Summary This paper discusses the field results of innovative K-Acetate or K-Formate mud formulations that have been used by ENI S.p.A./Agip Division to drill several wells through very plastic shales in South Italy. Field muds have been carefully designed and evaluated as far as drilling and waste disposal activities are concerned. While drilling, the integration between field observations, standard laboratory tests, and nonconventional rheological approaches provided the assessment of useful correlations between rheological properties, performance, and formulations of field muds. These findings permitted a gradual improvement of the mud effectiveness with a large reduction in dilution rates, bit balling, and hole cleaning problems. At the rig, the strict cooperation between all the people involved in field operations represented a key factor for the successful application of the know-how acquired. With respect to the previous wells drilled in the field with traditional dispersed muds, the optimization and careful management of the new muds contributed, in spite of the increased ROP, to a considerable reduction both in time spent to remove bit balling or reaming and in tons of wastes produced per hole volume. These improvements led to great savings in drilling and disposal costs that largely compensated the 8.4% increase in the mud mixing costs per hole volume due to the presence of potassium salts. Introduction Potassium acetate KC2H 3O2 or simply KAc) has been proposed and successfully applied since 1986 as a more environmentally acceptable alternative to potassium chloride (KCl) for drilling fluids.1,2 Potassium chloride, KCl, provides levels of potassium (52% by weight) similar to those provided by KAc (40% by weight) but the high chloride concentrations, associated with KCl, limit the polymer selection and have a harsh impact on plant life. In many areas, these environmental concerns recently imposed drastic restrictions on the chloride concentration accepted by the Italian local regulations in the drilling waste volumes. Recently, another potassium salt, potassium formate (KCOOH), has been proposed and applied in brines and drill-in fluids formulations. 3–10 The formate salts are of increasing interest because they are biodegradable and have a low toxicity to aquatic organisms. In addition to that, they display very little corrosiveness towards ferrous-based metals used in oilfield tubulars and ancillary hardware,8 they have an unusually high solubility in water and reduce the rate of hydrolytic and oxidative degradation of many viscosifiers and fluid loss agents at high temperatures.9 Drilling polymer muds for nonproductive zones, that include in their formulations low concentration of potassium formate as an alternative to the usual KCl, have been first extensively applied only to ENI S.p.A./Agip Division wells. In ENI S.p.A./Agip Division, K-Acetate and K-Formate polymer muds have been used to drill 11 wells located in two different fields (A and B) in south Italy. The muds have been sampled every hundred meters of penetration and analyzed with standard procedures and nonconventional rheological tests, such as low shear rate and oscillatory measurements. These tests helped correlate field muds suspension capability, carrying capacity and resistance to shale contamination to the initial mud formulation and, in particular, to the type and concentration of salts and polymers. The impact of mud formulation and management on the results of discharge operations has been evaluated with reference to some technical and economical indexes that have been defined by means of a statistical study performed on more than fifty wells. The improvements obtained in field B, with the introduction of the Scleroglucan biopolymer in the muds formulation and with the application of the zirconium citrate (ZrC) as a rheology modifier, have not been discussed in this paper because they represented the main subject of previous works.11,12 Experimental Nonconventional Rheology. All the field muds have been examined with the Fann 35 at the minimum and maximum well temperatures. Fann readings have been elaborated with numerical methods to calculate the Herschel Bulkley (H & B) rheological parameters. A Bohlin VOR has been used to perform low shear rate and oscillatory measurements with a couette geometry having the gap 100 times greater than the maximum dimensions of the solids particles contained in the samples. Measurements as a function of shear rate ("rate sweeps") have been carried out by varying the shear rates from 0.02 to 600 s?1 . Oscillatory measurements have been repeated every 30 s for a total time of about 10 min, at a fixed frequency (1 Hz), after the initial imposition of a shear rate of 100 s?1 for a period of 60 s ("time sweeps after a shear history"). Mud Formulations. The mud type and density used in the field A are illustrated in Table 1. The table also reports data relative to three wells previously drilled in the same area with traditional dispersed muds. As regards the new K-polymer systems, the typical mud formulations were prepared at pH 9/9.5, with Xanthan Gum biopolymer as primary viscosifier and suspending agent, polyanionic cellulosic (PAC) and Starch for filtration control, potassium salts (7-14 lb/bbl) and barite to the required weight. When necessary, rheology and filtration properties have been adjusted by controlling the mud degree of flocculation with dilutions or chemical corrections. Filtration control of the high salinity muds (with potassium salts over 7 lb/bbl) sometimes required an increase in biopolymer concentration.

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.

