Hole cleaning is still among the most important problems to handle in drilling operations. The difficulty in removing cuttings bed during drilling arises because of the drilling fluid interacts with the cuttings in cuttings bed to form a cuttings bed gel. The drilling fluid composition can be designed to minimize the gel formation in the cuttings bed. At the same time the drilling fluid properties are optimized to ensure a sufficient shear stress on the cuttings particles to be able to remove the cuttings. This technique has been used in several of Statoil's drilling operations. The paper explains the effect of the cuttings bed properties on hole cleaning in detail. Furthermore, the paper demonstrates how the drilling operations were improved compared to earlier drilling operations using conventional drilling fluids. From drilling operations in North Sea fields it is shown how the total drilling progress is improved. The torque curves of comparable wells are shown. These curves demonstrate that a significantly improved hole cleaning have been experienced in the wells drilled with the drilling fluid made to minimize cuttings bed gel formation. Introduction Although large resources have been spent on studying hole cleaning, there is still a debate going on how to optimize hole cleaning properties. The necessity of understanding hole cleaning is caused by the different operational problems that can arise if sufficient hole cleaning is not achieved. Poor hole cleaning may result in lost circulation or hinder the casing or liner to be run into its selected position. Therefore, it is important to use the correct operational practice to ensure optimum hole cleaning. Presently, recommendations to obtain good hole cleaning vary, depending on the different experiences or other sources that the recommendation is based. Recently, it has been recognized that the cuttings bed consolidation properties are important in evaluating the potential for hole cleaning1. Although these properties has not been fully implemented in practical evaluation during the drilling operation, the properties have been used indirectly in modeling hole cleaning ability through the use of necessary shear stress at the cuttings bed surface to remove cuttings2. Oil based and water based drilling fluids have been found to act differently in respect to hole cleaning even though their viscosity profiles may have been fairly equal. The different behavior is caused by the different ways these drilling fluids are constructed. Oil based drilling fluids are formulated using a continuous oil phase. The oil based drilling fluids are viscosified by the addition of emulsified water, typically in concentrations between 15 and 25% and by the addition of organophilic clay. There is no contact between the cuttings and the water when drilling with oil based drilling fluids. In water based drilling fluids a brine phase is viscosified by different polymers. These polymers will couple the fluid to the cuttings and thereby create a possibility for the fluid to generate a more or less consolidated cuttings bed. In the present paper there is an evaluation of the effect of the drilling fluid rheological properties of water based drilling fluids on hole cleaning properties in deviated wells. Although the effects on the cuttings bed are different when oil based drilling fluids are used, there are a few conclusions that can be generalized from the water based drilling fluid case to also be valid for drilling with oil based drilling fluids. These generalizations are also summarized in the article. The evaluation presented in the present article is based on results from several wells from different fields in the North Sea. In these wells the well paths include most sections from the more problematic 45°-70° deviations to horizontal sections were saltation flow could help simplifying hole cleaning. It is recognized from investigations that drill pipe rotation has positive effects on hole cleaning3. In practical operations, drill pipe rotation is well known to give better hole cleaning.

fax 01-972-952-9435. AbstractDuring the drilling and completion of the Huldra field in the North Sea, high temperature and high pressure conditions were expected and encountered in the reservoir section. The difference between the pore pressure and the fracturing pressure is small. Cesium formate had been evaluated as a potential drilling and completion fluid, but technical hurdles could not be completely addressed in time for the first well. As a result of well control problems occurring in the first well, with barite sag in the oil based drilling fluid as a contributing factor, it was necessary to use a drilling fluid with insignificant potential for sag. For the first time worldwide the cesium formate brine was chosen as a drilling fluid. This fluid could be delivered solids free with densities up to 2.2 s.g. The required down hole density in the well was 1.91 s.g. At the same time it was necessary to have as little contribution to the equivalent circulating density (ECD) from the flow as possible.The paper describes how the cesium formate brine was used successfully as a drilling and completion fluid. The effect of the fluid on well control, hole cleaning, rate of penetration (ROP), torque/friction, ECD, formation damage, casing wear and hole stability are covered. The paper also describes actions required to minimize losses of this very expensive fluid.The challenges acquiring adequate formations logs while drilling are also described. Finally, the use of cesium formate brine during the completing of the wells with open hole sand screens is outlined.

