Damage to core during acquisition and handling, as well as the high cost of some coring operations, have been major issues confronting the coring and core analysis industry. Core damage leads to analytical difficulties in the laboratory which can compromise the reliability of core analysis. It is necessary to conduct tests on undamaged core, because major reservoir evaluation issues and reserves estimates can be dependent 'on core-based data. The overall cost of coring is influenced more by rig time rather than direct charges for coring services. A dramatic drop in the number of cores being cut by operators over the last 10 years demonstrates industry's perceived value of core analysis. Two new tools, gel coring and coring while drilling (CWD), have been developed to provide geoscientists, reservoir engineers and drilling engineers with options to improve reservoir evaluation and reduce coring costs.
Downhole core preservation and encapsulation using high viscosity gel is an alternative to operator-intensive, wellsite core preservation. Standard downhole coring assemblies do not, preserve in situ reservoir properties because no provisions are made for core preservation prior to core surfacing. Low invasion coring systems help minimize drilling fluid invasion, but rock wettability and fluid saturations can still be altered by counter-current imbibition of mud filtrate and/or diffusion before core analysis begins. Core gel is a viscous, high molecular weight, polypropylene, glycol with zero spurt loss, which is non-soluble in water and environmentally safe. Because the Gel comes in direct contact with the core during and immediately after it is cut, further exposure to core contaminants is minimized. The high viscosity gel stabilizes poorly consolidated rocks with moderate compressive strengths and enhances core-integrity. Core gels can be customized to address most coring situations and rock types.
The coring while drilling (CWD) system is designed to provide operators with the flexibility of bottomhole coring or drilling with the same bit, without tripping-out of the borehole. In the drilling mode, the system is used in the same manner as a conventional bottomhole assembly (BRA). In the coring mode, a drill bit plug is replaced with an inner barrel and bearing assembly that transforms the drill bit into a core bit. After core recovery, the coring assembly is retrieved with a wireline and overshot assembly. Additional cores are cut or the retrievable drill (plug is quickly reconfigured for drilling ahead. CWD uses high-rate of penetration (ROP) anti-whirl polycrystalline diamond compact (PDC) bits and significantly reduces the time necessary to cut continuous full-diameter cores. The CWD system excels when core depths cannot be determined a priori and where thick non-reservoir sections separate zones of interest.
Drilling fluid filtrate invasion during coring can add uncertainty to the interpretation of core analysis results and may preclude fresh-state testing. In particular, in situ rock wettability can be altered by drilling fluid components and affect critical petrophysical and reservoir engineering parameters, e.g., residual fluid saturations, Archie saturation exponent "n," and relative permeability.