Many petroleum companies expand their activities further north towards the Arctic region, resulting in design temperatures down to −60• C, which is much lower than what is usual for most current petroleum installations. As properties of steels are temperature dependent, it is of great interest to evaluate the effects of low temperature on the crack driving force in steels. The present work investigates these effects numerically using finite element (FE) models of single-edge-notchedtension (SENT) specimens with crack depths 0.1 ≤ a/W ≤ 0.5. The effects of Lüders strain, yield strength and crack depth on the crack tip opening displacement (CTOD) and the relation between CTOD and crack mouth opening displacement (CMOD) are studied, and it is shown that an increase in yield strength and Lüders strain, as a result of Arctic temperature, intensifies the crack driving force. It is also shown that the crack depth has very little influence on the effect of Lüders strain on the CTOD. An approximate model that can be used to estimate the CTOD based on yield strength, Lüders strain and loading is proposed for gross stress levels σ G /σ y ≤ 0.5 and a crack depth a/W = 0.5. It is finally shown that the tensile properties have a more significant effect on the CTOD-CMOD relation than the crack depth for a SENT specimen.Mange petroleumsselskaper utvider sine aktiviteter nordover mot arktiske områder. Dette fører til designtemperaturer ned til −60• C, noe som er mye lavere enn det som er vanlig for de fleste nåvaerende petroleumsinstallasjoner. Ettersom egenskapene til stål er temperaturavhengige, er det av stor interesseå evaluere effektene av lav temperatur på den drivende kraften for sprekkvekst i stål. Dette arbeidet undersøker disse effektene numerisk ved hjelp av elementmetoden med modeller av single-edge-notched-tension (SENT)-bruddprøver med sprekkdybder 0.1 ≤ a/W ≤ 0.5. Effektene av Lüders tøyning, flytespenning og sprekkdybde på sprekkspissutvidelsen (CTOD) og sammenhengen mellom CTOD og sprekkmunningsutvidelsen (CMOD) er studert, og det er påvist at økt flytespenning og Lüders tøyning, som et resultat av arktisk temperatur, fører til økt drivende kraft for sprekkvekst. Det er også vist at sprekkdybden har liten innflytelse på effekten av Lüders tøyning på CTOD. Det er foreslått en omtrentelig modell som kan bli brukt tilå estimere CTOD basert på flytespenning, Lüders tøyning og last for bruttospenningsnivåer σ G /σ y ≤ 0.5 og sprekkdybde a/W = 0.5. Det er til slutt påvist at materialets strekkegenskaper har en større virkning på CTOD-CMOD-forholdet enn sprekkdybden for en SENT-prøve.i ii PrefaceThe work presented here is a Master's thesis written at Department of Engineering Design and Materials at Norwegian University of Science and Technology (NTNU). The thesis work is carried out as a part of SMACC (Studies of materials behavior for future cold climate applications), which is a research project with several industry partners, and with SINTEF and NTNU as research partners.The author wishes to thank SINTEF fo...

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Experience has shown that adding suitable particles to the drilling fluid can significantly improve the formation strength. This is especially beneficial when drilling wells with a narrow operational window as is typically the case when drilling depleted reservoirs. Successful operations have employed several types of particles, including graphite and calcium carbonate. For this kind of approach to work, it is important to establish an optimal particle composition in the drilling fluid. In a recent paper, SPE 107574, the possibility for continuous monitoring of the particle size distribution (PSD) and particle content in the drilling fluid, was demonstrated. This present work shows how these findings have allowed deploying a unique technique for running of solids control equipment in a North Sea offshore field, providing optimal particle concentration and size distribution when drilling a significantly depleted reservoir. We describe in detail an HTHP offshore operation where coarse shaker screens were used to allow relatively large particles to re-enter the well during circulation. These particles act just like particles deliberately added to the drilling fluid in order to enhance formation strength. The monitoring equipment allowed for close control of the PSD of the drilling fluid flowing into and out of the well during drilling. The particle size distribution is compared to that obtained with conventional additives previously used to achieve formation strength enhancement. Introduction Drilling through depleted reservoirs is a challenge since the pressure difference between the fracturing pressure and the pore pressure becomes small; or even sometimes negative. During the last decade a technique has been suggested that increase the fracture strength towards the original strength, and, in some cases the new fracture strength has exceeded the original strength. This method is based on fracturing the borehole wall with small fractures and then fill these with impermeable particles to stop further propagation of the fractures at the same time as the fractures remain increasing the formation strength of the remaining portion of the borehole wall. The concept of increasing the formation strength while curing lost circulation has been discussed by Messenger1 and Morita et al.2. Fuh et al.3 suggested using this method while drilling to prevent lost circulation. A suggestion of the selection of material and the theoretical treatment were later refined by others4–10. Theoretical analysis and all analysis of field experience on formation strengthening conclude that it is necessary to optimize the particle size distribution of the added solids. The fracture must be sealed by a non-permeable easily plugging material. The plugging is caused by arching or gel formation or a combination of these two items. Arching in pipes and conical sections has been a large subject for research the last century11,12,13, and the results are included in most textbooks on soil -or powder mechanics14. Although this subject is well established theoretically, there is still a need for experimentally optimizing the particle size distribution and particle content. Furthermore, for drilling fluid applications this particle size distribution must be optimized with respect to drilling fluid viscosity profile, gel formation and gel fragility, viscoelastic properties as well as chemical properties of the fluid and particles. In the laboratory there is therefore a need to develop proper equipment to evaluate different selections of added particles. An example of such a device has been presented by Hettema et al.15, and this device has been applied to further improve the particle size distribution.

