"DRILLING & EXPLORATION WORLD" (DEW) Journal, June 29, 2005

Horizontal sidetracks drilling in non-operating wells

One of the ways to increase oil production is to recover unproduced reserves from old oilfields with new technological applications. Zarubezhneft experts in this article shares Russian experience in Horizontal sidetracks drilling in ONGC’ s Assam Asset non-operating wells.

State-owned “Oil and Natural Gas Corporation Ltd.” being the largest Indian oil-producing Operator in recent years makes many efforts to increase volumes of oil production. One of the ways to reach this target is to recover unproduced

In the last decade in many oil-producing countries a method of workover on temporary shut-in and marginal wells by sidetracking got a shot in the arm. In respect to area of this method application, oilfields of ONGC Assam Asset, India, are of the great interest.

Result of such policy of the Company was a pilot project for sidetrack operations in 4 non-operating wells at Lakwa and Geleki fields.

General Contractor in the project of sidetrack operations in 4 non-operating wells at Assam State fields was JSC “Zarubezhneft” that has been realizing integrated oil-and-gas projects for 38 years in different regions of the world and has in the world’s oil community a reputation of reliable and highly-professional partner.

JSC “Zarubezhneft” sub-contractor under this contract became Russian Service Company “Techinformservice” that has wide experience in the field of directional and horizontal wells drilling. Upon its establishment in the early 1990th the Company was among first Russian Companies that started mastering method of sidetracking from earlier drilled wells.

Preparation phase and target determination

At the stage of geological materials study it was ascertained that petrophysical structure of Lakwa and Geleki fields is quite similar to oilfields of Western Siberia in Russian Federation. This fact had considerable influence on selection of drilling technology, type of drilling mud and was taken into account while preparing general work program under this project.

Following consultations with specialists of Assam Asset of ONGC it was concluded that one of the problems while drilling at these oilfields is a presence of thick formation called “Girujan Clay”. Distinctive feature of this formation – is high ability of clay to swell, that rather frequently results in drill pipe stucking. It offered problems at the time of technical offer preparation in prediction of well-bore behavior within Girujan Clay interval when drilling with big inclination angles.

Reservoir pressure values decreased significantly as a result of long term development. Consequently, a problem of selecting the drilling mud composition had arisen, simultaneously ensuring well-bore stability in its upper part and pay formation protection from contamination as a result of invasion of circulating fluid.

Classic drilling fluids on the basis of bentonite used for drilling vertical wells and small inclination angle wells in Assam Asset of ONGC were not able to solve this problem.

After thorough study of geological materials by Russian specialists a composition of biopolymer drilling mud was developed for drilling these wells that minimized contamination of pay horizons and a possibility of well-bore complications.

ONGC specialists set a task to obtain the largest production rate of the wells with the smallest level of water-cut in produced fluid. At the same time the situation with actual position of WOC (water-oil-contact) at the time of projection was not clear, thus impeding works’ planning.

Following consultations with Indian Party and analysis of field materials on earlier drilled wells standard work program was prepared taking into account all requirements placed on Russian Company.

Equipment and materials

At the stage of contract co-ordination by the parties the decision was taken that ONGC would provide its own fixed drilling rig of 250 tons capacity and JSC “Zarubezhneft” – telemetry and other auxiliary equipment, chemicals for drilling mud, bits and other consumables.

Russian specialists reached a decision to use telemetry system with cable data transfer channel from the bottom to surface that proved to be successful when conducting similar operations at the territory of the Russian Federation. Set of this equipment includes downhole telesystem, winch with geophysical cable and non-magnetic drill collar of 9m length. Also 4-3/16” and 3-7/16” downhole screw motors were provided with adjustable bent housing angle.

It was decided that the Russian Party shall carry out geophysical surveys with its own equipment in view of absence of geophysical complex in Assam Asset which can perform logging operations in high angled slim holes. For this purpose geophysical unit was delivered to well site with special cable and set of geophysical tools.

