The property of clay rocks to spontaneously disperse and swell complicates the entire drilling process. Inhibited fluids are used where the use of conventional clay fluids causes problems in drilling wells. Complications are observed in the form of screes and collapses, narrowing of wellbores, caverns caused by swelling of clayey rocks and their solution transition. Clay sludge is subjected to peptization and dispersion, which results in the solution’s thickening and parameter deteriorations [1].
Under these conditions acting as a prophylactic agent, inhibitory additives are used in solution with NaCl, KS 1, CaCl2, CaS04, silicates, fatty acid soaps, lime, etc. However, when they are used, the inhibition effect is lower, since inhibitors usually neutralize only one of the two lyophilic areas of the mosaic surface of clay particles: either along the planes - negatively charged areas, or along positively charged faces and fractures. Unilateral inhibition will be much less effective than multilateral inhibition, as clays are amphiphilic and strongly amphoteric [2, 3].
The developed inhibited additives PACS and PACS-T prevent hydration, swelling and disintegration of clay rocks.
PACS – provides multilateral inhibition of clay particles due to Portland cement and potassium chloride hydrolysates, containing simultaneously cationic and anionic forms of mineral inhibitors, enhanced by the inhibitory effect of potassium ions, capable of penetrating into the interplanar space of clays, and by the hydrophobic effect of combined surfactants based on their chemisorption on hydrophilic and hydrophobic exposed clay particles. In addition, the complex surfactant in lignosulfonate solutions completely degasses the foam and prevents its formation.
PACS-T (thermostabilized) is one of the homologues of the general additive of inhibited calcium-potassium solutions of PACS. PACS-T differs from PACS with a high level of inhibition and a large temperature range. These advantages are achieved due to the transition into a water-soluble condition of calcium and potassium chloride compounds present in Portland cements in the form of calcium and potassium monochromate and calcium.
This compound’s solubility is two orders of magnitude higher than the solubility of lime and is 16%. The content of water-soluble Ca+2 and K+ inhibitors in the PACS-T filtrates is Ca+2 - 900 - 1500 mg / l and K+ - 1000 - 2000 mg / l, against calcareous 400-600mg/l drilling mud systems as a result of the increased solubility and the subsequent transfer of calcium and potassium - chromates into mixed salts of lignosulfonate. Therefore, in the system of the inhibited additive PACS, the positive effect of inhibition is manifested up to a temperature of +70 °C, and in PACS - T, due to the use of chromates, the positive effect of inhibition is already manifested at a temperature of +30 °C, and not at a temperature of +70 °C and higher, which usually occurs in other types of solutions where chromates are used only as a heat stabilizer.
Drilling of well No. 19 pl. Altyguyi is because of the high content of clayey rocks throughout the well section, reaching 70% or more. These clays during drilling present certain difficulties related to the complication caused by narrowing and collapse of the walls of the wellbore. All this is due to unstable rocks that build up colloidal clays.
In order to have successful opening of the clay, a solution of the PACS system complexly inhibited by the additive up to a temperature of +70 °C and a thermally stabilized PACS-T system above + 70 °C was chosen by drilling to the projected depth 3950 м.
Drilling of the well from a depth of 655 m was carried out with a chisel Ø393.7 mm, the temperature at the bottom was +35 °C. Due to the fact that the temperature in the well increases intermittently and reaches the limits of +70 °C in the interval of 2500m, it was decided to transfer the drilling fluid to the inhibited solutions of the PACS system. The transfer was carried out without stopping during the well’s drilling in the range of 655–700m.
The main materials for transferring the drilling fluid with a complex inhibited additive PACS: Portland cement PCT1 - 100, potassium chloride (KS1), KSSB-2, caustic soda (NOH), surfactant СhT-48.
Drilling mud parameters before translation: density - p =1.47g/cm3; viscosity – T = 63 sec; water output – V = 3 cm3; thickness of clay crust – K = 1mm; static shear stress in 1min. – Q1 = 48dPa; in 10min. – Q10 = 96dPa; plastic viscosity – ηpl = =34sPz; dynamic shear stress – τ0 = 45dPa.
The drilling fluid’s parameters after transfer to inhibited PACS: specific gravity ρ = 1.45 g/cm3; viscosity T = 30sec; water loss B = 2cm3; mudcake thickness K = 0.5 mm; static shear stress for 1 min. Q1 = 6dPa; for 10min. Q10 = 9dPa; plastic viscosity ηpl = 15cPz; dynamic shear stress τ0 = 21dPa.
Well drilling from a depth of 2187 m was carried out with a bit Ø295.3 mm, the temperature at the bottom was +65°C. Due to the fact that the temperature in the well increases by intervals and reaches the limits of +96°C at the design depth of 3950m, it was decided to transfer the drilling fluid to the PACS-T thermally stabilized inhibited system. The transfer was carried out without stopping during the drilling of the well in the interval 2187 - 2234m.
The main materials for transferring the drilling fluid with the complex inhibited thermally stabilized additive PACS-T: Portland cement PCT1 - 100; potassium chloride (COP l); FHLS (ferro-chromelignosulfonate); Chrompeak (Na2 Cr2 O7 or K2 Cr2 O7), caustic soda (NOH), surfactant HT-48.
Drilling mud parameters before transfer: density – ρ = 1.45 g/cm3; viscosity - T = 71 sec; water loss - B = 3cm3; the thickness of the clay cake - K = 1mm; static shear stress for 1 min. - Q1 = 51dPa; for 10min. - Q10 = 108dPa; plastic viscosity - ηpl = 42cPz; dynamic shear stress - τ 0 = 57dPa.
Parameters of the drilling fluid after transfer to the inhibited thermally stabilized system PACS-T: specific gravity ρ = 1.45 g/cm3; viscosity T = 32sec; water loss B = 2cm3; mudcake thickness K = 0.5mm; static shear stress for 1 min. Q1 = 3dPa; for 10min. Q10 = 9dPa; plastic viscosity ηpl = 12cPz; dynamic shear stress τ0 =18dPa.
As the well deepened, complex inhibited solutions of the PACS system and thermally stabilized PACS-T were carried out by introducing PACS and PACS-T inhibitors into the solutions to maintain their properties. The oil was well emulsified in solutions, having lubricating properties. The pH values of the solutions were maintained in the range of 10-12 by the addition of caustic soda. The decrease in water loss was achieved by treating the solution with KSSB-2 and FHLS reagents. Viscosity was reduced by treating the solution with water or aqueous reagents KSSB-2 and FHLS. With an increase in the structural-mechanical properties of the solutions, the solutions were inhibited.
Drilling of the well on the inhibited mud of the PACS system in the interval of 655 - 2187m and on the thermally stabilized PACS-T system in the interval of 2187m and to the actual depth of 3922m was completed without complications, the casing strings Ø324mm, Ø245mm and Ø140mm were lowered without landing and cemented.