As the expense of deepwater drilling increases, efforts to eliminate or reduce non-productive time (NPT) can have a significant effect on overall costs. Current standard practices for installing surface casing strings involve multiple trips to first drill the hole section, and then run casing, land the wellhead, and cement. In many instances these casing seats are set shallower than is optimal due to anticipated hazards such as shallow gas or water flows and wellbore stability issues. Although with the current deepwater drilling model these practices are safe and reasonably efficient, they can limit the depth at which the casing can be set, as compared to the optimal depth with respect to pore pressures and fracture gradients. The use of current proven Casing While Drilling (CWD) techniques has effectively reduced NPT in several key areas including: flat-time reduction, shallow hazard mitigation, minimized/eliminated lost circulation, and improved cement placement and bond. Additionally, CWD provides the opportunity to push casing seats deeper, thus potentially eliminating contingency strings. Current CWD methods have not been applied to subsea surface casing applications, primarily because of the challenge of simultaneous drilling-in and landing the high pressure wellhead without damaging the seal surfaces on the wellhead housings. Additionally, many wellhead running tools are not designed to carry drilling torque. This paper discusses the benefits associated with drilling-in the surface casing in a subsea application. Introduced is a new CWD method and enabling technology that can significantly improve this phase of deepwater drilling operations. The complete process of drilling-in and running casing, landing the wellhead, and cementing can be achieved in a single trip. The benefits of CWD and features of the new technology are presented. When compared with current deepwater "best practice" models, an immediate 25% time savings is achievable. This technology also enables the utilization of less expensive floating rigs for cost reduction by batch drilling and setting of surface casing.

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.

In this paper are discussed the field performances of innovative K-Acetate or K-Formate mud formulations that have been used by Agip S.p.A to drill several wells through very plastic shales in South Italy. Field muds have been carefully designed and evaluated as far as drilling and waste disposal activities are concerned. While drilling, the integration between field observations, standard laboratory tests and non conventional rheological approaches provided the assessment of useful correlations between rheological properties, performances and formulations of field muds. These findings permitted a gradual improvement of the muds performances with a large reduction in dilution rates, bit balling and hole cleaning problems. At the rig, the strict cooperation between all the people involved in field operations represented a key factor for the successful application of the know-how acquired. With respect to the previously wells drilled in the field with traditional dispersed muds, the optimization and careful management of the new muds contributed, in spite of the increased ROP, to a considerable reduction both in time spent for bit balling or reaming and in ton of wastes produced per hole volume. These improvements led to great savings in drilling and disposal costs that largely compensated the 8.4% increase in the mud mixing costs per hole volume due to the presence of potassium salts. Introduction Potassium acetate (KC2H3O2, or simply KAC) has been proposed and successfully applied since 1986 as a more environmentally acceptable alternative to potassium chloride (KCl) for drilling fluids. Potassium chloride, KCl, provides levels of potassium (52% by weight) similar to those provided by KAC (40% by weight) but the high chloride concentrations associated with KCl have a harsh impact on plant life and limit the polymer selection. Recently, an other potassium salt, potassium formate (KCOOH), has been proposed and applied in brines and drill-in fluids formulations. The formate salts are of increasing interest because they have an unusually high solubility in water, at a wide range of densities, and reduce the rate of hydrolytic and oxidative degradation of many viscosifiers and fluid loss agents at high temperatures. In addition to that, they are biodegradable, have a low toxicity to aquatic organisms and display very little corrosiveness towards ferrous-based metals used in oilfield tubulars and ancillary hardware. However, drilling polymer muds for non productive zones, that include in their formulations low concentration of potassium formate as an alternative to the usual KCl, have been extensively applied so far only to Agip wells. In Agip, K-Acetate and K-Form ate polymer muds have been used to drill 11 wells located in two different fields (A and B). The data discussed in this paper only concern the 7 applications performed in the Field A. In this field, the mud performances have been carefully monitored with a special attention to hole cleaning, rate of penetration and mud tolerance to reactive clays. The field muds have been sampled every a hundred meter of penetration depth and analyzed with standard procedures and non conventional rheological tests, such as low shear rate measurements, oscillatory measurements and non conventional analysis of the Fann readings based on the Herschel & Bulkley (H&B) rheological model. These new approaches helped correlate field muds suspension capability, carrying capacity and resistance to shale contamination to the initial mud formulation and in particular to the type and concentration of salts and polymers. Afterwards, the impact of muds formulation and management has been evaluated with reference to some technical and economical indexes that have been considered of major interest for the given operative conditions. Some of the selected indexes have been defined on the base of a statistical study, performed on more than fifty wells, that will be more extensively discussed in a further paper. In this work, the major steps that led to the muds performances evaluation and optimization are discussed and condensed in several learning points. P. 661^