryogenic SEM analysis is performed there would be less associated risk. Engineers would be able to customise the design of the fluid(s) and / or design mechanisms for successful field deployment based on knowing the full scope of damage mechanisms (nature and distribution). Therefore, COREX (UK) Ltd recommends both dry and cryogenic SEM to provide a complete evaluation of all possible fluid and solid damaging mechanisms.

In essence, to clearly identify favourable non-damaging fluid candidates, remedial treatments and / or incorporate mechanisms or design operations to improve results for field use the permeability results, cryogenic SEM analysis, dry SEM analysis, thin section analysis and XRD are required.

Below are related papers published by COREX that may be of further interest.

"Return Permeability Measurements - Proceed With Caution"

"Return Permeability: A Detailed Comparative Study"

Drilling Fluids Screening

COREX offers a number of Drilling Fluid Screening Tests.

Drilling Mud Evaluation

For many years laboratory flood tests have been used to pre-screen various introduced wellbore fluids such as drilling muds and completion fluids. In order to fully interpret the results of such flood tests COREX continues research and development of recognised geological techniques which have a particular relevance to flood test interpretation.

Permeability versus throughput and fluid loss versus time measured at reservoir conditions are produced as standard. Using these results and integrating geological techniques such as SEM, Cryogenic SEM, Thin section, X-ray mapping and cathodoluminescence, it is possible to determine the causes of damage and potential solutions to formation damage problems. Combining permeability/fluid loss results with all of these techniques greatly enhances the value of reservoir conditions core flood tests.


X-ray elemental mapping can be used in conjunction with advanced core flood tests to predict and avoid formation damage problems. This particular example above shows how the technique can be used to identify elements from a backscattered SEM image (top left quadrant). A polished section perpendicular to the wellbore end face has been prepared and the examples shown identify the selected elements at the wellbore end of the sample and in the mud cake. The mud cake contains dolomite (calcium and magnesium) as the main weighting agent but considerable amounts of barium were also detected. The barium is thought to represent barite contamination from mud mixing tanks - this mud was a "live" or used mud sampled at wellsite. The mud solids do not extend deeper than the first pore throat into the sample.

Drilling Mud Filtrate Loss

drilling_mud_filtrate_loss2.jpgThe diagram illustrates a single pore showing coalescence of mud filtrate brine droplets and the development of a water in oil micro-emulsion due to mud filtrate loss into an oil leg Formation brine droplets are visible on a sand grain.


Filtrate Retention

filtrate_retention_001_2.jpgA high magnification view of the film of retained filtrate coating grains and draping across pore openings



filtrate_retention_002b.jpgBrine retained within pore lining and pore-filling native clays


sludge2.jpgA remnant pill cake attachment composed of sludge. Brine, oil and blocky fragments are observed within the sludge.

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