New Formulations of Potassium Acetate and Potassium Formate Polymer Muds Greatly Improved Drilling and Waste Disposal Operations in South Italy

Abstract

Abstract 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^