A complex inhibitor drilling mud (CIDM) has allowed Russian operators to complete former problem wells.

In drilling deep and ultradeep wells, the most common and difficult problem is the loss of hole stability when drilling through argillaceous-clay rock, especially at intervals past 3,281 ft (1,000 m), due to capillary, osmotic and absorption forces that cause moistening and argillaceous rock retrogression.
Numerous research projects and field experience have shown that when water-based muds are used, proper chemical treatment must be implemented to slow the hydration process and prolong the stability period of argillaceous-clay rocks. Inhibited drilling muds based on calcium, aluminum, magnesium, potassium and other salts, whose effectiveness is determined by a time factor of rock stability, have been used for drilling under complicated conditions.
It is possible to reduce the moistening rate of argillaceous-clay rocks considerably while increasing their stability period by using water-based muds through:
• further ionic strengthening of drilling mud by adding components that prevent interpacket swelling of clay minerals;
• use of hydrophobic agents that adsorb on the surface of clay mineral particles to create protective adsorption layers; and
• limiting mud filtration by application of polysaccharides, which have stability over a wide range of pH and salinity.
It is important to predict the period of the stable state of clay rock in order to plan drilling time and casing of intervals prone to bridging. Analysis of eight deep wells (Akberdinsky No. 3, Sosnovsky No. 3 and No. 5, Kulgunino No. 1, Kipchak No. 1 and others in the Urals) drilled to depths of 16,404 ft to 18,044 ft (5,000 m to 5,500 m) considered:
• lithological and petrographical characteristics, structural bonds and dip angles of bedding rocks;
• well construction;
• value of reservoir (pore) pressure;
• type of drilling mud used; and
• stability period of clay shale.
Drilling problem formations
In the process of drilling the Aputovo No. 33 well, caving and bridging of unstable rock occurred at 3,428 ft (1,045 m). According to stratigrapic and lithological characteristics, the rock may be referred to as the Abdrazyakovsky suite (Moscow stage) and represented by tectonically deformed argillites and aleurolites.
Different technological measures were taken to prevent hole problems, including weighting up the drilling mud up to 84.28 lb/cu ft (1.35 g/cu cm), raising specific funnel viscosity to 120 s and more, raising gel strength to 40 s, and reducing filtration value up to 0.31 cu in. (5 cu cm). All failed to give positive results.
The regular drillstring runs failed to reach the target depth of 10,006 ft (3,050 m) by 300 ft to 1,300 ft (90 m to 400 m). Required reaming took from 8 hours to 10 days. Severe hole problems caused sticking of the drillstring, which required excessive rig time. After running the 95/8-in. casing to a depth of 6,187 ft (1,886 m) the operator decided to use silicate-potassium drilling mud containing 5% to 7% sodium metasilicate and up to 3% potassium chloride (KCl) to prevent bridging. The silicate-clay mud was used because the concentration of KCl in filtrate could not be increased above 0.44% due to high gelling of the mud. After reaching a depth of 6,663 ft (2,031 m), insurmountable problems were experienced in the form of intensive caving and bridging of crushed argillite (with angles of dip from 20° to 65°) that made further drilling impossible. The drilled argillite thickness with deformation texture constituted 475 ft (145 m).
Innovative mud design required
A new mud was required that would extend the period of hole stability to enhance drilling performance in a thick mass of clay rocks with tectonically deformed texture. Since positive results were not always achieved using potassium muds for drilling thick masses of unstable deposits, the possibility of using complex inhibitors that affected interpackets and the outer surfaces of clay minerals was considered. A polyethylene glycol potassium-silicate drilling mud composition, or CIDM, was developed. An aqueous mixture of polyethylene glycols that imparts high hydrophobic, lubricant, surfactant and inhibitive properties to the mud was used as a dispersion medium (Table 1).
The dispersed phase includes high-colloidal bentonite clay (5% to 7%) or Kaolinite clay (10% to 12%). Nonadhesive active carbonate lime weighting material was used to control the density up to 0.009 lb/in. (1.6 g/cm). Barite was used to control the density above this level.
KCL and potassium metasilicate were used as the inhibitor base. K-ions diffuse into defects of the crystal lattice to provide a deep (interpacket) inhibition. Silicate ions adsorb on the clay surface to hinder hydration and swelling, and abruptly increase the plastic strength due to the formation of polyhydro silicates (CaSi2O5.(H2O)).
