“The story of finding oil in the South China Sea”: Oil discovery in White Tiger oil field

30 years since the discovery of oil in White Tiger oil field, Vietnam-Russia Joint Venture -Vietsovpetro has exploited more than 180 million tons of oil from this oil field. In addition to Bạch Hổ oil field, a series of other oil and gas field has been put into operation on the continental shelf of Vietnam. International colleagues often concern how Vietsovpetro find and exploit oil in the granitic basement rocks?

There are also many mysteries of the granitic basement rocks where the explorers in Vietsovpetro are continuing to find and exploit them. It took 40 hour year to understand about gravimetric anomaly of -2 miligal of White Tiger oil field (achieved by the Mobil Company in 1974) related to the density of fractured granitic basement rocks containing more than 180 million tons of oil; Exploration Consultance Company in 1979 explained that this is a salt dome. Vietsovpetro's a new seismal document 3D-4C in 2015 initially gives a detailed picture of the surface of the White Tiger oil field , provides new information explaining the horizontal wave field and the nature of seismic amplitude anomalies that have long been used by Vietsovpetro as drilling in the foundation.

White Tiger oil field

Unlike the "normal" way to find oil, just drills down layer of soft rock and stops when it hits layer of hard rock - "the granitic basement rocks", Vietsovpetro has designed and drilled the first exploration wells at Bach Ho oil field down to 20 – 30m of the granitic basement rocks and confirmed by core samples. However, the 2D seismic technology was very weak at that time, with the presence of an "indistinct" screen (SH-10 or "E" roof) hiding an Oligocene slice, Actual design depth of the wells is usually increased by 100-150m for redundancy. Drilling deep into the the granitic basement rocks allowed Vietsovpetro to study the formation before the third layer lying under the bottom of the Cuu Long Basin, which includes the "lifting blocks of the layer have Paleozoic and the Mesozoic covered by the third layer, is worthy of concern” (page 46, "Geological structure and prospects for oil and gas in the southern continental shelf of Vietnam", Author collective of Petroleum Company II, Vung Tau - 1981).

In fact, the comprehensive approach of Vietsovpetro, as well as the exploration and validation of shallow oil layers for quick commissioning and data collection, are deeply appreciated. In 1985, at the exploration-well BH-4 in the Northern White Tiger, Vietsovpetro has combined evaluation of the Miocene oil level with detection of Oligocene oil. In 1987, when Oligocene oil was measured at BH-6 well, Vietsovpetro has discovered the oil with high yield (flow 505m3 / day) in the granitic basement after explosion with bombs (torpedo) at a depth of 3550m - 3515m. This result is because Vietsovpetro has learned from the two failed tests in the previous foundation at the well BH-3. Both wells drilled over the 70m foundation, took core samples with oil marks and lost circulation of drilling fluid same as the BH-6 well.

BH-6 exploration well discovered the oil in the granitic basement rocks of White Tiger oil field on May 11, 1987

After that event, Vietsovpetro concentrates efforts to exploit Oligocene oil in the priority area and combine with explore and measure in the granitic basement rocks from fixed platforms. On May 25, 1988, Vietsovpetro designed the BH-47R exploration well in the center roof with vertical depth of 3.300m, was going to drill from MSP-1 to the base stand MSP-2. While waiting for the construction materials, Vietsovpetro implemented the proposal of the Chief of the Sea Drilling Department on 24/6/1988, testing the granitic basement rocks again at BH-1 well which is exhausted from Miocene.

As a result, on September 5, 1988 the oil from the roof was sprayed with a capacity of 407 tons / day tấn/ngày and BH-1 was immediately put into operation (September 11, 1988) through a drill with a diameter of 89mm. After that, Vietsovpetro drilled the well to measure BH-2 from MSP-2 in a vertical way instead of the well BH-47R and other wells such as BH-401, 402, 403. Particularly, the decision to prioritize BH-1 wells accelerated the pace of oil exploitation from the granitic basement rocks earlier than planned for about 4 months.

Design the exploration well BH-47R and recommend to re-test the BH-1 well

The necessary time to preparing for oil extraction at the White Tiger oil field was shortened to 16 months from the date of discovery. It showed that many of Vietsovpetro's intellectuals have been mobilized for this hard work and determination. At the same time, it described the great effectiveness of geological exploration with high professionalism in Vietsovpetro.

In the next period, measuring and drill for oil extraction was increased with higher depth such as: 4.450m, 4.650m and 5.000m. Difficulty and the most important when constructing wells is determination of the exact depth to meet the the granitic basement rocks to add the 245mm diameter pipe on the roof. If the pipe is too early to add causing the sediment remaining under the pipe will collapse. Sometimes it takes an extra pipes. If add them late, drill into the roof will lose all the solution causing blockage of the drill same as in the BH-430 well (1993), it took another 3 months to drill the new body into the granitic basement rocks.

The construction site always requires a detailed roof structure to add the pipes in time, but at 3 - 4km underground, the depth of the roof in 3D seismic is relative. The surface of the granitic basement rocks is rough, only a few meters away is the terrain is changing up or down, sometimes down to the deep. Vietsovpetro's geological survey experts have successfully explored and implemented the solution to build effectively. So, series of boreholes drilled by Vietsovpetro by the contractor or internally have all been drilled safely and cost-effective. Particularly at the well BH-9001 which was drilled in 2001, despite storm weather, Vietsovpetro does not have to wait to mobilize equipment and geophysical measurements, still determine the exact depth to add the pipe into roof of the granitic basement rocks. It saves about $ 1 million in rental of drilling rig and well geophysical measurements.

It has been 30 years with White Tiger oil field, Experts still have a lot of questions about the technology to increase the recovery of this precious resource, This issue was discussed vigorously at the workshop at the final stage held on 02/11/2017 at Vietsovpetro.

Nguyen Van Duc - pvn.vn