RECENT ADVANCES IN GORING TECHNOLOGY - NEW TECHNIQUES TO ENHANCE RESERVOIR EVALUATION AND IMPROVE CORING ECONOMICS

Damage to core during acquisition and handling, as well as the high co st of some coring operations, have been major issues confronting the c oring and core analysis industry. Core damage leads to analytical diff iculties in the laboratory which can compromise the reliability of cor e analysis. It is necessary to conduct tests on undamaged core, becaus e major reservoir evaluation issues and reserves estimates can be depe ndent on core-based data. The overall cost of coring is influenced mor e by rig time rather than direct charges for coring services. A dramat ic drop in the number of cores being cut by operators over the last 10 years demonstrates industry's perceived value of core analysis. Two n ew tools, gel coring: and coring while drilling (CWD), have been devel oped to provide geoscientists, reservoir engineers and drilling engine ers with options to improve reservoir evaluation and reduce coring cos ts. 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 reservoi r properties because no provisions are made for core preservation prio r to core surfacing. Low invasion coring systems help minimize drillin g fluid invasion, but rock wettability and fluid saturations can still be altered by counter-current imbibition of mud filtrate and/or diffu sion before core analysis begins. Core gel is a viscous, high molecula r weight, polypropylene glycol with zero spurt loss, which is non-solu ble 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 st abilizes poorly consolidated rocks with moderate compressive strengths and enhances core integrity. Core gels can be customized to address m ost coring situations and rock types. The coring while drilling (CWD) system is designed to provide operators with the flexibility of bottom hole 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 (BHA). In the coring mode, a dr ill bit plug is replaced with an inner barrel and bearing assembly tha t transforms the drill bit into a core bit. After core recovery, the c oring assembly is retrieved with a wireline and overshot assembly. Add itional cores are cut or the retrievable drill plug is quickly reconfi gured for drilling ahead. CWD uses high rate of penetration (ROP) anti -whirl polycrystalline diamond compact (PDC) bits and significantly re duces the time necessary to cut continuous full-diameter cores. The CW D system excels when core depths cannot be determined a priori and whe re thick non-reservoir sections separate zones of interest.