A new technique that can minimize section milling has been developed for plugging wells and achieving better isolation. An innovative perforating gun design produces slots in the tubing, without damaging an outer string or the wellbore, allowing cement or resin squeeze operations to create reliable zonal isolation that meets government regulations.

Proper well abandonment mitigates the risk of fluid migration that could damage freshwater zones as well as flammable natural gas seeping to surface. Safely abandoning wells to prevent migration of fluids and sealing potential leak points by setting cement plugs in the wellbore requires the removal of concentric casing strings, or when cutting and pulling casing are not possible, it must be milled away. Milling casing often is challenging and requires multiple trips into the hole, which significantly raises the cost to plug and abandon wells. Tungsten carbide has been used for downhole metal cutting since the 1930s, and today lathe-style cutters and downhole motors are used to speed up operations. However, milling still takes too long, and metal shavings at the surface are an environmental and safety hazard.

The new gun design perforates a helical pattern of overlapping horizontal rectangular slots in the tubing, allowing 360-degree access to the area behind the tubing or casing. The perforations (see Figure 1 above) can be designed to penetrate a single string of pipe without damaging an outer string or to penetrate a casing string to access the formation, covering all voids and microannuli between the casing and formation or within the cement.

The slotted perforating gun system outperforms big-hole squeeze guns because the radial pattern and depth of penetration avoid having to reperforate a second or third time when the first attempt is unsuccessful. This single-gun system prepares the well for the cement operation in only 12 hours versus conventional 360-degree access methods, such as section milling and slot cutting with abrasives, which take days or weeks to execute.

Section milling, squeeze cementing alternative

In many plug and abandonment (P&A) wells, downhole tubulars are milled and cement is pumped downhole to establish a barrier against the migration of fluids. Section milling can be challenging in some types of tubulars, and the tools must be carefully designed to avoid failures inside the wellbore. After a section of the pipe is milled, achieving reliable isolation depends on several factors that impact the cementing operation: equivalent circulating density, flow dynamics, thermal gradients, pipe corrosion, and pressure and temperature fluctuations. In some downhole conditions, resins have proved effective in annular fluid flow applications to shut off gas sources and squeeze a leaking plug. The low yield point of resin allows it to flow into micron-sized leaks without acid cleanup.

An important resin plug application occurs when bubbles are observed coming from the annulus after casing is cut. If the bubble stream is thought to be channeling through cement, resins may be an ideal squeeze application to stop annular leaks. Section milling guarantees perfect access to the area behind the casing, but this method can be expensive and time-consuming. In wells in the People’s Republic of China, squeeze operations have been demonstrated to be more efficient if perforating establishes adequate communication before section milling is attempted.

Slot perforating improves efficiency

The slotted perforating gun system (Figure 2) is designed to intersect more channels than conventional gun systems with standard big-hole charges because its rectangular slots overlap between the charges. Standard big-hole charges may not intersect all microannuli and channels behind pipe. Compared to conventional round-shaped charges, the slot gun uses charges that are rectangular. The slotted shaped charge provides greater horizontal coverage in the casing than is possible with round charges. By overlapping slot charges by 50% in a complete circumferential-vertical cross section, 360-degree coverage is achieved (Figure 3).

The slot gun employs a scallop design to create a rectangular slot without significant burr height protrusion. Slots can be created in the liner or inner casing without damaging the outer casing or the shaped charges, and guns can be designed to penetrate the pipe to access the formation. The perforating guns can be conveyed by wireline, slickline, coiled tubing or conventional jointed tubing and initiated by a pressure-activated firing head, or when conveyed by wireline and initiated with a firing panel from the surface.

From the wells attempted to date, the designers of the gun system believe wells with isolation challenges are candidates for using the slotted gun approach as the first attempt to establish communication without section milling.

Case study

The slotted perforator was used in a North Sea well as part of a well abandonment program. The operator wanted to safely abandon an existing wellbore and reuse the slot for a sidetrack to access bypassed oil. The challenge was to perforate 5½-in. 17-lbm/ft tubing above the cement top with no damage to external 95⁄8-in. casing. The well was deviated about 45 degrees, and there was concern that the perforator would lie on the low side of the pipe and damage it when the gun detonated.

The solution was to load dummy charges in a section of the gun and use an eccentric weight bar to orient the dummy charges to the low side of the hole where there was no casing standoff. Using a memory trigger device conveyed via slickline the oriented gun was detonated, saving cost and space. The slickline-deployed slot-shaped charge successfully produced large, overlapping punch holes while avoiding the risky low side of the wellbore, thus avoiding damage to the external string.

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