The CT annular fracturing method enables pinpoint communication between the production casing and the formation immediately followed by the frack operation. The frack operation is monitored via the annulus pressure and downhole memory tools at the point at which the perforation actually breaches the casing and where the hydraulic fracture takes place. In addition, the CT technique delivers a pristine wellbore; there is no remedial cleanout work required and no milling of plugs.

While CT annular fracturing has been used in Canada in relatively low-frack stage counts (e.g., 40 stages), the U.S. segment of the industry is pursuing this method for use in higher volume operations (e.g., 100 stages). Operators are finding that an important benefit to CT annular fracturing is that it provides the knowledge of frack and proppant-packing occurrence while having a contingency downhole flowback method in the event of screenout.

Specialized equipment such as a frack tree with a frack head containing an isolation sleeve is needed to protect the CT. While flowback equipment (choke manifold and flow iron) is necessary for standard frack operations, it also is required to maintain a hydrostatic head on top of the CT tool packing elements during the frack operation.

Benefits

Operators find the CT annular fracturing technique attractive for a multitude of reasons, including the ability to

• Push and pull during tool conveyance;
• Locate and frack precisely within the formation;
• Pinpoint proppant where it is most effective;
• Monitor real-time frack zone pressure to obtain the information needed to control fracture growth and avoid screenouts;
• Make a quick recovery from screenouts. The ability to circulate the frack zone enables sand removal; and
• Reduce the amount of water needed to fracture the well; and
• There also is no need to drill out plugs or remove ball seats.

Overall, CT annular fracturing offers a means to obtain a precise understanding of the frack operation downhole and knowledge of how the formation is taking the treatment.

Use of CT for well stimulation

Historically, the use of CT in fracturing became more prevalent with the inception of hydrajet fracturing technology. Before that, CT fracturing was limited to placement of small fractures in the formation and in some acidizing applications to bypass near-wellbore damage. Use of CT in fracturing stimulation is used to place large fractures by which the annulus now delivers primary fracturing fluid.

CT technology has evolved to produce fractures of more than 183-m (600-ft) half-lengths. In most formations, such fracture sizes would not be possible with fluid delivery originating from the CT alone, so the use of the annulus to deliver the primary fracturing fluid is a step change.

CT can be considered for many applications such as pipe cleaning, perforating, tool placement, logging, fishing, chemical placement, matrix treatment and drilling, among others. It is considered for so many applications because it can be rapidly deployed without the need for a drilling rig, it has the ability to fit through small restrictions like production tubing and it provides a seamless fluid conduit that has the rigidity for tool conveyance.  

Inception of managed pressure fracturing

In response to the CT annular fracturing trend, whether fracturing a new well or refracking an existing well, Cameron has developed frack trees for CT applications, isolation tools to protect the CT from abrasive and corrosive frack fluids and equipment used to manage well-bore annular pressure while fracturing.

Having considered all elements of the operation and possible efficiencies, Cameron has developed the CT Annular Frac Tree featuring the FracGUARD Coiled Tubing Isolation Frac Head. This system supports the requirement to upsize the tree and components to accommodate larger fracture and proppant fluid volumes in a compact package. Also, the company offers pipe, manifold and personnel for managed pressure fracturing and wellbore flowback. 

In CT operations there is additional pressure control equipment such as BOPs and lubricators along with the CT injector head that must be suspended above the well-head. Reduced height of the overall stackup improves the safety aspect of the operation. The company's vast selection of frack tree sizes allows operators to choose a solution so that there is enough annular space between the CT outer diameter (OD) and the inside diameter
(ID) of the frack tree.

FracGUARD is engineered to provide a flow area between the ID of the frack head and OD of the CT isolation sleeve. This enables high-volume flow rates ideal for "popping the toe." Because it is critical to protect the CT during the frack itself, FracGUARD functions as a barrier between the fluid entry ports of the frack head and OD of the CT. Such an isolation system allows the fracturing components to withstand today's high-volume fracture jobs while countering corrosive and erosive effects.      

It has been documented that 40% of all fractures are not producing hydrocarbons. The use of CT annular fracturing, coupled with innovative systems, represents a step forward for operators choosing to use this technique to fracture or refrack their wells.