High-Viscosity Gel Encapsulation for Core Preservation and Improved Reservoir Evaluation
- R.A. Skopec (U. of Aberdeen) | P.E. Collee (Baker Hughes INTEQ) | L.K. Shallenberger (Coring Services Consulting)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- May 1995
- Document Type
- Journal Paper
- 399 - 399
- 1995. Society of Petroleum Engineers
- 1.6.9 Coring, Fishing, 1.11 Drilling Fluids and Materials, 1.14 Casing and Cementing, 1.10 Drilling Equipment
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The goal of coring and core preservation should be to obtain rock that isrepresentative of the formation while minimizing physical and chemicalalteration of the rock during coring and handling. Low-invasion coring systemshelp minimize drilling-fluid invasion, but rock wettability and saturation canstill be altered by drilling-fluid imbibition and/or diffusion before coreanalysis begins.
New technology that uses high-viscosity gel for downhole core encapsulationand preservation is an alternative to operator-intensive wellsite methods.Existing low-invasion antiwhirl coring assemblies are easily retrofitted toaccommodate use of a simple inner-barrel piston for gel distribution and coreencapsulation. The viscous core preservation gel is a high-molecular-weightpolypropylene glycol that is insoluble in water and environmentally safe.Laboratory tests at 200 F and 250 psig on water-saturated gel-coated rocks withpermeabilities as high as 10 darcies indicate no spurt loss and zero waterloss. The gel is compatible with most water- and oil-based drilling fluids.Because the gel comes in direct contact with the core during and immediatelyafter it is cut, further exposure to drilling fluid is minimized. Once at thesurface, exposure of the core to air is reduced significantly. An additionalbenefit of downhole gel encapsulation of core is obvious when one considershandling poorly consolidated rocks with moderate compressive strength. Thehigh-viscosity gel stabilizes the core and enhances the mechanical integrity ofthe rock. Surface handling is improved and core damage is reduced duringtransport to the laboratory. For rocks with little cementation, theencapsulating gel can be replaced with a self-hardening plastic, thuseliminating the need for time-consuming surface resination.
Fig. 1a shows the downhole core preservation assembly before coring begins.Before delivery at the wellsite, 22 gal of gel is preloaded per 41/4-in.-diameter by 30-ft-long disposable inner barrel. The gel is contained inthe inner barrel before coring and is distributed by a core-activated floatingpiston valve after core begins to enter the core head (Fig. 1b). Excess gel isdisplaced from the inner barrel by the core through the extended pilot catchershoe, out the throat of the bit, and past the cutters, where it mixes with andis dispersed by the drilling fluid. At the cutter/rock contact, the geldisplaces drilling fluid and protects the core from flushing anddrilling-fluid-filtrate invasion. Static filtrate invasion of the core byoverbalanced drilling fluids in the inner barrel, an intrusive phenomenoncommon to conventional coring, is eliminated. A safety relief valve mounted atthe top of the coring assembly prevents excessive pressure buildup in the innerbarrel. Approximately 2 gal of gel remains in the inner barrel annulus andforms a thin protective layer over each 30-ft section of core. Fig. 1c shows agel encapsulated core before surfacing.
Gel encapsulation of the core is not known to affect surface gamma loggingor noninvasive core imaging (e.g., X-ray and nuclear magnetic resonance). Thepreservation gel can be peeled, scraped, or wiped from the core surface. Iffull-diameter core testing is to be performed, mechanical removal of the gel ormachining of the core may be necessary. Gel on the outer surface and in thenear-surface pore layers of the core generally presents no more problem to thecore analyst than mudcake in conventional analysis. Downhole core preservationcan be used in combination with pressure-retained core barrels if residual oilsaturation determinations or special core studies are to be conducted.
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