Enhanced oil recovery from a mature field in the Republic of Uzbekistan
The effectiveness of radial drilling technology has long been a contentious issue within our industry, and it was into this climate of controversy that Tethys Petroleum Limited in 2010 embarked upon a campaign of radial drilling to enhance oil recovery from a mature oil field in the Republic of Uzbekistan. This would be the first application of radial drilling technology in Uzbekistan.
Tethys Petroleum Limited is an oil and gas exploration and production company currently focused on Central Asia with projects in Kazakhstan, Tajikistan and Uzbekistan. It is the only independent oil and gas company operating in all three Republics.
The North Urtabulak oil field in southern Uzbekistan was discovered in 1972, with calculated oil in-place of approximately 124.2 million barrels (“MMbbls”). To date, 119 wells have been drilled on the structure, and remaining oil thought to be in the region of 7.3 MMbbls.
The field is a carbonate reef structure of Jurassic age, with an average reservoir thickness of approximately 320m.
In March of 2010, Tethys Petroleum Limited via its subsidiary Tethys Production Uzbekistan acquired the Contractor rights to the North Urtabulak Production Enhancement Contract, whereby the Contractor is obligated to implement new technologies and new techniques to enhance liquid hydrocarbon production from the North Urtabulak field. In return the Contractor receives an allocation of this increased (incremental) production.
A Petrel/Eclipse reservoir model of North Urtabulak commissioned by Tethys Petroleum in 2010 indicated the presence of significant volumes of remaining oil in place, much of which was ‘trapped’ between existing wells – a situation exacerbated by severe near-wellbore skin damage and the currently pressure-depleted condition of the North Urtabulak reservoir. A solution was therefore required whereby these remaining oil reserves could be accessed in a cost-effective manner.
Tethys Petroleum and its contractual predecessors to the Production Enhancement Contract with the state-owned oil company Uzbekneftegaz, had previously implemented horizontal drilling and sidetracking technologies as means of accessing additional oil reserves. However, these capital intensive and relatively high-risk technologies were no longer considered to be cost effective for the North Urtabulak reservoir in its current state of depletion. Similarly, acid stimulation of the reservoir had also been attempted at North Urtabulak, although it was found that the stimulation fluid tended to follow the path of least resistance, which is invariably into the lower productivity zones of the reservoir. It was therefore hoped that radial drilling might allow Tethys to more efficiently and more accurately access trapped and/or previously inaccessible hydrocarbons.
Radial drilling’s principal application to date has been in marginal and mature fields with low productivity and shallow (<2750m) wells. Radial drilling effectively applies modified coiled tubing technology to penetrate lateral holes of 50mm in diameter up to 100m from the original wellbore. The principal objective of this technique is to improve the production profile around the original wellbore by penetrating beyond the damaged skin zone and by accessing trapped pockets of hydrocarbons. At present this technology can only be applied in vertical (or near-vertical) wells, although research is ongoing to adapt this technique to deviated and horizontal wells. Suitable well candidates for radial drilling are mutually agreed between the operator and the radial drilling contractor, based principally upon the mechanical condition of the well and its production potential.
The principal factors to be taken into consideration when selecting suitable well candidates for radial drilling operations are as follows;
» Casing Size: The current radial drilling equipment can operate in production casing sizes of 5-1/2” OD or greater. The equipment can also operate in 4-1/2” or greater in-gauge open hole.
» Multiple Casing Strings: The current radial drilling configuration can only penetrate single strings of casing. Overlapping casing strings cannot be milled with the existing cutting system.
» Casing Grade: The tungsten carbide bits used to mill casing exits are limited to casing grades of N-80 (Russian grade ‘D’) or less.
» Casing Wall Thickness: In its current design, the maximum casing wall thickness able to be milled is 10mm.
» Casing Cementation: In order to successfully initiate a casing exit, the radial drilling equipment requires a good cement bond between the casing and the formation. A poor casing bond generally results in difficulty or failure in jetting operations. The competence of the cement bond is generally assessed by CBL.
