Results of New Drilling Technologies Applied at the Verkhnechonskoye Field
The field development plan for the Verkhnechonskoye (VC) oil and gas condensate field provides for over 500 horizontal wells to be drilled between 2007-2021. To meet this schedule and the production targets of up to 10 million tons of oil per year, it was necessary to significantly improve the well construction time and maximize the net-to-gross pay ratio of the horizontal sections. To do this, VCNG Drilling Dept. partnered with Schlumberger’s Subsurface Dept. to completely redesign the VC drilling programs and data acquisition methods.
The complexity of subsurface structure in Verkhnechonskoye (VC) oil and gas condensate field is unique: the VC reservoir is rather shallow (true vertical depth (TVD) is 1,650 m) and the productive horizon is less than 10 m thick. The reservoir is heterogeneous with areas of different permeability due to the mineral salt deposits; therefore, the net pay zone in the 10-meter thin section is even smaller reaching about 3 m. To make matters worse salt deposits, hard dolomites and unstable shales have to be drilled prior to penetrating the reservoir itself. These challenges affect drilling efficiency and interfere with the well construction schedules.
In the beginning, VC was developed with vertical wells only. Yet considering the thin net pay of VC formation those wells did not show good productivity. In 2007, an updated development plan was proposed that relied on directional and horizontal wells rather than vertical wells. This helped halve the number of wells while maintaining the overall productivity. However, traditional technology using positive displacement motors (PDM) and gamma ray measurements was not able to ensure the trajectory within the sweet spot of the reservoir and consequently some of the directional and horizontal wells had low flow rates.
To boost drilling efficiency and optimize well cycle time, the VCNG Drilling Dept. worked with Schlumberger’s Subsurface Dept. to develop a brand new drilling methodology that relied on rotary steerable systems (RSS), logging while drilling (LWD) and geosteering technologies. The use of RSS coupled with LWD measurements provided proper trajectory control and important information about rock geophysics while the use of geosteering more than doubled productivity of each of the new wells.
Thorough Pre-Job Planning
Push-the-bit type of RSS tool was selected to be used in VC (Fig. 1). This system steers by using hydraulically actuated pads to push against the side of the wellbore, thus displacing the tool and the bit in the desired direction. Moreover, RSS is a fully rotating system and therefore transfers more weight to the bit than a conventional motor, which allows the use of aggressive bits and improves hole cleaning.
Bottomhole assembly (BHA) design was analyzed from main aspects, steerability and reliability. To have good steering tendency during the landing of the well and then dropping at the end of the 152.4 mm section, RSS was run with 147.64 mm stabilizer. A flex collar was run as a precautionary measure to guarantee the buildup rate capabilities of 3° per 30 m.
Given the limited options of bits, the selecting of the right bit to try to drill the section in one run while minimizing torsional vibrations induced by the bit was a difficult process since the RSS service had never been used in reservoirs similar to VC. The hydraulics were designed to get the required 50 atm pressure drop across the pads to ensure the maximum RSS efficiency. Hydraulic simulations and time vs. depth analysis proved that all criteria were met to run the RSS successfully while at the same time meeting rig’s torque and pressure limitations.
Pilot Project Success
An RSS+LWD pilot project was launched in VC in September-October 2009 and provided for drilling of four 152.4 mm horizontal sections. Each job was supported by specialists in a remote operation support center located in Irkutsk and a Geological team to ensure trouble-free drilling and proactive geosteering.
While RSS allows the use of aggressive bits, the average rate of penetration (ROP) for the four wells was about 16 mph, which is a 100-percent increase over the average PDM ROP, while record RSS ROP reached 21.85 mph. The increased ROP enabled VCNG to drill horizontal sections in 3.62 days saving about three days over PDM (Fig. 2, 3). In other words, it takes half as much time to drill 100 m – this parameter reduced from 1.39 days to 0.65 days.
Moreover, RSS allowed geologists to effectively steer the well using LWD to ensure the trajectory within the sweet spot of the reservoir (Fig. 4). This improved the ratio of net length to gross reservoir exposure to over 70 percent (86 percent in Well 814) as opposed to 30 percent demonstrated by PDM. High net-to-gross resulted in doubled production rate that reached 200 tpd to 250 tpd (290 tpd in Well 814).
RSS vs. PDM analysis has also proved the advantages of the new technology. As shown in Fig. 5A, the PDM well path is a simple descending line from the middle of the pay zone to its bottom. Moreover, the drilling of the last 200 m of the horizontal section encountered significant difficulties in sliding. As a result of low slide efficiency and unpredicted dropping tendency, final TVD was reached 100 m before target depth. This well failed to meet its geological objectives, while the well drilled with RSS+LWD succeeded (Fig. 5B). Therefore, new technology opens new opportunities for improved development of VC1 and VC2 reservoirs.
RSS coupled with geosteering made a good show while drilling deep wells. When Well 640 was drilled to a measured depth of 3,270 m it was not possible to reach the bottom without rotation: the drillstring was buckling even with no weight on bit because of the complexity of the well path (Fig. 6A). RSS was the solution: after each connection the top drive rotated the drillstring to reach bottom by eliminating drag and friction. Thus, VCNG was able to drill another 90 m without any problems (Fig. 6B) and reach planned target depth; Well 640 became the longest well on the pilot project with measured depth of 3,360 m.
