Drilling with lightweight muds makes its way into multilateral wells in South America.

Rewritten with permission from Weatherford Magazine, September 2002.

Subsurface conditions, remote locations and the popularity of enhanced recovery in Latin American fields lend themselves to under-balanced drilling (UBD) technology, and that popularity has created a path to advance the technology.

Weatherford took on the task of drilling the first underbalanced multilateral well when it aimed the technology at six horizontal legs in a well in the Carmopolis field onshore in the Sergipe-Alagoas Basin, 50 miles (8 km) north of Aracaju, Brazil.

The field had been produced for more than 35 years with single-bore vertical and directional wells drilled to the oil-bearing fractured metamorphic rock in the basement sandstone and to the syntectonic conglomerates and fine clastic sediments in the Carmopolis formation. The Carmopolis formation contains four major oil-bearing zones with oil gravity ranging from 18° API to 28° API and a vicosity range between 30 and 80 miliPascal (mPas).

Improving recovery

Several enhanced recovery technologies, including waterflooding, polymer injection, steam simulation, steam flooding, steam injection and in-situ combustion schemes have been tested as pilot projects. Currently, steam stimulation is being applied to the whole field.

To increase oil productivity, a well was formulated that combines three new technologies. The well featured a level-two multilateral with four horizontal legs in the pay zone. The zone was drilled underbalanced to minimize damage to the large, exposed build and horizontal bores.
Medium-radius technology was applied to minimize the reservoir energizing required to reach pump level when producing.

The planning, detailed design, engineering and contingency planning took 4 months. The main criteria were:

• to maintain equivalent circulating density (ECD) under 4.2 pounds per gallon (ppg) to guarantee the UBD condition at the reservoir. Equivalent density to pore pressure was 4.75 ppg;
• to use normal paraffin mud to maintain the ECD target with the use of a single 1,500 NPU N2 Unit, allowing the driller to trip and connect on balance without formation impairment due to drilling and reservoir fluid compatibility;
• to inject the nitrogen gas in the 95/8-in. to 7-in. annulus, which allows horizontal steering by conventional telemetry;
• to calculate bottomhole pressure by fluid level and hydraulic simulation before every leg and readjust hydraulic drilling parameters when required to avoid the need for a pressure-while-drilling tool;
• to complete the legs with slotted liner and avoid cementing across the pay zone; and
• to use an open separation skimming system to manage the returns and oil production while drilling.

The 7-in. casing was provided with one sub having a pressurization orifice for N2 injection and two double-window joints. Once the 7-in. casing was set and cemented, mud circulation was started via the N2 injection orifice. This ensured the concentric casing annulus was open.

The first pre-milled window was opened and the medium radius build-up for this first leg was drilled with 3.28 ppg ECD, achieving 82° in 121 ft (37 m) of new 43/4-in. hole drilled.

Thereafter, the bottomhole assembly was changed to steer in the horizontal section. A hundred meters were drilled with 3.40 to 3.75 ppg ECD. Oil production was kept at expected rates for the 39 hours it took to drill this first leg, proving the UBD condition at the reservoir. Mud and nitrogen were then shut down to allow the well to self-balance. The fluid level was measured and the reservoir pressure calculated at an equivalent density of 4.29 ppg.

A 27/8-in. liner was run and the whipstock successfully retrieved.

Second leg

For the second leg, the whipstock was set at a 15° angle from the first window with a vertical displacement of 16-ft (5 m) from where the first pre-milled window was originally place. The well was maintained in a dynamic condition to avoid over-pressuring the first leg. The second window was opened and the same procedure used on the first leg was applied. Medium-radius build-up was completed in 177 ft (54 m), keeping the ECD under 3.8 ppg. The bottomhole assembly was again changed to steer in the horizontal section.

A total of 426 ft (130 m) was drilled in 37 hours maintaining production rates of oil under the same drilling parameters. Mud and nitrogen were shut down to allow the well to self-balance again.
The fluid level was measured and the reservoir pressure calculated at an ECD of 3.95 ppg. Thereafter a 27/8-in. liner was successfully run, and the whipstock retrieved without any problems.

Thus encouraged, the operator decided to drill two more legs in addition to the four legs already planned. These would be placed in the upper two windows and drilled under the same parameters and pressure conditions as the others.

Two legs were then successfully drilled from the third window using the same drilling procedures and parameters as those used in the lower two build-up and horizontal sections, again keeping the well in underbalanced condition.

The fifth and sixth legs were drilled in the fourth pre-milled window. The lower fifth leg was left in open-hole condition, as was the first leg drilled in the third window, but the sixth leg was completed with a slotted liner. The ECD was kept under 3.8 ppg and the production accounted during the 9 days while drilling the two legs.

Comments