The mud weight at which lost circulation occurs depends critically on hole deviation. Similarly, collapse of a high angle well may occur with a mud weight which would provide adequate support in a vertical hole. For this reason, a Mechanical Stability Log (MSL) has been developed which, for a given well plan, calculates the range of mud weights which could be used for drilling that well without causing lost circulation or hole instability problems (eg sloughing or caving). It must be noted that chemical effects are not considered: that is, lost circulation or hole instability problems are assumed to have purely mechanical origins. The MSL uses wireline logs obtained in wells drilled in the vicinity of the planned well as input data. Ideally, density, gamma ray, compressional sonic and shear sonic logs are used. If all of these are not available estimates of "safe" mud weight can still be obtained although with less reliability. Introduction When a borehole is drilled, the process may be thought of as one of replacing the rock which was originally in the hole with drilling mud. This causes a disturbance to the stresses local to the hole because a column of rock which supported three, probably different, principal stresses is replaced by fluid in which the three principal stresses are equal and, typically, lower than any of the stresses in the original rock column. This stress change causes deformation of the rock surrounding the hole and may lead to failure. In 1968, Fairhurst calculated the stress change that would occur local to a borehole drilled in a formation which deformed according to a linear elastic constitutive equation. This solution is given, for reference, in the appendix. It has been used by Bradley, amongst others, in considering wellbore stability issues. Figure 1 is calculated using Fairhurst's solution. The solid line defines the mud weight which if exceeded with result in tensile failure of the borehole. The two dotted lines define mud weights outside which (that is, above the upper line and below the lower line) Coulomb shear failure of the hole wall occurs. it should be noted that the figure is not in any sense a type curve: the graph shown is for a specific depth, with specific assumed in-situ stresses, and specific rock strength and failure properties. It will be assumed in what follows that if tensile failure occurs lost circulation may result and that, therefore, to be "safe", the mud weight must lie below the tensile failure curve. Furthermore, it will be assumed that shear failure may result in sloughing and hole collapse: to be "safe". The mud weight must he between the two shear failure curves. So in figure 1, "safe" mud weights for the particular lithology at the particular depth considered, are those in The shaded region. It is of particular interest to note from this figure that no "safe" mud weight exists for deviations greater than about 65deg. P. 275

A major operator has initiated the data-acquisition campaign in the southern North Sea for a future storage facility capable of holding 5 billion m 3 of gas. It is estimated this venture will double the existing gas supplies stored in the UK and represent more than 5% of its annual gas demand. As North Sea gas production decreases and the UK becomes more dependent on imports, the ability to store gas has become an important part of the UK energy policy.Drilling into depleted reservoirs for gas storage produces several major technical problems and issues that must be addressed. This field is a pressure-depleted reservoir with a differential pressure equivalent to 7.3 lbm/gal between the drilling fluid's hydrostatic pressure and the reservoir pressure. This differential must be controlled to eliminate the risk of differential sticking, downhole losses, and hole collapse.Because of the reservoir depletion, it would be impossible to backflow and clean up the near-wellbore region without a postdrill-in treatment fluid to remove the fluid filter cake and waterwet all the surfaces for gas injection. To ensure project success and usable fluid designs, reservoir conditions were simulated in the laboratory and fluid parameters were altered to provide the optimum properties to minimize the future risks.The paper discusses in full the laboratory design process, the verification of the drill-in and treatment fluids as being fit-forpurpose, and their successful application in the field. Initial well testing suggested that the expected injection rates of 500 scf/min at 300 psi were exceeded, with rates of 750 scf/min at 280 psi reported.Stephen Vickers is the Eastern Hemisphere Applications Engineering Manager at Baker Hughes, where he has worked since 2001. His main area of interest is drill-in fluid design with emphasis on minimizing fluid induced formation damage. He studied quarry and mining engineering at the Doncaster School of Mining and Mineral Resources.Stephen Bruce is a fluids service representative for Norway Operations for Baker Hughes. He studied chemistry at State University of New York at Binghamton and oilfield chemistry at RGU.Alistair Hutton is a technical sales representative for UK Operations for Baker Hughes. He studied chemistry at Aberdeen University and software technology at RGU.Paolo Nunzi is a well operations manager for Eni UK. He has been with Eni E&P for more than 25 years. He served as an engineer for 8 years in several different countries, primarily in northern Africa, northern Europe, and CIS. He holds an MS degree in mining engineering.

