Drill pipe Fatigue in a fast drilling environment: how to cope with the extreme!
Authors: Plessis Guillaume , Wright Christopher , Aranas Agapito , Jellison Michael J. , Muradov Andrei 
Published: December 2007

The Pattani gas field in the Gulf of Thailand is known for having marginally economic reservoirs and so the operating company, Chevron Thailand Exploration and Production LTD (CTEP formerly known as Unocal Thailand LTD), has always needed to drill wells as fast as possible and at the lowest economic cost. This quest for efficiency led to selecting a slim hole drilling program that is completed within days, and as such, puts a lot of stress on the drilling equipment and in particular the small size drill string.

Over the years, the size of the drill pipe used to drill the production interval has evolved from 3½ to 4 in. with the use of a streamlined second-generation double-shouldered connection. Selecting a drill pipe configuration with higher torsional strength, improved hydraulics and buckling resistance contributed to improved performance and drillers delivering wells faster. The move to a 4 in. drill pipe body increased the stiffness and fatigue resistance compared to the 3–1/2 in. pipe. As a result, the fatigue resistance of the premium drill pipe connection was considered to be an area of improvement for the special fast drilling operation.

This paper will present the various options considered to enhance the connection fatigue resistance, the results obtained in simulation, laboratory testing and ultimately in operation. This last section includes information on the inspection reject rates and impact on the frequency of inspection that directly relates to the level of confidence one has in the equipment.

Need for fast drilling in the gulf of Thailand The well design currently drilled in the CTEP operated Pattani gas field has evolved over the past 40 years to one that can now be regularly drilled and completed in less than 4.5 days. These wells are a three string, slimhole, well that is completed as a monobore. The reserves per well are marginal and are further complicated by a steep production decline so, CTEP has to drill hundreds of new wells each year to maintain production output. This requirement to drill and complete hundreds of additional wells means that the well design must be executed in the shortest time and at the cheapest cost so as to maintain the viability of the field.

With a now well refined well design and drilling program, the rate of penetration (ROP) and subsequent drilling time are the main variable that influence the overall days per well. For this reason the emphasis by the CTEP drilling department has always been on improving ROP through innovation of drill bit design, bottom hole assembly tools and design and drill pipe design and specifications.

A typical well profile drilled in the CTEP operated Pattani field is shown in Figure 1 of the Appendices. Build rates in the shallow section of the well are typically 8 degrees per 100 ft so as to maintain the 8½ in. tangent inclination below a nominal 60 degrees; this to ensure firstly that open hole logs can be run in the slimhole section and also that the well can be perforated on electric line. The high doglegs in the shallow section of the well greatly contribute to the stress placed on the drill string and factor in the fatigue life of the drill pipe.

The extreme nature of this fast drilling environment sees the drill pipe constantly rotated at between 180 to 250 revolutions per minute during the drilling of the entire slimhole section. It is common for the on bottom rate of penetration in the production hole section to exceed 500 ft/hr and overall section ROP including the time it takes for making connections can reach 300 ft/hr. In one year a string of slimhole drill pipe can drill as much as 300,000 ft of open hole formation.

Drill string selection and improvement Several drill strings are used for drilling the various sections of these wells.

Top hole and intermediate hole sections. The first two intervals, the 12 ¼ and 8 ½ in. hole respectively, are drilled with a string of 5 in. API drill pipe. The total on bottom drilling time for these two sections is typically less than 8 hrs. Drilling performance in this part of the well program is driven predominantly by bit selection and directional tool performance and thus can be drilled efficiently using standard drill pipe with conventional API rotary-shouldered connections. This paper does not cover these hole sections and will only focus on the slim hole section that enters the pay interval.