Development of a Commercial Wireline Retrievable Coring System
Authors: Warren Tommy , Powers Jim , Bode David , Carre Eric , Smith Lee 
Published: December 2007

A wireline retrievable coring system for use with conventional drilling equipment is described. The coring system was developed and tested by Amoco and Security DBS for application in evaluating coal bed methane prospects where a large quantity of core is required and it is essential that the core be processed soon after it is cut. A drill plug allows alternating between coring and drilling without tripping the drill string. The system is particularly advantageous where long intervals are cored, multiple zones relatively close together are cored, or the exact depth of the target zone is unknown. The system has been used to core more than 4940 m in Poland, Germany, and France, with a combined recovery of 94%.

Introduction

In 1994 Amoco obtained concessions in Poland covering 450+ sqkm for the exploration for coal-bed methane. The concession agreement included an obligation to drill fifteen wells within three years. These wells were of two fundamental types: "Core holes" were needed in order to provide core to evaluate both the quality and quantity of the coal bed methane resource and to better define the geological environment. Production test holes were required to test completion methods and production rates.

A combination of Polish Mining laws and certification issues dictated that both the core holes and production holes were to be drilled with rigs that were already available, in Poland. The initial concept was to drill the core holes with a small coring rig that had been previously used by a local Polish company to core the coal seams in the same area as the Amoco concession. These wells would be drilled as "expendable" holes with no intention of completing them. The production test holes would be drilled with conventional Polish oil and gas drilling rigs with a minimum amount of core taken in these wells.

There were numerous coal seams in the concession area that needed to be evaluated. The exact depth, thickness, and number of these seams varies considerably across the concession area. The overburden rock is quite diverse, ranging from very hard and abrasive siderite to very soft shales. Information available from previous coring operations conducted in the concession area in the mid-1980's indicated that slim hole core holes were drilled with a combination of roller cone core bits and natural diamond core bits. These wells, cored with conventional coring equipment, took 120 - 200+ days to core 800 - 1700 m in wells with depths ranging from 1500 - 1800+ m.

The available information for drilling times and costs for drilling the core holes with the same technology as previously used by the local Polish contractor indicated that there were several fundamental problems to be overcome. First, the cost of the overall evaluation program appeared to be prohibitive unless a way of providing a significant cost reduction could be found. Secondly, it appeared that the coring technology used in Poland at the time would be inadequate to provide an accurate measurement of the methane saturation.

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