» Wellbore Inclination: Since the system relies upon gravity to seat the radial drilling equipment, the well inclination should not exceed 60 degrees from vertical.
» Wellbore Depth: The present system is designed to operate at a maximum depth of 3000m.
» Wellbore Rathole: A rathole of 10m is required to allow the jetted debris to settle beneath the deflector shoe.
» Bottomhole Temperature: BHT should not exceed 120 Deg C.
» Bottomhole Pressure: BHP should not exceed 6500 psi.
Reservoir & Geological Factors
» Dipping Formations: Steeply dipping formations are generally not suitable for radial drilling operations, particularly where there are porosity differences between adjacent formations.
» Unconformities: Facial changes, pinch-outs and unconformities generally result in a stalling or stoppage of the jetting process.
» Mineralization: Calcitic or siliceous mineralization in carbonates and sands can result in zero porosity which renders the well unsuitable for radial drilling.
» Cavernous and Vuggy Formations: Since the forward penetration of the jetting system relies upon some lateral constraint, cavernous and vuggy formations are not generally suitable for radial drilling operations.
» Unconsolidated Formations: Jetting of unconsolidated formations generally causes washouts which impedes forward progress of the jetting assembly.
» Evaporites: Salts, gypsum and anhydrite are generally not penetrable by jetting action.
In the case of the North Urtabulak reservoir, core samples of the reservoir rock were sent to Radial Drilling Systems’ (RDS) laboratory for pilot testing. At North Urtabulak the RDS coiled tubing unit was operated in conjunction with Tethys’ contracted XJ-450 truck-mounted workover rig.
Upon selection of suitable well candidates the radial drilling procedure is as follows;
1. Kill well. Rig up workover unit. Retrieve existing completion string.
2. Run in hole with casing scraper and casing drift assemblies.
3. Connect deflector shoe to tubing and run in hole to required depth.
4. Orient deflector shoe (if required).
5. Rig up coil tubing unit and gooseneck.
6. Run in hole with casing cutter and initiate casing exit.
7. Run in hole with jet assembly connected to 100m flexible hose and coiled tubing.
8. Perforate 50mm diameter hole up to 100m in length.
9. Rotate deflector shoe through 90 degrees.
10. Repeat steps 6 through 9 until all four laterals are completed.
11. Raise deflector shoe to second level (if required).
This procedure may be repeated for as many additional levels of penetration as required.
Throughout the radial drilling process the coiled tubing string is circulated with filtered (< 10 microns) water, and penetration is achieved through high pressure jetting via nozzles matched to the compressive strength of the formation. By maintaining control of the coiled tubing from surface the jetting string is held in constant tension thus permitting only straight forward departure perpendicular to the original wellbore. In carbonate (and carbonate-cemented) reservoirs, a 10% HCl acid wash can be applied after drilling each lateral. On average, it took less than two hours to complete the radial drilling of each 100m lateral at North Urtabulak.
In total, five well candidates were selected for the radial drilling trial at North Urtabulak. These were a combination of cased-hole and open-hole completions located in different parts of the field. All wells were vertical or near vertical. In four of the well candidates one level of four laterals were attempted and in well NU-116 two levels of four laterals were attempted. With the exception of well NU-44 (where only two laterals were achieved), in all the remaining wells four laterals of ~100m were successfully achieved at each level. All laterals were treated with a 10% hydrochloric acid wash immediately after drilling.
The results of radial drilling at North Urtabulak are summarized in the chart above.
NU-87 – is an open-hole sidetracked well located slightly south of centre of the field. In total, four laterals (each of 98m in length) were radially drilled at a depth of 2450.9m. Oil production from this well has increased from 56.6 barrels of oil per day (“bopd”) prior to radial drilling to 69.8 bopd after radial drilling. This represents an increase of 13.2 bopd (23%).
NU-79 – is a vertical well located slightly north of centre of the field, and is the only cased-hole well candidate on which radial drilling operations were performed. In total, four laterals were drilled (each 100m in length). Due to difficulties in initiating these laterals, two were drilled at 2436.7m and a further two were drilled at 2476m. It is believed that the problems encountered in drilling the final two laterals may have been due to a poor cement bond between the casing and the formation. Oil production from this well increased from 10.6 bopd to 54 bopd after radial drilling. This represents an increase of 43.4 bopd (409%).