fax 01-972-952-9435. AbstractWater based drilling fluids have been used in many North Sea drilling operations during the last decades. Traditionally, a high focus has been directed towards creating inhibitive drilling fluids with high low shear rate viscosity for optimising cuttings transport and intact cuttings on the shaker. It has, however, been recognized that there is no direct connection between cuttings quality and hole stability. In the current field it was also found that a low gel/viscosity drilling fluid is effective in transporting cuttings out of the wellbore. Following this, the content of high molecular weight polymers have been reduced and exchanged with low molecular weight polymers to enhance hole cleaning throughout the last years. Fewer cuttings are entrapped in beds down hole and are transported up to the surface. The current paper demonstrates how this low viscous/low gel strength drilling fluid contributed to improved drilling performance, where an improvement in tripping time, casing running speeds and total drilling time was observed.In addition to changes in the viscosity a wide range in NaCl and KCl salt concentrations and mixtures have been tried and field tested. The project is currently using only a moderate concentration of KCl, which has improved hole stability. The frequency of unstable hole occurrences in a particular troublesome formation are today greatly reduced by using lower KCl salt concentration and improved well design.The current paper explains in detail how the drilling fluid design optimizes the chemical performance and the hole cleaning performance during the drilling operation. The drilling performance for this particular field improved significantly during the last three years where the total drilling time was reduced with roughly 50% compared to earlier practice. The reasons for the improvements are mainly the changes in well design combined with the use of an effective WBM system.

Over the years the industry has seen major changes in drilling fluids technology, especially in the field of organic-phase fluids (OPF) such as diesel, mineral oil and synthetic hydrocarbon base fluids. Environmental concerns have driven the development of ‘traditional’ oil-based fluids away from diesel and through to the less toxic, more biodegradable synthetics such as esters and olefins. Many companies are now considering the overall picture regarding the disposal of wastes and are looking for alternative uses for drilling by products, thus turning wastes into useful raw commodities. While organic-phase fluids evolved, research into water-based fluids (WBF), which are generally considered less harmful to the environment, concentrated on duplicating the technical performance of oil-based fluids, the absence of oily discharges being the environmental benefit. A more holistic view of the overall impact of WBF discharges and concerns about the persistence of some WBF chemicals has now focused development on alternative ways to further reduce the impact of water-based fluids on the environment and accelerate recovery of the impacted areas. The key to reducing the environmental impact of drilling fluids is typified by the standard waste management hierarchy. The areas to consider are source reduction, recycling/re-use of the product, recovery of useful or valuable materials and treatment prior to disposal. The focus of this paper is how new technologies can be used to bring about these changes and to discuss the various ways in which the amount of drilling fluid by-products can be reduced. The paper also describes ways in which new drilling fluid developments such as salt-free drilling fluids or the use of colloidal weighting agents can be designed to optimize waste management and reduce the amount of waste. These technologies also facilitate the re-use and recycling of drilling fluids and their components. Introduction To significantly reduce the environmental impact of drilling operations, the process of drilling a well needs to be viewed holistically and environmental benefits need to be tied to financial savings. For example, which is more advantageous, drilling quickly with an "expensive" fluid that saves four days rig costs and thus reduces overall CO2 emissions or using a "low-cost" fluid which does not perform as well as the expensive one and results in increased rig costs and emissions. It is equally important to ensure that the fluid used to drill the reservoir section maximizes recovery of the available hydrocarbons and reduces the need to drill more wells. This paper uses some of the concepts of life cycle analysis1 to consider the total impact of the various stages of drilling. Factors that should be considered when trying to optimize the overall waste management strategy include total materials used as well as solid, liquid and gas emissions. One of the key points of an effective waste management strategy is the waste management hierarchy. Figure 1 ranks the desirability of each stage of the hierarchy. The best solution is to avoid producing the waste, but if this is unavoidable, then the amount of waste produced should be minimized. Steps should then be taken to maximize the recovery, re-use and recycling of materials before reducing the amount of material for final disposal2. Avoid - Reduce Well Intervention Reduction of the total number of operations/interventions required to extract hydrocarbons will result in a reduction of both the total energy budget and the overall environmental impact. It is also important to recover the maximum amount of hydrocarbons (or energy) from the reservoir. This means that in addition to drilling to the reservoir quickly and efficiently, it is also necessary that drilling practices do not reduce the productivity of the reservoir causing an increase in the overall energy costs.

No abstract