Drilling mud technologies exemplified by Yen-Yakhinsk superdeep well sg-7 Печать E-mail


Yen-Yakhinsk superdeep stratigraphic well SG-7 is located on the western wing of Yen-Yakhinsk structure outside the investigated petroleum bearing outline in alternative geological conditions comparing with the previously drilled Tyumen SG-6.

The purpose of well drilling is investigation of the deep geological structure in the northern part of West Siberian PBP and estimation of petroleum bearing perspectives of Trais and Paleozoic deposits.

The upper intervals of the well- Quaitemary, Palaeogene and Cretaceous deposits are best investigated and presented by unstable soft types of clay complicated to the depth of 200 m by presence of perennial frozen rocks; the lower section is mainly presented by clay, argillites, siltstone with lays of sandstone.

The given section is characterized by lower gradients of stratum pressure (from 1,0 to 1,15), presence of considerable clay lays and argillite tending to hydro-swell and cave in.

The distinctive peculiarity of drilling intervals to the descending depth of the first and second casings/columns 426 and 324 mm in diameter is the necessity of longitudinal maintaining of steadiness of the borehole 480 and 393,7 mm in diameter 1000 and 2000 m long to their fastening.

To drill a borehole for a conductor the fust and the second intermediate casing columns, inhibiting polymeric potassic drilling mud of density 1,1-1,18 g/cm was used. Steady boreholes with cavernity coefficient not more than 1,02-1,03 were designed , which allowed descending of the conductor casing columns 508, 426 and 324 nun in diameter without complications. Shortening of the descending depth of the second casing to 3555 m (the planned depth 3750) was caused by the not confirmed geological section forecast and considerably high factual reservoir pressure versus the forecast in deposits in Tyumen suite and Achimovsk deposits. To compare, Tyumen suite is characterized by Ka=1,98 versus Ka 1,48-1,52 as foreseen by the project/plan, which caused complexity of the well structure by means of descending of an additional casing 273 mm in diameter (the third casing) at the depth of 3932 and significant alteration of composition and properties of the drilling mud. The temperature conditions were also different along the hole at the depth of 3930 m the bottom-hole temperature was 125° versus 105° as it had been foreseen.

In the course of drilling this interval the previously used polymeric potassic drilling mud was gradually substituted by inhibiting, mainly lignosulphanatpotassic with density 2,10-2,11 g/cm3.

As well as in the upper intervals to 3555m presence of clay-containing rocks required applying of flocculent of Praestol type and acryl-containing flocculent of complex effect such as hypan, unifloc. To regulate filtration and fluidity/fluctuation properties the common lignosulphanates and polycarbohydrates were used, inhibition was maintained by a complex of reagents of common usage in combination with potassic reagents.
After casing the well by a 273 mm casing at the depth of 3932 m, further descending in Tyumen suite was supposed with density of the drilling mud l,75-l-82g/cm3, nevertheless, the attempts to reduce the density lower than 2,06-2.08 g/cm3 led to penetration of gas to the borehole and degasation of the mud.

Considering the peculiarities of the stratigraphic well, commercial and geological concern about the lower horizons as well as a necessity of thorough investigation of the latter (complex GWL (Geological Investigation of Well), testing, selection of core) made the role of the flushing agent more significant than the other types of mud used in development and exploitation drilling.

Comparing the cost of drilling mud in estimates of construction of deep stratigraphic, development and exploitation wells, it appears that the deeper the more significant drilling mud is in stratigraphic drilling because of its multifunctional application as well as expenses on it should be increased.

Taking into account all these, the main requirements to drilling mud used in superdeep wells have been worked out as follows:

- forming a steady borehole and maintaining its condition for a considerably long time;

- providing passage of geophysical apparatus for conducting the required complex GIW and the necessary steadiness and carry out ability of the drilled rocks;

- minimal affection the productive lays, objected to excavating by geophysical methods, testing and exploiting;

- minimal friction properties;

- necessary thermo stability.

Thus, our further research was aimed at creating flushing drilling mud, meeting these requirements as much as possible. It should be noted that some of them are incompatible.

