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

Abstract

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 Specially Designed Organophilic Chloride-Free Reservoir Drill-In Fluid Meets the Challenges of the First Horizontal Well in Mozambique

“…Gillenwater and Ray (1989) documented the use of KAC in 30 wells in the South Texas area as an alternative to potassium chloride fluids and referenced that the amount of potassium provided by KAC to be 40% by weight. Gallino et al (1997) document the use of KAC for drilling plastic shales on five wells in South Italy instead of KCl. They concluded that the use of an inhibitive KAC base fluid was more environmentally acceptable than a KCl inhibitive one and resulted in lower disposal costs of per unit ton of waste produced and a large reduction of waste generated as compared to noninhibitive fluids.…”

Studies on the Damage Induced by Drilling Fluids in Limestone Cores

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Successful Shale Drilling Using a New Environmentally Compatible Potassium Carbonate/Polymer/PolyGlycol Mud System: Design and Field Experiences

“…Mud rings, plugged flowlines, low penetration rates, bit balling and hole instability have all been experienced. Escalating disposal costs led to the trial of a variety of chloride free mud systems and included the use of potassium acetate or potassium formate in concentrations of 25 to 30 kg/m 3 with various polymers 10,11 . Even with these formulations the muds became contaminated with fine clay solids and had threshold limits for the methylene blue test of 65 kg/m 3 above which rheology significantly increased.…”