With the purpose to lighten manual operations at the rig during tripping and connection making, the Russian party installed hydraulic and mechanical pipe-wrenches for pipes with diameter less than 5”, pneumatic spider.

While drilling horizontal side-tracks with 4-3/4” bits, circulation fluids rates are not more than 12-14 ltrs/sec and high capacity cleaning system is not required. That is why a compact cleaning system, manufactured in Russia under SWACO license, has been delivered, mounted and commissioned.

As it was noted earlier, it was decided to use biopolymer drilling mud. Together with polymer itself, delivery also covered typical chemicals, necessary for controlling reological and other properties of drilling mud.

Also delivered were casing cutting equipment, tools for liners setting, 4” casing, 2-7/8” drill pipes, 3-1/4” drill collars, 4-3/4” three-cone bits, PDC bits and perforation materials.

Description of typical well and operations

This article deals with the last L-412 well, drilled within the frame of the project; and this well is representative regarding well profile and approaches to drilling technologies, which came out as consequences of experience, gained after drilling of three previous wells. Well profile is represented in the figure.

All horizontal side-tracks have been drilled under typical plan, according to which a 10 meters window is cut in 5-1/2” production casing, than a cement bridge plug is set, side-track is kicked-off and drilled to project depth, well surveys are done, liner is set, cemented and perforated. Afterwards ONGC activated the wells with compressor.

In L-412 well the window was cut in vertical section of production casing in 2276-2286 m interval. During window cutting service water was used as circulating fluid, which subsequently was displaced with biopolymer drilling mud with 1,03 gr/cm3 density. Casing cutting was done by rotary method with three-blade casing cutter FKR-140, manufactured in Russia. The whole process took 20 hours.

After window quality control with microcaliper, cement bridge plug was set at 2270-2295 m interval.

Side-track kick-off was done by three-cone bit with minimal axial load, driven by down-hole motor with adjustable bent housing angle of 2,12 grad. Such assembly allows to gain azimuth deviation of 5 grad per 10 meters of penetration. Down-hole motor azimuth orientation was controlled by telemetry system. Drilling process was monitored by continuous sampling of cuttings. After drilling of first 6 meters cuttings constituted 100% of rock, which proved a successful operation, afterwards axial load was increased to 3-4 tons and another 20 m were drilled.

After reaching zenith angle of 13 grad and depth of 2300 m, three-cone bit was changed to PDC bit and bent housing angle of spindle section was reduced to 1,36 grad. The following interval down to projected depth was drilled with one bit in 5 days. Well trajectory was constantly monitored by down-hole telemetry system. Average rate of drift angle build-up while drilling with PDC bit was 2,2 grad per 10 meters. At projected depth of 2555 m zenith angle was 70 grad. Drilling was conducted with strict observance of special measures on accidents elimination, so as to avoid any complications.

After reaching TD, planned well surveys were conducted and interpreted.

After getting approval from geological department of ONGC Assam Asset, 4” liner was set at 2226-2555 meters interval. Liner assembly consisted of float collar, shoe, casing mechanical packer and special setting and cementing tool. Cementing was conducted with Class G cement with additives. After waiting-on-cement a final set of logs was run and the well was perforated with Dynawell guns.

Overall period of operations at L-412 well, starting from side-track kick-off till perforation, was 21 days.

Experience gained

As a result of executed works the following conclusions can be made:

1. It is necessary to employ screw down-hole motors with higher power parameters, which enables to increase penetration rates and makes observation of planned well trajectory easier.

2. It is viable to use PDC bits at the intervals of angle stabilization and build-up with the rate of 2,5 grad per 10 m. Such bits show higher penetration rates and meterage then cone bits, but cannot provide rates of angle build-up more than 3 grad per 10 m.

3. At well profile design phase it is necessary to consider accumulated field experience on changing of well trajectory, depending on geological section and bottom hole assembly.

4. For drilling of horizontal side-tracks it is necessary to modify existing mud cleaning system, which is currently a part of drilling units in ONGC Assam Asset.