The superior performance of invert emulsion fluids in challenging drilling operations such as HT/HP wells makes them the preferred fluid for such applications. However, there are instances such as drilling in environmentally sensitive areas, or where costs and logistics become prohibitive, where their use is undesirable. In such circumstances, a high-performance water-based drilling fluid is the only option available. A main challenge in developing a water-based fluid for such applications has been to maintain the stability of key fluid properties such as rheology and fluid loss at temperatures in excess of 130°C. Conventional additives, such as biopolymers or synthetic polymers, either become ineffective at such high temperatures or generate too high a rheology when used in quantities needed to curb fluid loss. This paper discusses a novel water-based drilling fluid system in which the synergy of the key fluid components produces a stable rheology and low fluid loss at temperatures approaching 200°C. Furthermore, by balancing the components of the novel system, fluid rheology can be controlled to meet the hydraulics and hole-cleaning demands of the drilling operation.1, 2 Introduction Today, an increasing number of drilling scenarios involve deeper and hotter reservoirs, complex well geometries and increasingly more stringent environmental requirements. These call for a new generation of high performance water-based drilling fluids, which can offer better drilling hydraulics (pressure losses and cuttings transport), high-temperature rheological stability, fluid loss control, shale stabilisation and lubrication. From a rheology point of view, the best fluid systems available to date are those that offer a high degree of shear-thinning to reduce friction losses and improve hole cleaning. These are based typically on biopolymers (e.g. xanthan and scleroglucan gum) or certain organic mixtures. While the most effective polymers for rheology control are those with high molecular weight (millions of Dalton), lower molecular weight polymers, frequently used for fluid loss control, can also have an effect on rheology. Thus, historically, control of rheology and fluid loss has required a fine balance between these two types of materials. At temperatures up to 130ºC biopolymers and organic mixtures are effective in controlling the rheology, but at higher temperatures stability problems set in and synthetic polymers must be used. Synthetic polymers, mainly polyelectrolytes, have good temperature stability but generate high plastic viscosities. This problem, which may be intensified by the high solids fraction of heavier drilling fluids, can lead to large pressure losses in narrower well sections. The conventional approach for generating and controlling drilling fluid rheology utilises various additives for the purpose of supporting and carrying the weight material and drill solids. To overcome the limits presented by such additives - biopolymers (temperature stability) and polyelectrolytes (poor shear-thinning and high plastic viscosity) - a different approach is needed. In the new approach described here, the polymers and the weight material are selected such that they both contribute to the generation and control of a highly shear-thinning, thermally stable rheology. Such improvement results from a synergistic interaction between the polymers and the weight material and requires polymers with: moderate molecular weight (to avoid excessive plastic viscosity), low charge density (charge on polymers increases backbone rigidity with impact on plastic viscosity; polyelectrolytes are more sensitive to salts, drilled solids and cement contamination) and stability at high temperatures (up to 180°C). Polymers that satisfy the above criteria produce relatively low viscosity if used on their own, which may not be adequate for suspending the weight material and for cuttings transport. However, when combined with a weight material with a specific surface charge, they produce highly shear-thinning aggregates with good suspending capacity. Figure 1 illustrates the conventional way of generating viscosity (1a) and the new approach (1b).

Thm paper was selected for presentation by an SPE Program Commmee following rewew of mformal!cm ccmamed m an abstract subnwled by the author(s) Con!ents of the paper, as presented, have nol been reviewed by the SO.aety of Petroleum Engineers and are $u@ect to correctmn by the author(s I The mafenal, as rrasented, does not necessarily reflect any pas!bon of the ?,ocaetyof Petroleum Engineers, Its offcers, or members Papars presented at SPE meetmgs am Sub)ect to pubhcat,on rev,ew by Edttorlal Committees of the S.xiety of Petroleum Engneem Permmmon to WY IS restrmted to an abstraci of not more than 3CY3words Illuslrabons may no! be copied The abstract should wtar conspu?dous acknOP.WdgMent of where and by whom !he paper was~esenled VWIte Llbrarlen, SPE P O Box 833636, Rchardson, TX 7W33-38M, U S A fax 01-214-952-9435 Abstract Xanthan Gum is the most popular biopolymer used today to provide hole cleaning and suspension capabilities to water based drilling fluids but it presents some limitations as regards temperature sensitivity and tolerance to field contaminants. In this paper a Scleroglucan is proposed as a better alternative to Xanthan Gum for drilling fluid compositions. Superior benefits offered by Sclerogiucan on hole cleaning, rate of penetration, temperature sensitivity and mud tolerance to shale have been demonstrated by successful field applications and by extensive laboratory studies. Low shear rate viscosities at 0.06 see-l and IJerschel & Bulkley yield stresses are used to quantify hole cleaning capabilities and critical MBT of lab formulations and field muds. Evaluation of cleaning efficiency and cutting concentration is performed with an in house developed program using the Walker and Mayes theory modified with the Herschel &Bulkley theological approach") The oscillation parameters Storage Modulus (G'). Loss Modulus (G") and Loss Tangent (G''/)') are used to characterize the "undisturbed" structure and thixotropy of the systems. According to existing theoriesq['-~) and to the results of' the authors field experience and laboratory studies with analogous form ulations~2, Loss Tangent and Storage Modulus are related to the degree of structuration and to the "strength" of the structure that develops in biopolymer solutions and muds under static conditions. As a consequence, these data are analyzed to determine the suspension capability and resistance to shale contamination of the systems, On the basis of field experience and laboratory studies the main advantages offered by Scleroglucan for drilling muds compositions are confirmed and underlined. The reported field data refer to the first application of Scleroglucan in a Potassium Formate based mud. In the same field other wells were drilled using Scleroglucan in different operative conditions and mud formulations and always with good results.The discussion of these data will be reported elsewhere~'. 105

