Wellbore instability can be attributed to several causes. The ones thought to be most important include: chemical interaction with the drilling fluid, high tectonic stresses, and insufficient mud weight. Drillstring vibration, although not traditionally addressed as a potential cause, might influence the stability of wellbores drilled in specific formations. Evidence of the strong correlation between severe vibration and wellbore instability has been reported in the literature. However, a more thorough understanding of the phenomenon is still lacking. This paper describes a study that has been developed by PETROBRAS focusing on how drillstring vibration impacts wellbore instability. Vibration has been monitored in some wells, and events related to borehole enlargement were observed. Four field cases are presented showing a strong correlation between high vibration level and wellbore enlargement in different lithologies. Other sources of wellbore enlargement have also been identified, and they can be clearly separated from vibration.

An experimental program was performed to investigate the fundamental fatigue mechanisms of aluminum drill pipes. Initially, the fatigue properties were determined through small-scale tests performed in an optic-mechanical fatigue apparatus. Additionally, full-scale fatigue tests were carried out with three aluminum drill pipe specimens under combined loading of cyclic bending and constant axial tension. Finally, a finite element model was developed to simulate the stress field along the aluminum drill pipe during the fatigue tests and to estimate the stress concentration factors inside the tool joints. By this way, it was possible to estimate the stress values in regions not monitored during the fatigue tests

This paper was prepared for presentation at the 1999 SPE/IADC Drilling Conference held in Amsterdam, Holland, 9-11 March 1999.

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This paper describes the tests and concept behind a new hydraulic hammer. The objectives of these tests were to evaluate penetration rate performance of this hammer when used in conventional drilling conditions. To date the usage of hydraulic hammers has been hindered due to limited compatibility with drilling fluids solids content. Therefore, this hammer was conceived to operate with all kinds of drilling fluid, including Lost Circulation Materials. Additionally, this hammer would be used with conventional tri-cone bits. This is significant because inter-bedded formations can be drilled without repeated changes to the bottom-hole assembly and drilling can continue in the event of hammer failure. The technology consists of a rotating valve system that alternately directs fluid to a piston that drives down a steel mass to strike the rear of the bit and then to a port that bypasses the piston. This action allows the mass to return to its original position ready for the next downward stroke. The valve operates at a known frequency that is directly proportional to flow rate. Due to these unique characteristics, the Hard Rock Drilling JIP decided to test the hammer, the results of which are discussed here. Introduction Compared to conventional techniques, percussion tools are capable of increasing penetration rates in hard rock. For example, while drilling granite, penetration rates of 30m/h have been achieved1. However, the usage of hydraulic hammers has been limited by solids content in drilling fluids and the requirement for specialized drilling bits2. Targeting this particular application, the development of a new hydraulic hammer was initiated by Andergauge in 1996. Key operational objectives were 1) Penetration rate increase (Figs 1 and 2) 2) compatibility with all kinds of drilling fluid, including lost circulation material 2) compatibility with conventional tri-cone bits. The technology consists of a rotating valve that alternately directs drilling fluid to a piston that drives down a steel mass to strike the rear of the bit (Figure 3). Drilling fluid then flows through a port by-passing the piston enabling the mass to return to its original position ready for the next downward stroke. The valve operates at a known frequency that is directly proportional to flow rate (Figure 4). The rotating valve continuously generates pressure pulses, driving the steel mass eight times a second, which delivers 80,000 - 100,000 lb impact force at the bit. The impact improves penetration rates, but is of a sufficiently low magnitude to avoid damaging bit journal bearings. The rotating valve concept is proven in another established drilling tool. It has been used to withstand harsh down-hole drilling conditions in over than 250 runs, some of which have exceeded 200 circulating hours. Three tests had already been conducted using the same type of bit, BHA configuration to drill identical formations with and without the hammer. The first occurred in Stavanger, Norway. In this test, the rate of penetration using the hydraulic hammer was recorded as 50% to 100% higher than without. The second test was performed in Indonesia and the rate of penetration with the hydraulic hammer was 84% higher than without. However, both tests presented durability problems. A third test, considered an endurance trial, was conducted in Oklahoma, USA. The longevity of the run determined the test to be a success, independent of the 22% rate of penetration increase achieved. This run was considered a breakthrough as the hammer proved that it was capable of:tri-cone bit compatibility;drilling fluid compatibility;life expectancy in excess of 60 hours; andincreasing rate of penetration. Consequently, the Hard Rock Drilling JIP decided to evaluate the performance and life expectancy of the hammer through a series of tests in Stavanger, Norway.