Considering the project geological data on the well section they are presented by Jurassic deposits: terrigenous rocks- clay, argillite, siltstone and marl comprise more than 85% of the section. The further drilling was conducted with weighted lignosulphanate polymeric inhibited mud.

Choice of components and technological properties of the drilling mud was made on the basis of estimation methodology of influence the drilling mud (filtrate) on destroy of the rocks. Methodology based on the diffusion theory of destruction in a tension field allows forecasting and controlling behavior of the borehole throughout time with the account of physic-chemical composition of the drilling mud by means of evaluation of the coefficients of steadiness and rates of borehole destruction.

The peculiarity of conducting the interval 3932-6032 is frequently arising problems with the passage of geological apparatus in the open borehole 244,5 and 270 nun in diameter. The troubles started at the depth of 4200 m. We refer it originally to the temperature coagulation and peak increase of STS (Static Tension of Shift) of the drilling mud. These difficulties led us to major substitution of the drilling mud by lignosulfanate, reducing processing by polycarbohydrate and acrilate in particular. Nevertheless, analyzing the light tool fit and apparatus non passage in the intervals 4200-4400 m, we came to a conclusion that forming a stable filtrating crust opposite productive lays, creating established hydro-dynamic balance (not depending on SPO and other technological operations) in the "well- lay" system is from 30 to 40 days, that led to breaching the schedule and completeness of the geophysical work in the well.

Fulfillment of the interval (planned) geophysical research turned to be possible only after injection a portion of the drilling mud with lower density (1,7- 1,75 g/cm3) and respectively reduced structural-mechanical and thixotropic properties to the lower part of the borehole (500-1000 m). It should be noted that the experience of carrying out GDW at such density properties of the drilling mud at the given depths under the conditions of super-high temperatures (temperature at a depth of 600 m is 178-180 °C) doesn't exist.

hi these conditions the great importance was laid on lubricating properties of the drilling mud. which were estimated by friction and cementing coefficients of the filtrating crust. To regulate these indices a lubricating addition on the bases of monoethanamino salt of tallow oil and derivatives of polyatomic spirits, which also influences inhibition effectively, and in combination with high molecular cellulose reagents and lignosulphanates thanks to synergetic effect reduces filtration at high temperatures, are recommended. In addition, the lubricating addition impedes foaming of the drilling mud while processed by lignosulphanates.

Thus, the developed complex of technological solutions and interval recipes of the drilling mud provided formation of an open borehole 270 mm in diameter more than 2000 m long and steady during a few months. The last interval was steadied by the fourth casing -liner 219 mm in diameter in the interval 3709- 6031 m.

The peculiarity of the further deepening of the borehole in Trias sediments/deposits is necessity of considerable core selection, testing of the lays both in the open borehole in the casing, as well as increase of the amount of geophysical investigations connected with high temperature (180- 215 °C) and density of the drilling mud and also lessen of passage clearance of the apparatus (the nominal diameter of the hole 190,5 mm).

As a consequence, the composition and technological properties of the drilling mud were altered: it was transferred to on the lignosulphanate bases with minimal component of colloidal (clay) phase (3,5- 4,0 %). Control of filtration was carried out by high-molecular ethers of cellulose, protected (inhibited) from thermal destruction combined with acrylic polymers.

Control of fluidity parameters was maintained by combined processing of the mud by lignosulphanates and siliconorganic combines (methyl sodium silicate) along with ethers of cellulose and combines of monoethanoloamine salt of tallow oil and derivatives of polyatomic spirits on the bases of synergetic effect.

Thus, thechnological parameters of the drilling mud. related to the drilling interval lower than 6031 m were maintained within necessary limits.
At the depth of 6910-6920 m effusive deposits were discovered, presented by changed (foamed) basalts. According to the state by 20.09.04 the depth of the hole is 7075 m, the full complex of geophysical investigations and core selection are carried out. The temperature in the hole is 210 °C. According to the caliper logging data deflection (increase) from the nominal is 3-5 %.



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