Summary Well 33/9-CO2, located in the Statfjord field in the Norwegian sector of the North Sea, currently holds the world record in extended-reach drilling. A well of this type obviously requires an efficient drilling fluid to both suspend the weighting material and clean the annulus of drilled cuttings. The ester-based mud system used in the 12 1/4-inch and 8 1/2-inch hole sections exhibited excellent hole cleaning capabilities. This paper describes the fluid's performance in the field and in the laboratory where the fluid was tested wider downhole conditions. Fluid rheological behavior is described using the more accurate yield-power law (Herschel-Bulkley) model. Introduction The Statfjord field in recent years has been in the forefront of the development of extended-reach and horizontal drilling. The use of extended-reach drilling has eliminated subsea development of the northern part of the field. Previous wells drilled in the field include well Cl0 with a horizontal reach of 5,003 m (16,410 ft) and C3 with a horizontal reach of 6,086 m (19,962 ft). Experience gained on these and other wells has provided the drilling team expertise in the drilling of extended-reach wells. This expertise was applied in the drilling of well 33/9-CO2, a record-breaking well that had a horizontal displacement of 7290 m (23,911 ft), most of which was drilled at 70-84 degrees from vertical. Hole cleaning in the highly deviated intervals was expected to be a challenge, but actual problems experienced while drilling were few. A number of factors such as well design, drillpipe rotation, special tools, and the use of drilling models and simulators contributed to the success of this well, which have been described by others. One important factor remaining to be discussed is the contribution of the drilling fluid system and its properties to the success of this challenging well. This paper will document the mud properties used in drilling this well and explore the physical and chemical properties of the ester-based mud system (EBM) that enabled it to efficiently transport cuttings out of the deviated annulus and suspend weighting materials. Extensive FANN 70 testing of EBM was performed at varying temperatures and pressures to simulate downhole conditions.
Accurate predictions of annular frictional pressure losses (AFPL) are important for optimal hydraulic program design of both vertical and horizontal wells. In this study, the effects of drillpipe rotation on AFPL for laminar, helical flow of power law fluids are investigated through theoretical, study, flow models were developed for concentric and eccentric pipe configurations assuming that pipe rotates about its axis. A hybrid-analytical solution is developed for calculating AFPL in eccentric pipe configuration. Computer simulations indicate that the shear-thinning effect induced by pipe rotation results in reduction of AFPL in both concentric and eccentric pipe configurations. The pressure reduction is most significant for concentric pipe configurations. For conventional rotary drilling geometry and pipe rotary speeds, the reduction in AFPL is small. A number of laboratory experiments conducted on the full-scale TUDRP flow loop are generally in good agreement with the results of modeling. Available fileld data, however, consistently show an increase in AFPL. This behavior is explained by pipe lateral movement (swirling), which causes turbulence and eventually an increase in AFPL.
Wells drilled in the Andean Mountain region of South America present significant challenges as a result of both operational and environmental factors. Wells located in the foothills along the basin are particularly difficult due tectonic stresses and unstable, probably, micro-fractured shales. Operators have experienced difficulties drilling wells using both water-based and oil-based muds (OBM). Environmental regulations hinder the use of OBM in many of those areas due to the potential environmental impact and costs associated with waste disposal. In many cases OBM has not prevented wellbore instability problems. This paper explains how a lack of understanding of regional geology and the practice of using successful drilling fluid design and drilling practices from other areas has led to wellbore problems. The water phase salinity of OBM and the use of the appropriate inhibitors in the drilling fluid play a key role in the minimization of wellbore problems. Although reactive clays are present in all the shales along the basin, they represent only 30 to 40 percent of the clay fraction, while non-expandable kaolinite clays are the major clay components. This paper explains how physical/mechanical effects are more important than inhibition in controlling these shales. Moreover, in some cases "excessive inhibition" due the presence of shale inhibitors such us potassium and high water phase salinity in OBM exacerbate the problems. Pore pressure transmission caused by fluid invasion is a major contributor to the observed problems. A combination of operational practices and improved fluid design minimizes mud and filtrate invasion. Troublesome shales in the Andean basin include, from north to south, the La Rosa and Icotetea in Venezuela, the Carbonera, Leon and Villeta in Colombia, the