Since the development of the drilling industry the process has culminated in using shaker screens as the primary or only device for removing drill solids from the drilling fluid. Therefore it is necessary to optimize both filtration efficiency as well as the screen life to hinder drilled solids entering the drilling fluid. Optimum solids control can be obtained by using the understanding on how damage of the filtration cloth arises, on how to reduce it and finally on how the particles in the circulation system influence the general picture of the drilling process. When this knowledge is accepted in the industry, established and implemented in the drilling organisation; it is possible to maintain proper drilling. This paper describes in detail the theory and field examples on how wear arise on the shaker screen cloth. As will be shown, this knowledge has been used to increase the solids control efficiency at the same time as the screen wear has been reduced by more than 90 % in field cases were 17 ½″ sections have been drilled with OBMs. The documentation is based on practical offshore result from these drilling operations including data from the drilling log, laboratory analyses of the drilling fluid and of the Particle Size Distribution (PSD) of the drilling fluid. The particular focus of this article is the application of double deck shakers. Introduction Correct use of solids control equipment is essential to maintain drilling fluid within its desired properties and to avoid generation of unnecessary waste streams during drilling1. Since the early 1930s the shale shaker has been the dominating device for primary solids removal2. Additional equipment like desilters, desanders and centrifuges were often used in the past to maintain proper solids control. Although it is dependent on the choice of correct shaker screens, at present most shakers have a sufficient performance to be able to act as the sole solids control device without the use of desanders and desilters. The optimum solids control design for a particular drilling fluid may not be generally valid for all fluid types3. A combination of shaker and screens applicable for treating water based drilling fluids may for example not be suitable for treating oil based drilling fluids. Furthermore, the suitability of the screen and shaker combination may change during drilling because the drill cuttings morphology changes. Throughout the last decades major shaker design improvements have been made. The circular motion shakers used up to the 1980s have been replaced by elliptical motion and linear motion shakers. Furthermore, double deck, or even triple deck shakers have been implemented in the industry. Alternatives to shakers have been tested to improve HES issues like hydrocarbon vapour in the shaker room, although this type of equipment has not yet reached the marked4. Sinusoidal formed screens have been implemented on some shakers to increase the flow capacity5. Shaker operation has also been automated6. Scott6 claims that use of this automated system leads to an increase in shaker screen life. However, Scott6 do not reveal the screen selection for this case. Therefore it is difficult to use this information in the present analysis. Removal of solids with a particle diameter larger than 120–150µm, can be achieved without problems on most shakers today by the application of the correct screen size7. There are many types of screen on the marked. The following analysis is general and do not compare any products or designs. Screen opening and mesh sizes Typically, a shaker screen may consist of a single metal cloth or be constructed as a series of superpositioned cloths. In some cases these cloths are tensioned in all directions while being melted onto a frame, while in other cases the cloths are melted onto frames without being tensioned. Some screens are tensioned onto the shakers directly without being attached to frames. The screen cloths are woven with warp wires running along the cloth and weft wires running across the cloth as it is woven. The warp and weft wires can be equal or different, giving a large variation in possible screen cloth designs.

Major advancements have been made in the past decades to determine the effects from particle additions to drilling fluids. These additions affect wellbore stability, hole cleaning, sag stability, formation damage and back-production capabilities. Furthermore, optimum drilling fluid performance is strongly dependent on knowing the properties of the formation such that correct selections of drilling fluid additives can be made. Likewise, it is important to know if produced solids are drilled cuttings or cavings originating from unstable holes.In field applications, there has been reluctance towards trusting solids control equipment as the only method for controlling the particle size distribution (PSD) and particle content of the drilling fluid system, since no real-time monitoring equipment has been available to produce the necessary measurements. The present paper describes a technique based on image analysis, which makes it possible to obtain such information in real-time. The method also provides valuable information for characterizing drilled cuttings, creating the basis for caving logs. A method for obtaining mineralogical data from the formation by analyzing the drilled cuttings by using Raman spectroscopy is also described. Field tests and laboratory studies demonstrate the potential of the techniques for improved drilling process control by continuously monitoring the particles in the system while drilling. Some of the elements in a drilling fluid process which are affected by the presence of particles are also described. fax 01-972-952-9435.