Crossover Service Tool

Key Takeaways

• The alpha-wave gravel packing process relies on the crossover service tool's ability to direct slurry flow (gravel-laden carrier fluid) down the annulus between the work string and screen while simultaneously providing a return path for carrier fluid that has dropped its gravel and is returning from the formation face through the screen: the crossover tool in circulate position connects the work string bore to the annulus below the crossover (sending fluid downward to carry gravel into the casing-screen annular space) and connects the screen interior to the work string return bore (allowing carrier fluid that has passed through the screen to return up the work string to surface); as the gravel fills the annular space from the bottom upward (alpha-wave), the flow resistance increases and the carrier fluid is forced into the formation and back through the screen more aggressively; when the alpha-wave of gravel reaches the top of the perforated interval, the crossover tool is shifted to the beta-wave or reverse-circulation position to pack the remaining annular space from the top down; the crossover tool must be robust enough to handle the highly abrasive gravel-laden slurry flowing through its ports at the high velocities required to maintain turbulent flow for gravel transport, and its internal seals and flow diverters are engineered for the specific erosion environment of a sand control completion.
• The mechanical design of crossover service tools varies among service companies (Halliburton, Baker Hughes, SLB, and Weatherford all have proprietary crossover tool designs) but all share the core functional requirement of providing multiple reconfigurable flow paths through a single tool body: the Halliburton TRS (Treating and Reversing Service) tool uses an upward or downward set of packing elements that seal against the packer bore above the gravel pack packer, with an inner sleeve that rotates or slides to align ports in different positions; the Baker Hughes GPak tool uses a similar mechanical shifting mechanism; all designs must provide a positive mechanical indication at surface of which position the tool is in (typically through the weight or force required to shift the tool, observed as a change on the surface weight indicator), because operating in the wrong position during a gravel pack can result in gravel going to the wrong location, screen plugging, or the work string becoming stuck in the well.
• The crossover service tool in a frac-pack completion (a combined hydraulic fracturing and gravel packing operation) must handle both the high-pressure, high-rate conditions of the fracturing phase and the lower-pressure, slurry-circulation conditions of the packing phase: during the frac phase, the crossover tool directs high-pressure fracturing slurry (up to 15,000 psi and 40-60 barrels per minute) through its circulate ports into the formation via the gravel pack packer; after the fracture tip screen-out occurs and the fracture closes around the proppant, the tool is shifted to the pack position and the near-wellbore annular space is packed with gravel at lower rates and pressures; the tool must maintain its mechanical and seal integrity through the transition between these two very different operating conditions, and the position shifts that occur during this transition must be reliable under the wellbore conditions of temperature, pressure, and wellbore fluid that exist at the time of shifting; tool failures during the critical transition phase between fracturing and packing have been one of the primary sources of frac-pack completion failures in offshore deepwater operations.
• Intervention crossover tools (as distinct from completion service tools) are run in existing production wells to reconfigure the flow path during workover operations without requiring the permanent completion equipment to be pulled: a through-tubing crossover tool run on wireline or coiled tubing can establish communication between the tubing interior and the casing annulus below a packer, allowing remedial operations (scale treatment, annular pressure relief, chemical injection to a specific zone) to be performed without removing the production packer; these through-tubing crossover devices are typically much smaller in diameter than the permanent service tools used during initial completion (they must pass through the production tubing bore) and have correspondingly lower flow capacity, limiting them to relatively low-rate operations; the advantage of through-tubing crossover intervention is that it avoids the cost and risk of a workover rig operation to pull the production tubing and packer, and it allows certain remedial operations to be performed on live wells without killing the well with heavy fluid that could damage the formation.
• Quality control and pre-job function testing of crossover service tools is mandatory practice before running them in a critical completion operation because a tool failure during gravel packing or frac-packing can result in an incomplete or failed sand control completion that requires extensive (and expensive) remediation: the standard pre-job test includes function-testing all mechanical shifts (circulate, squeeze, reverse, and closed positions) with the tool submerged in fluid to verify that each position provides the expected flow path, that the seals hold pressure in each position, and that the shifting force is within the expected range; the tool is then disassembled, inspected, reassembled, and re-tested after any anomalous result; pressure-rated tool components (packing elements, sliding sleeves, port covers) are verified against their ratings for the anticipated wellbore pressure and temperature; any tool that cannot be positively confirmed in all positions during surface testing is rejected and replaced before the job — the cost of a surface test failure is trivial compared to the cost of a crossover tool failure at 10,000 feet underwater in a deepwater completion operation.