Modifications to the CT-conveyed Sand-Vac tool enabled its use to even through compacted fill in under-pressured North Slope wells.

Operators are increasingly developing new fields and redeveloping mature fields by drilling multilateral wells to extend reservoir access and enhance the effective drainage from minimal surface facilities. Stimulating or cleaning out these multilateral wells can be challenging without technology designed to navigate and properly enter the various branches.

Navigating multilateral junctions

For several years, BJ has accomplished these operations using the CT-conveyed Lateral Entry Guidance System (LEGS) tool, an intelligent system for navigating the junctions of a multilateral well. At the end of 2005, the tool was sent to Kazakhstan to aid in performance of acid treatments in two wells that were deeper, at more than 16,405 ft (5,000 m), than any prior well treated using the tool.

Due to the common depth uncertainties associated with running CT and uncertainties in the accuracy of the well diagrams provided, locating and running into the lateral can be challenging, even with ideal operation of the intelligent tool.

The tool is run into the hole on CT to the expected depth of a junction between the main well bore and a lateral. Applying pressure through the CT string activates the LEGS kick-off assembly, pivoting the wand to about 15°. A sweep assembly then rotates the wand around the side of the well bore. Rotation continues until the bottomhole assembly (BHA) senses the entrance in to the lateral. The wand then pivots further into the entrance of the lateral, opening a valve that allows a controlled flow through the tool, sending a pressure and flow signal to the surface indicating that the lateral has been found. If the tool rotates without finding a lateral, the tool is reset and the BHA is pulled up hole to continue the search.

Wand length is critical to success: If it’s too short, it will not extend far enough into the lateral; if it's too long, it will miss the narrow window of the lateral. Modeling software is used to determine the ideal wand length and increment based on the well diagram and junction geometry.

In one of the initial Kazakhstan wells, it was thought that the column of typical “search” fluid (brine or water) in the CT might create enough differential pressure (compared with the bottomhole pressure) to exceed the LEGS tool's valve opening pressure. This would unintentionally switch the tool to flow mode and indicate the location of a lateral when none had actually been found. Options included use of a pressure-reducing valve above the LEGS tool; however, there is a potential for wellbore interference. For the successful job in Kazakhstan, a less dense fluid (diesel) was used to reduce the hydrostatic pressure in the CT.

The fluid choice required a change in o-rings, however, as they swelled in the combination of diesel and wellbore fluids at the bottomhole temperature and pressures.

In addition, to improve depth accuracy and lateral searching time, depth correlation runs with a collar locator and logging assembly were completed, and a dual-wheel, dual-encoder depth counter was used.

In the first successful Kazakhstan implementation of the LEGS tool, an acid stimulation was performed in two laterals of a well in the Karachaganak field in a single intervention, without the need to return to surface for deflection devices. The operation saved several days compared with a typical completion and resulted in the best production from any well in the field to date (Pumnea et al., SPE 107124).

Depleted and low-pressure wells

The BJ Sand-Vac tool removes proppant and formation fines from new or mature, depleted wells with very low bottomhole pressure. Based on a jet pump, the tool comprises a high-pressure power nozzle, a suction port and a diffuser. To run the jet pump, power fluids are pumped down the center of a concentric coiled tubing (CCT) string, with some portion of the fluid exiting external swirl nozzles to fluidize the wellbore solids. The high-pressure power nozzle creates a pressure drop with in the tool that draws in wellbore fluids, jetting fluids and the suspended solids via the suction port. These returns flow through a diffuser that reduces the flow velocity, increasing pressure enough to drive the return fluids back to surface through the annulus between the inner and outer strings. This arrangement does not expose the well bore to the return pressure.

The tool has been used in low-pressure wells worldwide for some 15 years, but recent operations suggested some opportunities to improve cleanout efficiency. For example, hard fill and stubborn sand bridges presented challenges for the original tool during some cleanouts in Alaska in 2006 (Rafferty et al., SPE 107010).

In July 2007, a redesigned tool (modified to improve its ability to remove compacted fill) and a record-length (14,529 ft or 4,431 m) reel of 2 by 1-in. CCT were moved to the Kuparuk River Basin of the North Slope, Alaska, to remove proppant flowback and formation fines in a number of low-pressured oil wells. Prior attempts at conventional CT cleanouts had been unsuccessful in these wells because of the low bottomhole pressure, which resulted in unacceptable lost returns even with nitrogen assistance.

Of 21 well bores cleaned out during the summer 2007 series, 16 required the use of the new high-pressure, switchable jetting assembly to remove sand bridges or compacted fines. The jetting switch is activated from the surface, and the tool can be switched from vacuum to jetting mode as often as necessary without pulling out of the hole. Without this advancement, multiple CT runs would have been required to penetrate the solids in many of these wells.

For example, in one well, the top of fill was measured by slickline at 7,583 ft (2,312 m). Upon running the Sand-Vac tool into the well, the first sand bridge was encountered at 7,598 ft (2,317 m). The crew was able to quickly switch the tool to forward-jetting mode to break up the bridge, and switch back to vacuum mode to continue the cleanout. The well was cleaned out to 7,691 ft (2,345 m), recovering about 4,200 lbm of proppant.

Horizontal cleanouts

Cleaning solids from large-diameter, deviated and horizontal well bores can be extremely challenging because of the equilibrium solids beds that form behind the BHA while tripping in with conventional CT cleanout tools.

The company’s Tornado cleanout process uses a tool with a powerful forward jetting nozzle and is switchable to backward-facing jets. When running in the hole, the high-energy forward-facing jets break up and fluidize any compacted fill. When pulling out of the hole, the tool is cycled from surface so that fluid flows through low-energy backward-facing jets that disturb the equilibrium solids beds, re-entraining the solids in the fluid and resulting in a much more efficient and complete cleanout than was possible using older technologies.

Tripping speed and rates are critical to achieving a successful cleanout operation. The company uses proprietary software with a unique solids transport module to optimize such job designs. Staged treatments in horizontal 7-in. well bores up to 2,296 ft (700 m) long have been safely and efficiently performed.

To further advance the tool's effectiveness in the Norwegian sector of the North Sea, it has recently been combined with a new positive displacement motor and mill bottomhole assembly to remove harder solids deposition. This allows the cleanout tool to penetrate even very hard deposition (such as scales) to reach the desired penetration depth (Sach and Li, SPE 106857).