Methodology for Testing Casing Drilling Connections for High Pressure/High Temperature Applications
Authors: Hegler M.A. , Lu Q. , Griffin R.C. 
Published: October 2008

The three major requirements for casing drilling connections are for the connection to be fatigue resistant, be able to withstand high torques, and be economical. As casing drilling becomes more common in the industry, the requirements for casing drilling connections will become more critical and will add additional requirements of high pressure/high temperature resistance during and after drilling operations. A methodology for verifying connection performance capabilities will need to be developed to simulate the unique downhole conditions encountered while drilling with casing.

Introduction

Casing, by definition, is a steel lining run into the wellbore and cemented in place to permanently protect the wellbore and the environment. It is installed in the wellbore after the drilling has taken place and is meant to be installed in a mostly static condition. The loads that are concerns with a typical casing string are tension, compression, bending, internal pressure, and external pressure. Two other factors to be considered are the inside diameter (ID) of the wellbore which will dictate the outside diameter (OD) of the casing connection and the OD of the tools that will be run in the inside of the casing which will dictate the casing drift. Casing connections have been designed to meet these requirements in the way of flush, semi-flush, and threaded and coupled connections.

Testing of casing connections has been a common practice in the oilfield for decades. These tests were designed to verify performance ratings specified by the connection manufacturer. There are many different testing criteria that have been used including industry standard test procedures, i.e. American Petroleum Institute (API) Recommended Practice 5C5, International Organization for Standardization (ISO) 13679, and customer written test procedures. All of these tests, differing in philosophies, were designed to test the casing for its traditional usage.

Drill pipe, by definition, is high-grade steel pipe providing a closed flow system, through which torque is transmitted from the surface to the drill bit for rotation. Drill pipe, unlike casing, is used repetitively under high torque applications from one rig to the next. Drill pipe connections are typically subjected to cyclical fatigue, high torsional loads, and multiple make-ups and breakouts. Most of the time, drill pipe is tested per customer requirements or per API Recommended Practice 7G.

Finite Element Analysis (FEA), used to determine stress profiles in connection designs by means of numerical methods, is a shared resource between both drill pipe and casing connection designers. FEA can determine stress distributions in connections by applying simulated loads.

Casing drilling technology is today being evaluated as an economical alternative to conventional drilling that eliminates the process of retrieving a drill pipe string by utilizing the casing string for both drilling and wellbore completion. The bottom hole assembly (BHA), consisting of downhole tools including the drill bit, is attached to the casing. This allows the wellbore to be drilled out and the casing to be cemented in place in one convenient process.

With the emerging technology of casing drilling, casing connections were designed to meet the different types of loads that this type of drilling requires.