Increasing Core Quality and Coring Performance Through the Use of Gel Coring and Telescoping Inner Barrels
Authors: Whitebay Lee , Ringen Jon K. , Lund Torger , van Puymbroeck Luc , Hall Larry M. , Evans Rob J. 
Published: December 2007

Recent advancements in coring technology are improving core quality and reducing costs for North Sea Norwegian operators. Combing low invasion anti-whirl core bit technology with Gel Coring TM and telescoping inner barrels is resulting in recovery of high quality core. As a result, accurate saturation, petrophysical and rock mechanics data are being obtained. Cores provide the ground truth for calibrating logs and verifying assumptions about reservoir properties, and are critical for accurately forecasting project economics. Pressure to balance the cost of coring against the need for high quality core material for analytical studies led to the application of these emerging technologies.

This paper reviews coring programs conducted in the North Sea, offshore Norway, focusing on challenges encountered and solutions found. Historically, core quality and recovery had been less than desired. Core barrel efficiencies were running approximately 70%, including sections of damaged and poor quality core. Case histories reveal how combining several emerging technologies led to reducing overall time spent coring, and improving core quality for saturation determinations, special core studies, and rock mechanics testing. Specifically, the use of Gel Coring TM and telescoping inner core barrels significantly improved core quality and recovery in formations prone to jamming. Results of coring operations are presented to show the impact combining these technologies are having on coring programs. Significant progress has been made in reducing coring cost per foot, improving core quality, increasing core recovery/efficiency, and enhancing overall coring performance.

Introduction

Core samples provide insight into oil and gas reservoirs that is unavailable from other sources, Keelan D., 1985. Used to calibrate geophysical, petrophysical, geological, and reservoir models, cores yield a wealth of invaluable information. Core provides details on geologic setting, a direct means of resource assessment, and a source of reservoir, drilling, and completion engineering data. The utility of core is rarely doubted, but high drilling costs forced operators in the North Sea to look for ways to cut cost while continuing to acquire high quality core samples. Striving to improve the cost effectiveness of coring programs, operators are employing new technology as soon as it emerges from the laboratory, in some cases before it has been perfected through field trials. They are using increasingly longer core barrels, anti-whirl low-invasion coring bits, high quality drilling fluids, gel, motors, and telescoping inner core barrels in their quest to drive down costs while accomplishing core analysis goals.

Prior to the development of low invasion coring bits, drilling mud filtrate invaded deeply into core during the coring process. This changed fluid saturations, and often altered the wettability of the rock, complicating reservoir evaluation. Sharma, and Wunderlich, 1987, reported that most drilling mud filtrates had the ability to alter the wettability of core, so keeping filtrate out of cores became an important factor in many coring programs. Rathinell, et. al. 1990 and Tibbetts, et. al. 1990, pointed out that most filtrate invasion into core takes place in the throat of the bit as the core is being cut, unless the rate of penetration is very slow. When the ROP was low they saw a bank of mud filtrate develop ahead of the bit. The designers of low invasion coring bits made two significant changes. P. 419^