Along with exorbitant costs and safety considerations, drilling an exploratory well where conventional mud systems could not maintain wellbore stability and trouble-free drilling, also posed the additionally difficult challenge of selecting a fluid system stable against multiple contaminants and flexible for a broad range of densities. The demands are compounded dramatically when designing the system and information to support the decision is poor. This paper details the design and performance of a silicate-based fluid, which is an inhibitive mud system formulated with a soluble silicate for maximum shale inhibition. The system has been developed to drill water-sensitive reactive shales and dispersible chalk and illite formations. The degree of inhibition provided by the silicate system is significantly greater than any other water-based system, truly approaching the level of an oil-based system. The authors will discuss the design and the application of the drilling fluid, which was selected to drill a formation containing anhydrite, claystone, dolomite and salt. The system was formulated to be salt-saturated to avoid dissolution of salt layers; to withstand anhydrite contamination; to be flexible for densities up to more than 2.0 kg/dm3 while maintaining its high level of inhibition. Though comprehensive lab tests were performed, the possible depletion of silicate while drilling the anhydrite-containing formation remained. Nevertheless, this challenge was accepted with tight wellsite engineering planned to maintain the fluid properties. Six wells have now been drilled with the proposed silicate system, with each showing very good performance and stable behavior in the above mentioned environment. The authors will present its effectiveness, formulation, properties, maintenance and lessons learned, along with the coordination and pre-planning that contributed to its successful application. Introduction Using sodium silicate in water-based muds was first undertaken in the 1930s.1,2 These systems, known as protective silicate muds, were successful at drilling very reactive shales but the control of their rheology proved difficult and they were superseded by the introduction of natural organic dispersants to treat bentonite muds. Further field trials were undertaken in 1960s by Darley which again failed to establish silicate-based muds as accepted systems.3,4 In the 1980s, Wingrave's research on shale stability found that silicates, used in conjunction with the potassium ion and specific polymers, combined for an effective shale-stabilizing package.5 During the last decade the industry has effectively used sodium silicate in conventional polymer fluid formulations to provide an effective water-based shale stabilizing system.6–9 Since the reintroduction of silicate fluids, more than 450 wells have been drilled around the world.10 From the North Sea to India to Australia, silicate-based drilling fluids have demonstrated superior performances: optimum inhibition characteristics, high penetration rates, reduced trouble time and superior wellbore integrity, optimum solids removal performance, minimal environmental impact.11 The outstanding inhibition of the silicate system comes from three directions: through chemical bonding of silica oligomers onto surfaces of drill cuttings and formation, through precipitation of the silicate with divalent ions, and through silicate polymerization that results in complex inorganic polymer structures, forming a protective layer on all surfaces (Fig. 1). Silicate-based fluids also generate firm inhibited cuttings, which contribute to optimum performance of the solids-control equipment and greatly enhanced formation evaluation. With the contamination of solids minimized, dilution rates, in turn, are reduced considerably, as are the costs associated with waste management. The unique cuttings integrity provided by silicate-based muds is reflected in Fig. 2, showing the cuttings of a KCl-based mud, before and after treating it with sodium silicate. Silicate chemistry Soluble silicates are the metal salts of silicic acid that exist in multiple polymeric forms in solution. Amorphous solids and powders are also manufactured.

In the last years the quest for hydrocarbons findings moved into extreme drilling condition, such as ultra-deep waters, ultra-high temperatures and drilling fluids densities. Adding to these the very narrow density windows (reservoirs with very low differential fracture pressure - pore pressure), both operators and drilling fluids service companies faces increased challenges to deliver the well for production. It is common that for such applications the bottom-hole static temperature is over 180°C/356°F, even reaching temperatures in excess of 250°C/482°F in some fields, acid gases (CO2, H2S) are also present, so the list of challenges is formidable. In this paper, the authors details the common challenges for such extreme conditions wells and best practices, present a new generation synthetic-based fluid capable to withstand temperatures in excess of 500°F/260°C, as well as a unique hydraulics engineering calculations software which enable real time equivalent-circulation density (ECD) calculations. The design and development of the mew ultra-high temperature mud system is presented and its components. At the core of the system it is a new amine-free emulsifier, which is stable to temperature above 300°C/572°F. In addition, the authors will present the extensive lab work required to optimize the formulation for high densities and extreme high temperatures, detailing also the critical fluids engineering guidelines for drilling in such harsh conditions. Using laboratory, field, and computer data, the authors will demonstrate the effectiveness of the new fluid in delivering optimum drilling in extreme HTHP conditions. Also, it is illustrated how the rheology is maintained at minimum level with a micron-size weighting material, and the engineering software used for pre-planning and while drilling to accurately calculate the ECD and maintain it within the required range.

