What options exist for toe initiation, and how can I increase my reliability?

Toe Initiation

After drilling a well, the operator runs casing into the well and cements it in place. Once the cement sets, the well is closed, creating a problem for the completion operation that requires pumping down wireline BHAs. Several options are available for creating a flow path at the toe of the well to enable the pump-down operations. The three main types are Wet Shoe, Toe Sub, and Toe Sleeves. 

Wet Shoe

During the cement job, a volume of water and a second wiper dart follow the cement wiper plug. After the cement wiper plug lands, an internal rupture disc bursts, allowing the water above to pump around the shoe of the completion. The second plug lands and locks into the first plug.

The toe is "wet," and well control at the toe is reliant on the float equipment and a rupture disk in the second dart.

Before the frac, a Toe Prep operation includes rupturing the second dart, setting a bridge plug at the toe, and pressure testing the casing. The frac job then starts after.

The system is simple; other than an extra dart, it has no moving parts. Also, the system can be more economical than other options.

The toe prep operation is the most challenging environment for a composite bridge plug. Pumping the plug through a well with cement debris to the furthest point in the well increases the risk of issues. Plus, the toe prep operation is an added cost that should factor into the cost of the Wet Shoe System.

Leaving the toe wet can affect well control and zonal isolation in the first few stages.

Toe Sub

A toe sub is a small tool installed on the casing at the toe of the well. The sub has a series of rupture discs designed to burst at a specific pressure. The operator can use these in a couple of ways. First, they can conduct a casing pressure test below the sub-burst pressure and then burst the discs when ready to start the completion. Or, the operator can burst the discs at a lower pressure and conduct a toe prep with a bridge plug, like with the wet shoe system. The challenge with toe subs is that if one rupture disc opens prematurely, it can cause a problem during cementation. Also, the small rupture disks do not provide much flow area when open; all discs must open to create a flow path acceptable for the pump-down operation.

Toe Sleeve

A toe sleeve will have a moving component (piston) that shifts to expose open ports to the formation, providing a large flow area, decreasing the pressures needed for the pump-down. The sleeve shifts when exposed to wellbore pressure, usually when a rupture disk bursts. Mainly, the operator will pressure test the casing below the opening pressure of the sleeve and then open the sleeve when ready to start completion operations. There are three main types of toe sleeves available.

Types of Toe Sleeves

Single Open

A single open toe sleeve will open after the rupture disk bursts. The piston has well pressures on one side and atmospheric pressure on the other. When exposed to the well pressures after a rupture disk bursts, the pressure difference will shift the sleeve into the atmospheric chamber, opening the sleeve.

Time Delayed

Time-delayed toe sleeves operate similarly to the single open. However, there is a fluid on the atmospheric chamber side of the piston. When activated, the sleeve has to force this fluid through a small port during the opening sequence. The fluid slows the opening of the sleeve and provides a time delay. The time delay allows the operator to activate the sleeve at a lower pressure and then test at a higher pressure during the timed opening sequence.

Cycling

Finally, a cycling toe sleeve provides multiple pressure cycles before opening the sleeve. Rather than relying on a specific time delay or moving fluid, the mechanical design of the sleeve enables multiple (non-timebound) pressure cycles. The added processes provide fail-safes in the event of unexpected pressure spikes during the cementation operation.

ZoneStarter Gen II

Reliability is the most critical feature regardless of the technology chosen for toe initiation. The most reliable tool with the most features should win the work. Over the 5+ years, the WellBoss ZoneStarter Gen II has proven to be that tool. The two pressure cycles have saved operators multiple interventions after a pressure cycle during cementation, which caused it to perform the first cycle of the tool but not open the completion until necessary. The full-bore toe sleeve also eliminates wiper dart concerns and will work with cementation equipment. With over 2,500 runs, the tool has proven to operate with over 95% reliability.