A major operator has initiated the data acquisition campaign in the Southern North Sea for a future storage facility capable of holding 5 billion m3 of gas. It is estimated this venture will double the existing gas supplies stored in the UK and represent over 5% of its annual gas demand. As North Sea gas production decreases and the UK becomes more dependent on imports, the ability to store gas has become an important part of the UK energy policy. Drilling into depleted reservoirs for gas storage produces several major technical problems and issues that must be addressed. This field is a pressure-depleted reservoir with a differential pressure equivalent to 7.3 lb/gal between the drilling fluid's hydrostatic pressure and the reservoir pressure. This differential must be controlled to eliminate the risk of differential sticking, downhole losses, and hole collapse. Due to the reservoir depletion, it would be impossible to backflow and clean up the near-wellbore region without a post drill-in treatment fluid to remove the fluid filter cake and water-wet all the surfaces for gas injection. To ensure project success and usable fluid designs, reservoir conditions were simulated in the laboratory and fluid parameters were altered to provide the optimum properties to minimize the future risks. The paper discusses in full the laboratory design process, the verification of the drill-in and treatment fluids as being fit-for-purpose, and their successful application in the field. Initial well testing suggested the expected injection rates of 500 scf/min at 300 psi were exceeded, with rates of 750 scf/min at 280 psi reported.

Shales are widely blamed for drilling problems. Such problems are frequently attributed to mud/shale problems are frequently attributed to mud/shale chemical interaction, abnormal pore fluid pressures and excessive overburden or tectonic earth stresses. Problems experienced include 'lost time' due to tight Problems experienced include 'lost time' due to tight hole, sloughing, and even loss of the hole. This paper attempts to evaluate the practical application of mechanical stability studies to directional well planning and emphasizes the importance of a planning and emphasizes the importance of a 'combined approach' when tackling specific shales, where problem severity is frequently best understood in terms of both borehole stress and chemistry. An analytical technique has been developed, which utilizes wireline sonic, bulk density and gamma ray data from offset wells, to estimate the mechanical integrity of the wellbore with depth and hence the mud weights required to support the borehole wall without fracturing weaker beds. Increasing hole inclination usually narrows the choice of safe operating mud weights. The log was evaluated in CONOCO's Ikan Pari development field in Block 'B', South China Sea Indonesia, where shale 'bridging' and 'sloughing' had plagued earlier drilling campaigns. These recent plagued earlier drilling campaigns. These recent efforts to identify the causes of hole problems in Block 'B' demonstrate the cost saving potential of stability studies to development well planning and should be applicable to many other areas. A stability log from the vertical well in an offshore development field can be re-run at the planned well trajectories; this can provide valuable input to casing seat and mud weight provide valuable input to casing seat and mud weight selection, cement program design, fracture gradient calculation, drilling procedures through problem sections and directional well planning, - (particularly high angle profiles). Introduction Predicting the behavior of rock under applied stress is still an inexact science. Despite expanding research efforts in recent years, mechanical borehole instability is still responsible for a large percentage of todays drilling problems are probably often identified as either pore pressure or filtrate invasion problems, and the pressure or filtrate invasion problems, and the symptoms are treated rather than the disease in the case of overpressure the cure is, luckily, increasing [he mud weight, but in the case of filtrate invasion extensive water-loss treatments may be implemented needlessly. As the search for oil ventures into more structurally complex traps, at higher wellbore angles and with greater lateral displacements than ever before he importance of preventing borehole instability has become increasingly urgent. The application of laboratory derived rock constitutive models to the imprecise and hostile environment of real borehole deformation is advancing to meet this challenge. With the increasing ingenuity of wireline logging methods and the rapid growth and improvement of measurements while drilling (MWD) and logging while drilling (LWD) technology wellbore conditions and processes are daily becoming less mysterious. processes are daily becoming less mysterious. Logbased estimates of in-situ rock properties are essential to the improvement of current models of rock behavior in that they go beyond the 'macro' approach of trend lines, derived from laboratory tests on cored samples, to explore the 'micro' world of foot-by-foot variations in rock strength. Serious drilling set-backs can occur because of narrow units which may deviate from the general trends of pore pressure or rock strength. P. 283