fax 01-972-952-9435. AbstractAlong with exorbitant costs and safety considerations, drilling in a high-temperature, high-pressure (HTHP) environment also poses the difficult challenge of protecting a reservoir that is all-too-often depleted. In areas where environmental restrictions and compatibility issues in gas fields prohibit the use of an oil-base drilling fluid, engineering a water-base fluid system that is free of potentially damaging solids, stable at very high temperatures, and able to withstand acid gases (CO 2 , H 2 S) or other contaminants is a very difficult proposition. The demands are compounded dramatically when drilling a deviated or a re-entry deviated slim hole well in an HTHP environment. Furthermore, since pressure and temperature heavily influences the rheological behavior, it is extremely difficult to calculate, predict and control pressure losses, ECD, and/or ESD in real time to avoid total losses or kicks.This paper details the design of a unique water-base reservoir drill-in fluid and its successful application on five HTHP wells in the Kalinovac and Molve gas fields of Croatia. Four of the wells were high-angle re-entry slim holes. Using laboratory, field, reservoir investigation and computer data, the authors will demonstrate the effectiveness of the new fluid in delivering zero skin damage and subsequently higher production rates than other wells in the field. Further, the system significantly reduced operating costs by eliminating costly stimulations, while simplifying the generation of clear imaging logs. The system also remained stable at bottom hole temperatures of 180 -200°C.During the drilling operations, a unique software program was employed that accurately predicted the rheological behavior, pressure losses, ECD and ESD, which contributed heavily to the wells being drilled trouble-free. The authors will detail the formulation of the new system, along with the coordination and pre-planning that contributed to its success in Croatia. The new water-base system has shown the effectiveness in drilling HTHP wells in areas where invertemulsion drilling fluid systems are prohibited.

High torque, friction factors, and pick up weights were major challenges encountered by a major operator in Abu Dhabi while planning to drill challenging extended reach development (ERD) wells with complex 3D profiles. Well torque and drag simulations showed that planned depths were not reachable with water-based muds. This paper describes the implementation of a mechanical lubricant, which resulted in significant decrease of the friction factors and turned an ERD well from not drillable to drillable with water-based mud. After analyzing several possibilities, the solutions were narrowed down to two: use either a new generation mechanical lubricant or a reservoir non-aqueous fluid (NAF). The complexity was amplified by the necessity to re-design a filter-cake breaker for NAF, were this option to be selected, due to the type of completion. This second option would also create a substantial cost increase for the operator for products and rig time; therefore, the decision was made to introduce a mechanical lubricant. A comprehensive study and lab tests were conducted to ensure compatibility and stability of the lubricant with a planned mud type at downhole conditions. The results of this study were promising enough for the operator to introduce this lubricant, aiming a substantial reduction in torque and drag to enable drilling of the longest horizontal section in the history of the project. Before addition of the mechanical lubricant, drilling continued with a conventional type of lubricant, noticing an increasing tendency of torque and drag tracking the predicted trends. At a certain stage, drillstring buckling was observed and drillpipe started to reach their limits. To mitigate these impediments, the mechanical lubricant was introduced into the drilling fluid. After reaching the optimum concentration, the mechanical lubricant eliminated buckling and provided significant reduction in torque, pick-up, and slack-off friction factors, respectively by 27%, 52%, and 42%. These parameter improvements facilitated continued drilling the well to final depth without reaching the drillpipe limits. Additionally, the well and bottomhole assembly (BHA) designs allowed for significant margins in case of a stuck pipe event, and based on the new friction factors, the well could be extended by 3,000 ft without reaching the drillpipe limits. The impact of this exercise in future ERD wells is considerable. It will simplify well and completion designs, improve logistics by reducing the amount of chemical movements, facilitate drilling fluids selection, and optimize the well cost. The paper covers the gaps related to drilling complex ERD wells with water-based drilling fluids. It provides detailed methods and procedures covering the suitable application of the mechanical lubricant and the extensive laboratory tests done during the planning stage, as well as the field application and results. The proposed solution can be used during the well planning process in any other area of the world.

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