A solution of a mono-ethanol-amine salt of light tall oils (LTO) in a polyethylene glycol (PEG) medium was suggested as the hydrophobic component (under the technical code of DCB-4TMP). In this system, the PEG molecules act as oligomeric matrixes due to "donor-acceptor" interactions in the form of hydrogen bonds (H-bonds) between oxygen of the PEG and acid carboxyl groups of the LTO. With the selected ratio of components, the hydrated oxygen atoms in the PEG remain free - not tied with H-bonds - imparting hydrophilic features to part of the chemical mix, the rest of which is represented by hydrophobic groups of LTO fatty acids. Changing the PEG-to-LTO ratio may easily vary the balance of hydrophilic and hydrophobic constituents. This creates the possibility of dedicated control of surfactant, lubrication, filtration and other features of water-based drilling muds treated with PEG and LTO base.
Research established that among known types of drilling muds, CIDM had the lowest moistening ability (1.6), which resulted in an extended stability time in argillaceous-clay rock of 82.9 days. The low moistening value and the higher stability value are explained by the synergistic effect of KCl, K2SiO3, polyethylene glycol and hydrophobic DCB-4TMP, which provided the high inhibiting effect.
CIDM in a base of polyethylene glycol, KCl, K2SiO3 and hydrophobic DCB-4TMP was used in drilling the Leuzinskaya No. 1 with a projected depth of 14,763 ft (4,500 m). This well, along with Aputovo No. 3, was drilled to estimate the oil and gas content in Paleozoic and Riphean-Vendian deposits. The highest bridging risks are the deposits of Abdrazyak suite with layer thickness of 4,921 ft (1,500 m), represented by crushed argillites with calcite and clay spots, which act here as cementing material, and with formation dip angles from 20° to 65°.
Application of CIDM was carried out at the depth of 3,021 ft (921 m). The mud was prepared in a hydraulic mixer with displacement volume of 118 gal/sec (35 cu m/min) by adding in succession polyethylene glycol, KCl, K2SiO3 and Poly Anionic Cellulose Celpol SL from Noviant (Finland).
No problems were observed during the drilling process. Entering the well for routine drilling was carried out without any reaming. However, a caliper log run before setting the 12¾-in. casing string to a depth of 4,270 ft (1,302 m) revealed the existence of caverns in some interlayers that exceeded the drilled hole diameter by 40% to 50%. In order to strengthen hole stability, the KCl concentration was increased to 6% with simultaneous addition of 1% to 3% K2SiO3. A hydrophobic composition of mono-ethanol-amine salt of LTOs in polyethylene glycol medium (DCB-4TMP) was continuously added into the CIDM from 4,330 ft (1,320 m) to consolidate the drilling fluid's inhibition and hydrophobic features, which prevented the packing and sticking situations.
In the drilling interval 4,265 ft to 6,076 ft (1,300 m to 1,852 m), the bit reached total depth without reaming. At 6,102 ft (1,860 m), the inclinometer missed the intended depth by 36 ft (11 m) during its regular running. The content of KCl in drilling mud filtrate was 3.95%. The mud parameters were well defined (Table 2).
The KCl content in the filtrate was increased 7% to 8% to improve hole stability. For better cuttings-carrying capacity and exclusion of cuttings settling, the drilling mud density was increased to 87.4 lb/cu ft (1.4 g/cu cm), and the parameters of structural-rheological features were increased as follows:
• viscosity was raised to 68 s to 75 s; and
• gel strength after 1 second was 8 to 9 Pa, and after 10 minutes 13 to 14 Pa.
The hole remained stable, only occasionally failing to reach the achieved depth by 5 ft to 15 ft (1.5 m to 5 m). Neither hanging up nor sticking of well surveying instruments was observed while logging. The logs indicated considerable cavern severity over the whole compared interval in Aputovo No. 33.
Cavities increased from 8% to 18% in comparison with the bit diameter in the interval of 5,495 to 6,070 ft (1,675 m to 1,850 m) in Leuzinskaya No. 1, but generally the well's hole size was close to nominal. During drilling the interval from 4,275 ft to 6,080 ft (1,303 m to 1,857 m), the mud density and values of its structural and rheological features remained stable
This proves the absence of cuttings dispersion and their transition into mud. This also fixed the formation of clay-conglomerates from drilling cuttings, facilitating their release from the drilling fluid by vibrating screens.
These facts characterize the hydrophobic inhibition features of polyethylene glycol potassium-silicate drilling mud. The mud provides effective hole drilling in thick masses of argillaceous-clay rocks with deformed structures. By June 11, 1999, the Leuzinskaya No. 1 well was drilled to a depth of 11,598 ft (3,535 m), and the exposed thickness of crushed argillites had exceeded 4,921 ft (1,500 m). Drilling continued with CIDM flushing to ensure hole stability.