I.P. Zaikin, K.V. Kempf, O.L. Gotlib, S.V. Yefimov, S.V. Vykhrystyuk, A.M. Nasyrov
Rosneft Oil Company OJSC, Udmurtneft OJSC
The following article looks at the experience of Udmurneft when using sidetracked wells to enhance oil recovery. Various configurations of sidetracked wells are looked at, including multibranch sidetracks. The efficiency of different well construction methods, such as drilling a horizontal sidetrack with various profiles, are compared. The relevancy of multihole (branched) drilling is also discussed in the article.
The present article contains information about experience of in oil recovery enhancement in mature fields. Various configurations of wells sidetracking have been presented, including multi-branched sidetracks. Wells efficiency after multiple-hole horizontal sidetracking was benchmarked and analyzed. The article concludes that development of multi-lateral sidetracking technology is extremely important on the field of JSV “Udmurtneft”.
The majority of oil fields owned by Udmurtneft OJSC are mature, however considerable hydrocarbon reserves are concentrated in stratigraphic traps that haven’t been covered by the existing well spacing. Due to this, the well selection requirements for sidetrack drilling method are becoming candidiates demanding. It should be emphasized that under present economical conditions drilling of deviated sidetracks is often unprofitable due to their low efficiency. Average flow rates of deviated wells are approximately 3-3.5 tons of oil per day. For that reason, starting from 2009, the staff of Udmurtneft OJSC have stopped drilling deviated sidetracks and have begun to drill horizontal wells. As a result, the average flow rates of wells with horizontal sidetracks substantially increased and reached levels of about 10-15 tons of oil per day.
Horizontal sidetracked drilling began at Udmurtneft OJSC in 1993. The drilling service and geological survey specialists have wide experience in implementation of such geological and engineering operations; they are constantly improving the technology of drilling and sidetracked completions. The background of experience in this field became the basis for further enhancement of horizontal sidetrack efficiency.
The result of two interlayers at the Vereiskian facility of the Krasnogorskoye field, where a sidetrack in the producing horizon was arranged with a so-called double-level profile, became the first step towards the increased flow rate of the horizontal sidetracked wells. Double-level profile requires drilling in the upper interlayer of the sidetrack, 100-120 m long and passage to the lower interlayer followed by the drilling of a horizontal section 50 – 80 m long. Total length of a sidetrack with double-level profile is 450 – 600m, with up to 130-150 m when drilling the productive section of strata. Through the application of double-level profile the specialists of Udmurtneft OJSC managed to increase the average flow rate to up to 18-20 tons per day.
Fig. 1-3 show the examples of sidetracks with various profiles on Krasnogorskoye oil field developed by Udmurtneft OJSC.
History of Multibranch well Drilling at Udmurtneft OJSC
In recent years technologies of multibranch and multilateral well drilling in the oil and gas industry have been subject to rapid development, allowing substantially increased flow rates. The first two multibranch wells in Udmurtia were drilled at the Bashkir facility of the Gremikhinskoye field in 1996. Three deviated holes were drilled in well No.673; two holes were drilled in well No.1287 meaning that all productive zones from А4-1 to А4-6 were covered. However the practice turned out to be unsuccessful in terms of increased well flow rates: well No.673 was put into operation with a water content of 83%, and well No 1287 – of 90%.
In 2010 geologists and drilling technicians of Rosneft OJSC and Udmurtneft OJSC carried out a feasibility study of subsequent multibranch well drilling. Multibranch well drilling technologies seemed more advantageous for the Krasnogorskoye field, where it was possible to perform pilot work using the existing drilling equipment and technologies.
The Vereiskian facility bearing the thin carbonate multizone reservoir (zones from B-0 to B-IIIB), complicated by the presence of a widespread gas cap, was selected as a target zone for drilling. The average porosity is 0.17 d.fr., oil viscosity under reservoir conditions is 9.7 MPa·s, and the average thickness of the producing zone is 3.5m. The bulk of underlaying layer transit stock is within the gas cap limits, therefore to recover reserves it is necessary to form a separate well network by means of sidetracking towards the oil producing zone.
Multibranch well drilling technology
The basic concept of multibranch well drilling was to increase the effective length of the producing horizontal section of the well. By sidetracking, it became possible in the upper interlayer to increase the length of the horizontal section, followed with a shearing into the middle of the horizontal section and then drilling the second horizontal in the lower interlayer.
The basic stages of multibranch well construction based on this technology, using the example of Well 1 at the Krasnogorskoye field were as follows (for the actual sidetrack profile refer to Fig. 3):
1. After the elimination of perforated intervals a “window” is cut in the 146 mm production casing from a whipstock wedge in one run.
2. Drilling the main hole with a 123.8 mm diameter with an inclination angle increasing up to the point of entry to the producing zone. At 50 m before the entry to В-II reservoir, reference logging is carried out to specify the geology.
3. A hole 170 m long is formed along the producing reservoir. Entering the producing zone and drilling in strata is performed with a constant gamma ray & mud logging. On completing the drilling a final well log is run by the self-contained geophysical instrument on drill pipes.
4. On completing the first well, drilling on the additional well is carried out into B-IIIa reservoir 90m away from the point of entry into the producing zone by means of making a keyseat, descending by 180 degrees.
5. Drilling of the additional 40m long sidetrack in the В-IIIa zone was also performed with a constant gamma ray & mud logging. Since the В-IIIа zone is relatively, thin the final length of horizontal section is determined on the basis of the actual geological and technical conditions during the drilling process.
6. Following this, a 102mm liner was run into В-IIIа zone. The producing section of the main hole within В-II zone and В-IIIа additional sidetrack is walled off by 102mm casing pipes with a filter. Space between the unstable shales of the В-II and В-IIIа zones was covered by blind pipes without cementing.
7. In order to avoid a negative effect on the reservoir, a cement packer is installed at the top of В-II in compact rock. An additional packer combined with a liner hanger is installed in the production casing for the purpose of sealing the hanger. The hanger cementing is carried out by direct pumping, followed by the removal of excessive cement.
The first well reconstruction on the Krasnogorskoye field was accompanied by 20% appreciation, occurred mainly due to obtaining the experience and loss of time. Subsequent reconstruction of two wells 2 and 3 of Krasnogorskoye field resulted in less downtime with only a 10% increase in expenditure, whereas the well flow rate was doubled.
In 2010, four multibranch wells were drilled at Vereiskian field of Udmurtneft OJSC. The results obtained brought us to the conclusion that due to an increase in the length of horizontal sections, well flow rates were increased substantially following sidetracking. Moreover, such a method of well reconstruction allows operators to increase the drainage coverage by both the area as well as by strata different by their filtration and capacitative characteristics and thereby to increase reservoir production rate.
This technology is an innovative method of thin carbonate reservoirs development on the fields of Udmurtneft OJSC.
Proven experience of multibranch wells drilled in the Krasnogorskoye field makes it possible for Udmurtneft OJSC to employ this technology extensively during the development of similar reservoirs at the Yeseneyskoye, Karsovayskoye and Chutyrsko-Kiyengopskoye fields.
Basic instructions for development of multibranch well technology are as follows:
» Improvement and implementation of the equipment for injection at sidetracks connection points with possibility of selective recovery and study in each of the sidetracks
» Drilling the wells with drift angles higher than the existing sidetracks
» Balanced and underbalanced drilling on producing horizons for skin effect reduction.
To increase well efficiency, Udmurtneft OJSC, in association with SamaraNIPIneft LLC have developed a construction project of multilateral operating wells on the Listvenskoye oil field scheduled for 2011.
The authors will publish the continuation of their work in a later edition of ROGTEC Magazine.