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Core stabilisation by resin, foam or gypsum techniques

Corex has unparalleled experience in the handling and analysis of unconsolidated and poorly consolidated sediments, both at wellsite and in the laboratory. This experience includes over 50000 feet of core from “Alba”, “Gryphon”, “Balder”, “Tay”, “Sedgwick”, “Frigg” and "Balmoral" fields and many other unnamed plays.

Corex have developed and adopted a range of techniques to stabilise cores to prevent mechanical damage during transportation and processing of core samples. Each method has advantages and we recommend that Corex representatives are involved in pre-spud discussions to allow the most appropriate technique to be selected for each coring program.

The principal methods of preventing movement of the cores within the core liner have been to introduce a supporting material into the annular space between the liner wall and the outer core surface. Three materials are offered by the core analysis industry for placement in the aforementioned annular space – resin, gypsum and foam. Each method has advantages and Corex can offer advice on the most appropriate method for each project. The 3 methods are briefly described below.

Typically each method involves cutting the cores into shorter (1 metre or 3 feet) lengths. However foam can be injected prior to cutting the liners to further maintain the integrity of the cores. Once cut into shorter lengths the cores are placed on a drainage rack to allow the drilling mud to gently flow out of the core barrels. The picture below shows a typical set up for using the resin technique; here the cores have been cut into 3 feet lengths. Once the drilling mud has been drained the cores can be placed in an almost vertical position to allow the resin to fill the annulus. The pump that is used delivers the mixed resin via a nozzle directly into the annulus. No pressure is used to deliver the resin into the barrels. The mud cake on the core prevents the resin entering the pore space. Within 5 minutes the resin will set to a solid plastic thus protecting the cores from mechanical damage.

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Resin Technique

The resin method works best on very unconsolidated cores with low levels of cementation between grains. The setting time for the resin is usually 5 minutes however a range of resins can be used to allow for extremes of ambient temperature. As with most chemicals resin can be shipped by air, sea or road in accordance with the relevant legislation. All unused chemicals are returned to Corex for appropriate safe disposal.

The picture below shows an example of how resin has been used to stabilised friable sandstone cores. Sedimentary features are clearly still visible on the slabbed core surface.

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Foam Technique

The foam stabilisation method is similar to the resin technique. A 2-part expanding polyurethane foam is used instead of resin. Foam packs are provided complete with dispensing nozzles. Where appropriate small access ports can be drilled in the core liner to ensure an even distribution of foam along the full length of the core. Foam stabilization maintains a degree of flexibility and is therefore better able to adsorb shocks imparted during the transport of the core between wellsite and laboratory. In most cases it readily peels away from the core once the barrels have been opened. Foam will not alter the fluid saturation of the core material.

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Gypsum Technique

Gypsum is introduced in a liquid form by pouring/injecting under low pressure into the annulus, after draining the resident mud. Both gypsum and resin methods require there to be continuous access to all available space for them to be fully effective. Any barriers caused by slumping, slippage along fracture planes or grain/ grain movement (in the case of unconsolidated sand with gas expansion) will prevent the passage to the liquid, thereby potentially leaving portions of the core unprotected once either the resin or gypsum has solidified. Foam will bypass such blockages in part due to its expansive qualities and also because of its multiple placement points.

One further comment with respect to the gypsum method concerns the problems with mixing and subsequent modification of the water phase. Too much water and the slurry will solidify slowly, with the attendant possibility of water invasion into the pore space and the modification of the water saturation profile. Too little water and the pump will jam and/or the slurry will set before all reachable annular space is penetrated. Gypsum also has a tendency to extract liquid from the pore space where there is continuity in the water phase, once it has solidified.

The application of core stabilisation through annular space filling is also being applied to fractured carbonated core material, with the purpose of reducing displacement during handling. However, all three materials will be intrusive in terms of fracture (and vug) fill during the placement phase. It may be argued that the gypsum can be dissolved as part of the subsequent sample preparation phase, but this is by no means certain and residual solids will impact on gas permeability, porosity and grain density.

It is vital that the best technique is selected for each project.

 

 

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