Speaking of the entire project, LWD coupled with geosteering proved its worth in VC since reservoir uncertainty is very high and a well may be very much unlike its neighbors. Geosteering significantly improved net-to-gross in the high reservoir uncertainty due to timely adjustment of the designed well trajectory. This helped increase the length of the borehole in the net pay, thus improving initial flow rates and reducing the payback period.
Thus, the pilot project execution helped clearly determine major advantages of RSS+LWD from both technical and economical viewpoints:
» Well construction cost savings
» Decreased drilling time and associated risks
» Improved well placement and borehole quality
» Early production with higher oil production rates
» Increased well step out option
Large-Scale RSS Drilling
Good results of the pilot project led VCNG to expand RSS+LWD application across VC field. Since autumn 2009, about 90 percent of all 152.4 mm sections were drilled with utilization of the new methodology.
Further advantages of RSS+LWD were demonstrated when sidetracking from the original 152.4 mm section. Two successful sidetracks have been performed to date. In one case, the openhole sidetracking was initiated even without pulling the BHA out of the hole. The manner of ledge creation can be explained as time drilling: pick up from sidetracking point for 1 m, then time drilling with about 0.3 mph, then pick up again for 2 m and start time drilling with 0.6 mph. Drilling continued in such a way for the next 7 m to 10 m. The sidetrack was completed in about one day, saving time and money on the cement plug installation and pulling and running different BHA’s.
Azimuthal sidetracking with RSS is possible since the RSS’s pads help push the bit to the side helped by gravitational forces, while the bit cutting action acts on the low side of borehole. At the same time, the first sidetracking experience in VC shows that time drilling approach should be applied for at least the length of the RSS tool.
On the other hand, this sidetracking experience opens up future applications of RSS for multilateral drilling to help improve oil production from both VC1 and VC2 sandstones with proper fluid dynamics.
Construction Cycle Optimized for 215.9 mm Sections
In the typical well design for VC wells, the longest section is the one of 215.9 mm. Drilling of this section is often associated with low ROP in hard dolomites, risk of sticking in unstable shale located just above the pay zone and sliding efficiency decreases in the lower part of the section. Most of these issues can be resolved with the use of a power rotary steerable system (PRSS) (Fig. 7).
PRSS has a fully integrated high-torque power section that converts mud hydraulic power to mechanical energy. This energy combined with rotation provided by the rig’s top drive significantly increases downhole power at the bit. The additional torque capacity allows more aggressive PDC (polycrystalline diamond cutter) bits for directional applications and high weight on bit resulting in increased ROP and reduced well cycle time (Fig. 8).
To date, VCNG has drilled four wells with PRSS and the new technology has proved very efficient. The smoother borehole drilled with PRSS helped reduce torque and drag. Higher rotary speeds at the bit and slower drillstring rotation reduced the vibrations in the drillstring, hence improving penetration rates by 58 percent on average. Moreover, PDC bits were pulled out of the hole in good condition; therefore, PRSS can prolong bit life and this opens an opportunity to finish the section in one run.
One of the most important lessons learned from the drilled wells was the positive effect of PRSS on BHA stability, since very low vibrations were observed while drilling each well. This stability was achieved by decoupling the surface rotation from the bit rotation with greater torque PDM. Other benefits offered by PRSS were reduced tortuosity and better hole cleaning. Therefore, the key components for drilling optimization in VC are further PRSS BHA design optimization and improved PDC bit selection. While less days in finishing the 215.9 mm section and casing running optimization will significantly improve the overall project efficiency and profitability.
Gold Standard
RSS and LWD significantly contributed to overall drilling optimization in VC field. As compared to the initial well cycle for a development well (63 days) and net-to-gross ratio (30 percent) achieved with a conventional PDM, RSS helped reduce well construction cycle to 22.3 days (including an addition of 100 + m of lateral section) and boost net-to-gross to exceed 70 percent by July 2010. This resulted in a significant production increase. Rotary steerable systems and LWD significantly contributed to overall drilling optimization in VC.
VCNG is going to further optimize its drilling performance in the next two years, which would not be possible without rotary steerable systems:
» Increasing well length up to 3,600 m of measured depth and above
» Piloting extended reach drilling (152.4 mm horizontal section of 1,500 m)
» Decreasing the number of pads by increasing wells step out
VCNG experience with rotary steerable systems, logging while drilling and geosteering has set new standards and a benchmark for drilling efficiency and cost in East Siberia. The well designs, technical limits and new technologies tested in Verkhnechonskoye field may also be applied in other fields of the region.
This paper on «Results of New Drilling Technology Application on the Development of Verkhnechonskoye, a Complex East Siberian Field» was prepared for presentation at the 2010 SPE Russian Oil and Gas Technical Conference and Exhibition; paper number is SPE 135969. Published with thanks to TNK-BP and Innovator Magazineand Innovator Magazine
Kevin Wilson, Drilling Director, VCNG
Ivan Shokarev, Drilling Engineer, Schlumberger
John Small, Marketing Director, Schlumberger
Elnur Akhundov, Drilling and Measurements Sales Manager, Schlumberger
