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TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractHistorically, wellbore instability problems have been mainly attributed to the physico-chemical interaction between the rock and the drilling fluid, especially when this is a waterbased fluid (WBM). However, recent studies have shown that these chemical effects may be irrelevant in comparison with other energetic events which are present during the drilling operation of a well. Excessive drillstring vibration has been identified as such an event that can cause a great amount of damage to the wellbore. Although some studies present clear indications that the reduction of drillstring vibration has led, in many occasions, to an improvement on instability problems, still today this item only deserves attention from equipment divisions, aiming at reducing drillstring fatigue problems.The main objective of this paper is to show the clear existing relation between excessive drillstring vibrations and serious well instability problems. It is shown, in energetic terms, that drillstring vibration, when having enough lateral amplitude to hit the wall, is responsible for irreparable damage to the borehole. Simple ways of quantifying vibration effects in terms of energy are presented. The paper also presents field measurement data, obtained with surface vibration sensors, which clearly allow identification and correlation between vibration problems and wellbore instability, through the comparison of these vibration records with caliper logs.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractIdentification of the correct cause of wellbore instability is the key to quickly solve the problem. This paper presents some results of tests conducted to check the behavior of preserved shale samples when immersed in different solutions, trying to identify the importance and relevance of the osmosis as well as the effect of other components in the mud, such as the emulsifier. Results showed that not only the salinity of the mud is important. Also, in some situations, rock swelling did not follow what states the osmosis theory, leading to a conclusion that other phenomena are relevant and with a similar order of importance.The paper also discusses the mechanism of micro-cracking with subsequent fluid penetration, and the use of a truly noninvasive fluid to avoid the instability. Field cases are reported, demonstrating the effectiveness of using such approach. The paper also presents the problems ofcurrent rig site monitoring tests that can induce wrong interpretation of the real cause of the problems.

TX 75083-3836 U.S.A., fax 01-972-952-9435. AbstractDrilling wells for oil/gas has been increasingly challenging with the companies moving towards difficult environments. The problems faced in these locations range from very narrow margin between the pore (or collapse) and fracture pressures, pore pressure uncertainty, to high pressure and high temperature wells. Wells drilled in these scenarios using the conventional drilling method often do not get to total depth (TD), and even so, drilling can be extremely risky, with several kick/loss situations.A new drilling method 1 has been developed to overcome these problems, allowing a much safer condition, reducing the risks, and also permitting the wells to be drilled to TD much cheaper. Drilling is taken to the limit in a safe manner, extending each phase as much as possible, using the entire available mud weight window for that well.The method uses the new concept of micro-flux control, which is based on detecting a minimum loss or influx of fluids, and instantly adjusting the return flow and, consequently, the bottom-hole pressure to regain control of the well. The well is drilled closed at all times, and the return flow from the well is compared to the predicted and ideal one, allowing detecting the discrepancies in a very short time.This paper describes the basis of this new method and steps taken so far in the development. Field tests will be done very shortly, after the method has been tested in a simulated well condition. The use of this method allows wells to be drilled where today it has been impossible, extending today's technological limits way beyond the current boundary.

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