Thai partners tread new ground with branched completion
Bongkot, operated by Total, lies 180 km off the coast of Songkhla province in the Gulf of Thailand. The field's natural gas and condensate accumulations are found in multi-faulted reservoirs composed of channels and bars in a fluvio-deltaic and coastal environment.
In the northwest of the field, the 12-slot, WP-4 wellhead platform is connected to the process platform via a seven-km long, 18-in. diameter pipeline. WP-4, installed early in 1995, was designed to be tender-assisted for drilling operations. Maximum flow attainable is 120 MMcf/d, A first series of wells was put onstream in May 1995. The second well batch, including the dual-horizontal well BK-4-M, was drilled from May to June 1996. This particular well began producing in July.
The objective of BK-4-M was to maximize production from Bongkot's biggest proven oil rim in the 10-90 sandstone reservoir at a maintained rate of 4,000b/d over two years. Estimated oil in place in 10-90 is 22 million bbl with 8 bcf of free gas. The trap is an anticline of about 42 meters vertical closure underlain by a 15-meter active aquifer.
Hydrocarbon distribution at the top of the structure consists of a column of 17 meters of gas and 10 meters of oil. BK-4-G and BK-4-L, two multizone deviated wells previously drilled and completed through 10-90, confirmed the depths of the gas-oil contact at 1,089 meters and the oil-water contact at 1,099 meters. BK-4-G had experienced 25% water cut after four months in production and 45% after one year. The aim for BK-4-M is to delay water breakthrough by two years.
For BK-4-M, reservoir simulations determined that using a dual-horizontal well instead of a standard deviated well, oil recovery could be tripled to 20% of the oil in place. To achieve this, each branch would need to be 1,000 meters long and located 2 meters below the gas-oil contact.
It was a considerable challenge for Total, not having undertaken a dual-horizontal well before. It would also be the first such completion offshore South East Asia. However, drilling operations started only three months after the decision was taken to employ this technique. Implementation had to match the classic requirements of operations in the Bongkot Field, namely speed, low cost and quality (zero default approach).
BK-4-M was spudded on May 20, 1996. Two horizontal branches were drilled using an instrumental motor (Navigator). This FEMWD tool has the inclinometer a short distance behind the bit (3.9 meters). The ensuing additional control of inclination made it possible to remain in the upper third of the oil rim, far enough from the OWC to delay water breakthrough.
For drilling of the second branch, a sidetrack was performed in the 9 5/8-in. casing 253 meters above the shoe. A retrievable whipstock was employed to allow milling of the casing in order to cut a window. Also in this second branch, the well azimuth was turned through 60 degrees from 142 degrees in the first branch to 82 degrees in the second one.
Following retrieval of the whipstock, a guide was set to enable the liner to be run in the second leg. This guide was perforated to allow production from the first drain. seven-in. slotted liner was set in both the horizontal drains.
The main difficulty on BK-4-M was to set the 9 5/8-in, casing shoe at the correct depth. Since the 7-in. liner is slotted, the whole 8 1/2-in. open hole phase is produced. An error on the 9 5/8-in. shoe depth would have increased either the gas cut or water cut, thereby jeopardizing the well's economics.
Logging while drilling provided precise geological information on the formation being drilled and helped set the 9 5/8-in. casing shoe two meters TVD below the gas-oil contact. Correct orientation of the whipstock was important as it determined the initial orientation of the second branch.
The critical maneuver was to maintain the two horizontal branches of the well in the upper third of the oil rim. Tolerance was a mere two meters TVD between the required depth and the gas-oil contact. This necessitated extremely accurate piloting of the well.
In view of the tight constraints on branch position, drilling equipment was selected to maximize real time information. During the drilling operations, increasingly frequent surveys were implemented to ensure that the drains remained within the target window.
Milling of the window was performed in four steps:
1. Setting of the packer, approximately 250 meters above the 9 5/8-in. casing shoe
2. MWD run, to check the orientation of the packer and allow pre-orientation of the whipstock on surface
3. Setting of the retrievable whipstock. This sealed whipstock prevented the first drain from producing or being accidentally plugged by milling fragments
4. Milling of the window, which required two trips, owing to blinding of the mill with debris.
The second branch was then drilled using the Navigator instrumented motor. After reaching TD the whipstock was recovered and replaced by a permanent guide stock. This allowed setting of a 7-in. slotted liner in the second branch as well as production from the first branch.
BK-4-M's completion, which was effected within 18 days, comprises the following elements:
- 7-in. slotted liner set across each of the two horizontal branches
- 3 1/2-in. tubing from tubing hanger to top of second branch liner
- Single production packer in the 9 5/8-in. casing at a depth of 1,179 meters
- Tubing retrievable safety valve at a depth of 251 meters
- Xmas tree.
Both horizontal branches produce simultaneously through the 3 1/2-in. tubing. BK-4-M came onstream on July 28, 1996, at a cost of $2.1 million. The well produces through a stainless steel 3 1/8-in. 5,000 psi Xmas tree with Axelson actuators via a 4-in. flowline to the production manifold. Wellheads are Kvaerner National SMS 5,000 psi unitized types. The self-erected tender-assist drilling barge was the Smedvig Robray T-1, which can remain safely anchored in waves up to 13.7 meters and wind speeds up to 70 knots. Its top drive is a Varco TDS3, while the drawworks is a National 1320 UE with maximum line pull of 365 tons.
Other contractors on this exercise included Baker Hughes Inteq for directional drilling, MWD and LWD; Baker Oil Tools for whipstock, completion and liner products; Geoservices for wireline services; and Houston Well Screen for the slotted liners.
Copyright 1997 Oil & Gas Journal. All Rights Reserved.