High-Viscosity Gel Encapsulation for Core Preservation and Improved Reservoir Evaluation

Abstract

The goal of coring and core preservation should be to obtain rock that is representative of the formation while minimizing physical and chemical alteration of the rock during coring and handling. Low-invasion coring systems help minimize drilling-fluid invasion, but rock wett ability and saturation can still be altered by drilling-fluid imbibition and/or diffusion before core analysis begins. New technology that uses high-viscosity gel for downhole core encapsulation and preservation is an alternative to operator-intensive wellsite methods. Existing low-invasion antiwhirl coring assemblies are easily retrofitted to accommodate use of a simple inner-barrel piston for gel distribution and coreencapsulation. The viscous core preservation gel is a high-molecular-weight polypropylene glycol that is insoluble in water and environmentally safe. Laboratory tests at 200 F and 250 psig on water-saturated gel-coated rocks with permeabilities 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 immediately after it is cut, further exposure to drilling fluid is minimized. Once at the surface, exposure of the core to air is reduced significantly. An additional benefit of downhole gel encapsulation of core is obvious when one considers handling poorly consolidated rocks with moderate compressive strength. The high-viscosity gel stabilizes the core and enhances the mechanical integrity of the rock. Surface handling is improved and core damage is reduced during transport to the laboratory. For rocks with little cementation, the encapsulating gel can be replaced with a self-hardening plastic, thus eliminating 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 in the 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 is displaced 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 and is dispersed by the drilling fluid. At the cutter/rock contact, the geldisplaces drilling fluid and protects the core from flushing and drilling-fluid-filtrate invasion. Static filtrate invasion of the core by overbalanced drilling fluids in the inner barrel, an intrusive phenomenon common to conventional coring, is eliminated. A safety relief valve mounted at the top of the coring assembly prevents excessive pressure buildup in the inner barrel. Approximately 2 gal of gel remains in the inner barrel annulus and forms a thin protective layer over each 30-ft section of core. Fig. 1c shows a gel encapsulated core before surfacing. Gel encapsulation of the core is not known to affect surface gamma logging or noninvasive core imaging (e.g., X-ray and nuclear magnetic resonance). The preservation gel can be peeled, scraped, or wiped from the core surface. If full-diameter core testing is to be performed, mechanical removal of the gel or machining of the core may be necessary. Gel on the outer surface and in the near-surface pore layers of the core generally presents no more problem to the core analyst than mudcake in conventional analysis. Downhole core preservation can be used in combination with pressure-retained core barrels if residual oil saturation determinations or special core studies are to be conducted.