Summary Selection of the best drill-in fluid for drilling horizontal wells is essentially based on the characteristics of the filter cake formed in the near-wellbore region. Minimizing filtrate loss into the formation by forming a thin filter cake with low porosity and permeability is the key to managing formation damage problems. In this paper, the effects of drill-in fluid components and their interactions in the formation of a filter cake, which provides an effective caking process with reduced filtrate loss, have been evaluated. Statistically designed experiments carried out with a dynamic filter press apparatus were used to identify which variables had the most significant effect on filtrate loss and cake permeability so that the optimum fluid composition could be determined. These variables include the formation characteristics (permeability and pressure values) as well as the type and amount of the fluid components. Data obtained in this experimental design study were analyzed with the multiple-linear-regression method, which allowed us to model the filtrate loss and cake permeability. The analysis showed that the most important factors are the type and the concentration of the polymer. In particular, when the formation temperature is as high as 90°C, we can reduce the cake permeability and the filtrate loss by increasing the concentration of scleroglucan. An interaction effect between the concentration of scleroglucan and the more highly crosslinked starch was also identified, and none of the other variables showed any significant effects within the range of experimentation. This fact can be considered as an advantage because it allows flexibility in the choice of other factors when appropriate for the cost reduction. Introduction Control of fluid filtration has long been recognized as a part of good drilling practices.1 Adequate control of drilling-fluid filtration characteristics, both the filtrate volume that enters the formation and the quality of the resulting filter cake formed in the wellbore, is needed to limit borehole instability, excessive torque and drag, pressure-differential sticking, and formation damage. This concern is even more important in drilling horizontal wells with water-based muds where the fluids remain in contact with the pay zone for a long period. Therefore, selection of the appropriate drilling-fluid composition is considered key to minimizing drilling problems and obtaining desired productivity levels.2 Development of specialized drill-in fluids3 (i.e., fluid formulations suitable for drilling reservoirs) has allowed better control of filtration phenomena during drilling operations. To limit filtration, these drill-in fluids can build up a thin filter cake with low permeability near the wellbore. As a consequence, the invasion of solid particles and mud filtrate into the formation is reduced to a minimum, and the permeability impairment is minimized. To evaluate the drill-in fluid candidates for field application, a rapid laboratory investigation was undertaken using a statistical approach. It allowed us to quantify the effects of fluid components and some operating conditions on filtrate-loss and filter-cake properties. Selecting the correct chemicals and defining their concentrations is paramount to obtaining the desired properties. Until now, unfortunately, relatively little attention has been given to the mutual interactions that may occur unintentionally between drilling-fluid components and the consequences for the properties of these complex fluids. This paper focuses on a new, experimental method to approach the selection and optimization of drill-in fluid compositions to minimize wellbore problems associated with filtration. The investigation concerns the biopolymer/CaCO3 drill-in fluids proposed as candidates for field operations in Italy. Statistically Designed Experiments By the phrase "statistical design of experiments," we refer to the process of planning experiments that will ensure that the collected data can be analyzed by statistical methods. The statistical method for the experimental design is usually the most efficient approach when the effect of several variables has to be estimated simultaneously, minimizing the number of experiments. It allows us to draw correlations when a lot of data are available, and it is useful in understanding what variables involved are the most important and if there are interactions among them. For every experimental problem, it is essential to consider two aspects - the design of the experiments and the statistical analysis of the results. The two basic principles of experimental design are replication and randomization. Replication allows us to estimate the experimental error and have a measure of the precision; randomization guarantees inferential validity in the face of unspecified disturbances. The traditional one-variable-at-a-time (OVAT) approach modifies one variable with the remainder held constant and requires a lot of experiments before obtaining conclusions. The method provides an estimate of the effect of a single variable with selected, fixed conditions of the other variables. Additional details on experimental design can be found elsewhere.4,5 Experimental Water-Based Polymer Systems. The investigations of the biopolymer/CaCO3 fluids have been conducted on both scleroglucan- and xanthan gum-based drill-in fluids. The fluid formulations can be represented as:Brine.Viscosity-inducing polymer.Fluid-loss reducer.Solid bridging particles. The viscosity-inducing agents are xanthan gum and scleroglucan, the most common biopolymers (especially xanthan gum) used in drilling fluids. The concentration levels are 0.25 to 1%; the minimum polymer concentration was chosen on the basis of the minimum effective amount necessary to form a good gel structure. Two starches with different crosslinking degrees and excellent fluid-loss performance were selected as fluid-loss reducers, and their concentration range is 1 to 3%. The crosslinking degree of the two products was characterized from a semiquantitative point of view by means of solid-state1,3 nuclear magnetic resonance (C-NMR). Sized acid-soluble calcium carbonate (CaCO3) was chosen as the solid bridging particle, with a particle-size distribution suitable to the permeability of the rock-filter medium employed in the dynamic filtration tests. The two particle-size distributions were optimized following the D1,2 rule6 for high and low rock permeability (1,000 and 100 md, respectively). The concentration range of CaCO3 is 5 to 20%.