NU-92 – is an open-hole sidetracked well in the east of the field. In total, four laterals were drilled (each 100m in length) at a depth of 2457m. Oil production from this well has increased from 64.1 bopd prior to radial drilling to 79.2 bopd post radial drilling. This represents an increase of 15.1 bopd (24%).
NU-44 – is an open-hole sidetrack in the north-west of the field. Some considerable difficulties were encountered in initiating laterals in this well and this thought to be because the well had been previously acidized. This acidization and consequent hole enlargement undoubtedly affected the ability of the deflector shoe to centralize in the wellbore. However, we did still manage to drill two laterals in this well (1 x 94m and 1 x 23m) at a depth of 2451.3m. Despite only having achieved two laterals in this well, oil production increased from 50.9 bopd prior to radial drilling to 94.9 bopd after radial drilling, resulting in an increase of 44 bopd (86%).
NU-116 – is a newly-drilled vertical well in the north-west of the field. This well has never really produced any significant quantities of oil and the consensus of opinion is that the well is located in a part of the field with exceptionally low porosity and permeability. It was hoped that laterals drilled from this well might extend beyond this poor poro-perm area and access reserves in a more productive part of the field. In total, two levels of laterals (each of 4 x 100m) were radially drilled from open hole. However, despite having drilled a total of 8 laterals from this well, results have been somewhat disappointing. Oil production prior to radial drilling was 1.25 bopd, and after radial drilling it increased to 5 bopd.
Overall, the radial drilling trial at North Urtabulak can reasonably be judged to have been a success. With the possible exception of NU-116 (for justifiable reservoir-related reasons) all well candidates in the trial demonstrated a significant uplift in production as a result of radial drilling. It must however be said that whilst production from many of these radially-drilled wells has subsequently dropped off, we believe to be due more to reasons of an overall decline in reservoir pressure than to any radial drilling related factors.
By far the single most important lesson-learnt from the radial drilling trial at North Urtabulak is that careful field and well selection are paramount in assuring the successful outcome of radial drilling operations. For example, we were advised by our radial drilling contractor during the planning phase that radial drilling technology is most effective in fields which retain at least 70% of the original reservoir pressure.
The North Urtabulak field certainly does not fulfill this criterion, which is probably why the post-radial drilling uplift in production was not sustained. However, Tethys Petroleum was well aware of this impediment but was keen to prove the effectiveness of the technology to Uzbekneftegaz as a means of securing additional field rehabilitation projects in Uzbekistan and elsewhere. The only well candidates in which we had problems initiating and progressing radially drilled laterals fell foul of one or more of the well selection criteria outlined above. Specifically, well NU-44 suffered from hole enlargement as a result of a previous acid stimulation, which resulted in an inability to centralize the deflector shoe in the wellbore. Similarly, we had problems in initiating laterals in NU-79 due to a poor cement bond between casing and formation. Again, this re-emphasizes the need for careful planning and well selection prior to the commencement of radial drilling operations.
In conclusion, and despite the pressure depleted condition of the reservoir, the trial at North Urtabulak proved emphatically that radial drilling can be a cost-effective and time-efficient application to increase production and to access trapped hydrocarbons. It allows accurate placement of laterals and extended horizontal penetration over conventional perforating. It is a technology which Tethys Petroleum Limited fully intends to implement elsewhere, both in Uzbekistan and within the Tethys Petroleum Group’s other assets in Central Asia.
STEVE ELLIOTT is currently Project Development Manager of Tethys Petroleum Limited (Part of The Tethys Petroleum Group of Companies). Prior to joining Tethys, Steve was with the Baker Hughes Group of Companies for 22 years, working in the North Sea, Africa and latterly Central Asia. Steve has detailed knowledge and experience of drilling operations, with recent focus on project management and the execution and co-ordination of Integrated Services contracts.