Innovative Concepts in Wireline Continuous Coring
Authors: Carpenter Chris 
Published: January 2017

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 180017, “Innovative Concepts in Wireline Continuous Coring,” by Rahman Ashena, Walter Vortisch, Michael Prohaska, and Gerhard Thonhauser, University of Leoben, prepared for the 2016 SPE Bergen One Day Seminar, Bergen, Norway, 20 April. The paper has not been peer reviewed.

Coring in unconventional reservoirs with typical unconsolidated formations has introduced additional challenges requiring more-innovative concepts to be developed. These requirements have recently led to widespread attention to wireline continuous coring (WCC), which was modified from the mining industry for petroleum-industry applications. This paper is aimed at identifying and addressing complications experienced in the use of WCC. In addition to the challenges and solutions discussed in this summary, others are covered in the complete paper.

Basic Bottom-Coring Methods These methods, in the context of petroleum exploration, comprise conventional and WCC methods. Conventional coring is a method of rotary coring by which the inner tube containing the core is retrieved along with the outer-tube assembly to the surface following a conventional drillstring trip.

WCC is a method of coring or drilling by which the same outer assembly is used as the bottomhole assembly and the same core bit is applied for both drilling and coring modes; however, changing the inner assembly can be performed easily by wireline in order to switch to the desired mode. WCC has been identified recently as a less-time-consuming coring method to replace conventional coring in certain circumstances.

WCC Challenges and Solutions

Flushed Inner Tube While Run in Hole. Problem. As a common practice in conventional coring, approximately 90 ft before reaching the bottom (off-bottom), mud circulation is started, still without string rotation. This is because the mud circulation that passes within inner tubes contributes to conditioning and flushing the inner tubes to ensure that the inner tube is free of debris/fill. The most problematic issue with debris/fill is the high risk of jamming off in the core bit, core catcher, or inner tube. Following tagging of the formation to be cored, immediately before beginning the coring, the ball is either dropped from surface or hydraulically actuated by virtue of a drop-ball sub in order to change the mud path from within the inner tube to the annulus between inner and outer tubes so that mud does not flush the core while entering the inner tube. After cutting enough cores or reaching coring termination, the drop ball enables any gas and trapped pressure to be released through the top of the inner assembly. It is noted that, for coring systems with closed inner-tube systems, there is no possibility for drop balls to be used; instead, an enclosed ball and seat.are used.

In WCC, an enclosed ball in place is also commonly used in the inner coring assembly (above the inner tubes) instead of dropping a ball from surface or hydraulic actuation using a drop-ball sub. Therefore, before beginning the coring, the mud never passes through the inner tube and thus never flushes the inner tube.