Site-specific challenges related to water depth, geological and environmental conditions, rapid technological development and increased demands with respect to cost effectiveness and environmental consciousness are all factors that impact the uncertainty picture related to drilling and well operations. Thus risks related to undesirable outcomes such as kick and blowout are also influenced. It is however questionable whether the ruling practices for use of quantitative risk analyses in planning of the operations keep up with this development. As the nature of the operations change drastically, use of overall statistics from earlier incidents hardly provides a satisfactory basis for decision making. Such statistics does not reflect the exposure of the specific well to the various kick and blowout mechanisms or the effect of measures introduced to reduce the probability of these mechanisms to occur. KickRisk, a new risk analysis tool with focus on kick and blowout is developed in order to improve the basis for risk-based well control planning. The idea behind this tool is a more detailed system modeling, which enables the planning to include more well- and operation-specific information in the analyses and to obtain a more differentiated risk description. Uncertainty is expressed through probabilities related to factors at a detailed system level and is propagated in kick and blowout probabilities through the logic of the model structure. The intention of this paper is to discuss the requirements for risk analysis methods with respect to kick and blowout in well planning and how these can be met in practice. The principles of the new tool KickRisk are described and the experiences from case studies on real wells are presented. Introduction Having the potential to result in catastrophic consequences with respect to the lives and health of personnel, the environment, assets and economic values, blowouts represent the most feared and unwanted phenomena that might result from drilling and well operations. In operations not involving production, taking a kick represent the first barrier lost in the development of a blowout. Even if well control is retained, kick incidents in themselves often imply considerable costs in terms of:increased rig costs and lost production due to several days delayadditional operation costs related to the well control activitiesreduced production or recovery actions as consequence of well or formation damage inflicted by the kill operationloss of credibility in the marked. Avoiding kicks and blowouts are basic objectives in well design and operation planning. Characteristic for probabilistic planning against these events is dealing with uncertainty associated with a great number of factors related to local geology, the performance of equipment and human actions. This uncertainty implies that some probability of taking a kick, which again might develop into a blowout scenario, follows any well operation. In planning of traditional operations in areas with normal pressure regimes the uncertainty has to a large extent been handled by good industry practices involving application of rules of thumb and safety factors developed through experience and standard engineering methods. Even for more complex wells the industry has so far mainly used deterministic approach, applying physical models for kick and blowout development. For example, advanced deterministic kick simulators1,2 are used to plan casing setting depths and for kick tolerance evaluations in the design phase as well as during operations. Some evaluate possible scenarios by playing "what-if" games with realistic simulators. Probabilistic methods taking uncertainty into account more explicitly are also applied in various parts of the planning process. Examples are use of standard reliability analysis in design of critical well control equipment and methods of structural reliability in casing design.

fax 01-972-952-9435. AbstractThe superior performance of invert emulsion fluids in challenging drilling operations such as HT/HP wells makes them the preferred fluid for such applications. However, there are instances such as drilling in environmentally sensitive areas, or where costs and logistics become prohibitive, where their use is undesirable. In such circumstances, a highperformance water-based drilling fluid is the only option available. A main challenge in developing a water-based fluid for such applications has been to maintain the stability of key fluid properties such as rheology and fluid loss at temperatures in excess of 130°C. Conventional additives, such as biopolymers or synthetic polymers, either become ineffective at such high temperatures or generate too high a rheology when used in quantities needed to curb fluid loss. This paper discusses a novel water-based drilling fluid system in which the synergy of the key fluid components produces a stable rheology and low fluid loss at temperatures approaching 200°C. Furthermore, by balancing the components of the novel system, fluid rheology can be controlled to meet the hydraulics and hole-cleaning demands of the drilling operation. 1, 2