Development of a Mud-Motor-Powered Coring Tool
Authors: Miller J.E. , Huey D.P. 
Published: October 2008

A new wireline retrievable mud motor powered coring tool has been developed. A significant aspect of this development project was a "design by analysis" approach whereby the performance of the tool was completely analyzed prior to performing the detailed mechanical design. This paper describes the methods used in the project and includes discussions of concept studies, computer modeling, design, testing and deployment. The features and observed performance of this tool are also highlighted.

INTRODUCTION

Ocean Drilling Program (ODP) operates the dynamically positioned drill ship SEDCO BP471, also known in the scientific community as JOIDES Resolution. With this vessel, ODP has performed scientific coring at locations around the world. The goal of the coring activities is to further earth sciences by retrieving core samples for scientific analysis. More information about ODP appears in (1).

Many types of materials are encountered while drilling for core samples and each geographical location may have a different lithology, but a "model" lithologic profile may be composed of unconsolidated sediments near the sea floor which change to firmer clays or sandstones at intermediate depths and ultimately transition to hard basalt structures at deeper depths. ODP has pioneered and adapted a variety of technologies for coring in each of the common lithologies encountered offshore since no single technique or tool is effective in all conditions. Generally, however, the tools of ODP are more efficient and well developed for coring sediments than for the hard basalt structures.

The bottom hole assembly (BHA) fielded by ODP accommodates the various types of coring tools and the BHA design allows coring to continue for long distances without tripping the main 5 inch drill string. Commonly, the coring tools are designed for deployment down the inside of the main drill string (4-1/8" min. diameter) and are sized around the "HQ", 3-1/2" nominal core barrel systems from the mining industry. These tools and core barrels are retrieved via wireline. The BHA provides a landing shoulder (3.8" min. diameter) for location and support of the coring tools and includes a sub to allow latch-in and torque transfer between the BHA and the contained coring tool. The entire drill string, BHA and nominal 11-7/16" roller cone bit are designed to pass slim-hole coring equipment of a 3-3/4" maximum diameter.

The generally accepted method for coring in hard rock, developed in the mining industry, uses thin kerf diamond bits at relatively high rotational speeds, usually between 200 and 700 rpm. Successful coring operation requires close control of weight on bit (WOB) and flow rate to the bit. The driller has the ability to modulate WOB and water flow rate to keep the core bit running effectively. A good overview of slim hole coring operations appears in (2). ODP is continuing development of a system called the Diamond Coring System (DCS) that mimics this operation by running a 31/ 2" mining string inside the main drill string and which utilizes a high speed electric top drive to turn the mining string.