Napo in Ecuador, the Chonta in Peru and the Los Monos in Bolivia and Argentina. Case histories involving these shales are presented. Contrary to experiences in many other parts of the world, high water phase salinity OBM and potassium based water-based mud (WBM) are not the answer to shale stability problems. Rather, mud sealing properties, correct chemical composition and appropriate drilling practices are the key factors in maintaining wellbore stability. Introduction The challenges of drilling in the Andean Mountain region of South America are well documented. The presence of tectonic stresses combined with over pressures1 makes this a particularly challenging region. The stresses in this region were generated by the Andes Mountain orogeny. The geology is typified by steeply dipping sand/shale sequences. Many faults have been documented in this area. Claystones and shales dominate the lithology in this region. These clays and shales can be "sticky"2 at times, requiring the use of inhibitive drilling fluids to minimize the associated problems. The factors discussed above lead to wellbore stability being a major challenge when drilling in the Andean mountain region. These problems present the greatest challenge when drilling directional wells3. The drilling problems experienced in Colombia are well documented and include stuck pipe, high torque and drag, tortuous wellbores, twist-offs, poor cementing, and unplanned back-offs. Many of the difficulties encountered have been attributed to poor hole cleaning in enlarged hole resulting from wellbore instability. The cavings generated during the hole enlargement process have also presented hole cleaning challenges. Success in drilling wells in this region has been attributed variously to simplifying well design, understanding the tectonic stresses and their orientations, drilling fluid design4 and sound drilling practices.
SPE and IADC Members Abstract Efficient hole cleaning is vitally important in the drilling of directional and extended-reach (ERD) wells. In previous works, the importance of fluid velocity/pump output has been emphasised, while in others, the importance of individual fluid rheological parameters has been stressed. Drillpipe eccentricity and rotation directly affect hole cleaning as well. Recent work on particle settling velocity has accelerated the understanding of the behaviour of drilled cuttings in wellbores. This paper presents a new model to the industry, which combines recent developments in the fields of particle settling and rheology. This model provides a useful tool for the planning of hole cleaning for highly deviated wells through the use of Lift Factors under the eccentric drillpipe Information from deviated wells in the field is used to illustrate the usefulness of this hole cleaning model. Importantly, analysis of the data indicates that fluid 'n' factors calculated using the yield-power law [Herschel-Bulkley] rheological model play a greater role in promoting good hole cleaning in ERD wells than has previously been recognised. Introduction Several hole cleaning models have been published. It is worth reviewing these works prior to describing the new model. Bern et al include in their model a dimensionless Rheology Factor based on the API plastic viscosity [PV] and yield point [YP] from the Bingham plastic rheological model. Included in the model are calculations for critical flowrates (CFR) required to drill deviated wellbores It is stated that under laminar flow conditions, muds with either very high or very low YP values are preferred for hole cleaning and that YP is the dominant factor in deviated wells. In their earlier paper, fluid rheologies were described using the power law model, but as the PV and YP terms are simpler and more commonly used, the authors applied them in the later paper. It is noted that this model assumes a rotating drillstring. Clark and Bickham use a mechanistic model to describe hole cleaning. This model considers the various mechanisms involved in the transport of cuttings out of a well (i.e., rolling, lift, and particle settling). This paper concurs with industry opinion in classifying flow rate as the most important factor in hole cleaning, and it considers fluid density and rheology as the most important drilling fluid properties that affect hole cleaning. The Herschel-Bulkley rheological model is used in this paper with the fluid's yield stress being the dominant factor. The influences of the other rheological parameters (ie. consistency index and flow index) are unclear. Rasi investigated the rate of cuttings bed development and in his modeling efforts focused on predictions of cuttings bed height in eccentric wellbore. In an undefined manner, Rasi used a dimensionless friction factor (which included factors for fluid rheology), well geometry and design, and pump output to calculate the height of the free zone above the cuttings bed. The API PV and YP and the viscometer 6 rpm term (which approaches the yield stress term in the Herschel Bulkley model) were mentioned as input parameters. It is uncertain whether the author used the three terms to develop a flow behaviour curve across the shear rate spectrum. P. 479
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