Previously inaccessible high pressure, high temperature zones can be isolated and successfully treated using coiled tubing-deployed inflatable packers.

For years inflatable packers have been used to isolate individual zones for treatment. Unfortunately, conventional inflates have been limited in their ability to operate under extremes of temperature and pressure and in harsh chemical environments. Now, underperforming zones can be successfully treated using coiled tubing (CT) deployed large inflation-ratio packers, even in hot corrosive environments.

The CoilFLATE HP/HT from Schlumberger is a coiled tubing conveyed intervention service that includes a bottom hole assembly that can be either permanently set or that can be retrieved in the same run. The tool has several features that improve its operational reliability and effectiveness:

• A Circulate/Inflate Orifice Tool (CIOT) inflation control valve makes circulation of the well possible while the packer is being run into the hole.
• Inflation and deflate mechanisms that are set precisely and reliably by shear screws and burst discs.
• An inflatable packer element (carcass) that is sheathed in stainless steel slats to prevent wear during run-in.
• A Carcass Restraint System (CRS) that ensures uniform inflation using a crush sleeve that adds tension to the slats while inflating, reducing the possibility of bladder deformation and potential bursting.
• A chemically-resistant sealing compound that provides pressure isolation from above or below the packer. This chemical resistance makes the packer ideal for selective placement of treating fluids required for acid stimulation, water control and chemical treatment.

The service provides a reliable high-pressure seal at large inflation ratios and can be run in 27/8-in. tubing. The packer can expand to more than three times its 21/8-in. run-in diameter to seal in 75/8-in. casing. It can operate at bottom hole temperatures as high as 375?F (190?C).

Additional benefits of the system include economical production optimization in harsh environment wells and a significant safety margin over conventional systems at similar injection pressures. In one application, acid stimulation using the tool resulted in more than 300% increase in production from the targeted zone.

While the system can be run on either coiled tubing or jointed pipe, coiled tubing provides several advantages, including quicker and easier control of the fluids, and fewer people required on site. Additionally, use of coiled tubing saves money by eliminating the need for a workover rig.
Another advantage of using coiled tubing over jointed pipe in low bottom hole pressure wells is that nitrogen can be pumped to unload the well and begin production while pulling out of the hole. Coiled tubing can go into live wells and then quickly go to production from the treated zone.

Measuring depth accurately

Correlating the bottom hole assembly position to well logs prevents errors in measuring tubing length and deformation and in cases of incomplete or inaccurate well references.

The packer system can be run with the DepthLOG CT depth correlation service that determines accurate depth with a traditional casing collar locator (CCL), but with flow-through capabilities. The tool uses fluid pulse telemetry to transmit the CCL data to surface, eliminating the need for an electric line in the coiled tubing. DepthLOG overcomes limitations of other downhole depth-correlation methods by providing a supporting tool to correlate depths in real-time replacing standard practices such as tagging bottom and setting down; use of nipple and tubing end locators; memory logging tools; or electric line.

Accurate packer inflation

The CoilFLATE packer is inflated with the internal CT fluid. For example, if water was pumped through the coiled tubing to achieve the DepthLOG signal, then water would be used to inflate the packer. The packer can also be inflated with brine, diesel or methanol as well. The CRS ensures that uniform inflation of the bladder is achieved to maximize anchoring and sealing to the tubing/casing wall.

Once the packer is fully inflated, the fluid inside the packer is completely trapped. A poppet valve is opened with about 250 psi of pressure, and the fluid flows around the valve to inflate the packer. When the packer is fully inflated, the spring in the poppet valve closes the valve to isolate the fluid inside.
The packer can be configured as a bridge plug or cement retainer, then disconnected from the bottom hole assembly and left downhole to permanently abandon a lower zone. The packer can also be retrieved at a later date after providing temporary isolation of wellbore areas, such as for tubing integrity testing, wellhead testing and general pressure testing applications in addition to zonal isolation for acidizing or cement squeeze jobs. To deflate and retrieve the packer to surface, tension is applied on the coiled tubing to shear a predefined number of shear pins.

Software aids accuracy

Final job design is aided with InflateAdvisor, a computer-based optimization program that is a part of the Schlumberger CoilCADE coiled tubing design, execution and evaluation software. It is used to predict downhole hydraulic conditions to ensure precise inflation pressure and maximize packer performance. The program provides calculated optimum outputs regarding the shear screws and burst disc ratings from various inputs of the coiled tubing string and completion data as well as known pressures, temperatures and fluid information.

Case Study

In Algeria, a well required isolation between two separately perforated zones in order to stimulate the lower interval, which was believed to be underperforming. Only a 10-ft (3.3-m) window was available in the nearly 10,000 ft (3,280 m) well to set a packer and successfully isolate the high permeability top interval, from the low permeability bottom zone. To minimize costs, the operation had to be conducted without a workover rig.

During a previous attempt, the packer was set without the use of the DepthLOG correlation device. As a result, the window was missed and the packer was set across the lower perforation. This resulted in the treatment being misplaced. Even with the misplaced packer, the CoilFLATE tool held pressure without failure. It was decided to re-stimulate with mud acid through the coiled tubing and below the packer when the two zones were completely isolated. To ensure that critical depth control requirements were met, DepthLOG and CoilFLATE were run together. The tool's flow-through feature allows enough circulation to support the CCL pulse telemetry but prevents premature inflation of the packer.

The bottom hole assembly was run into the hole below the setting depth. Two upward passes were made while continuing to pump fluid in order to receive pressure pulses from the DepthLOG tool to clearly indicate where the casing collars were located. Once the depth correlation signal confirmed the targeted setting depth, the second challenge was to inflate the packer to hold the high stimulation pressure. The packer was inflated to an internal pressure of 4,000 psi inside the 41/2-in. OD liner. Set down weight applied to the coiled tubing verified complete packer inflation. Mud acid was pumped through the coiled tubing creating a maximum differential pressure of about 3,500 psi across the inflated packer, a significantly higher pressure than many currently available packers are capable of handling.

Between 100 and 120 bbl of mud acid treatment were successfully pumped at maximum differential pressure across the packer. Upon completion of the treatment, the packer was deflated and nitrogen was pumped to immediately flow back the acid while pulling the tool string out of the hole.

Following the successful treatment, production rose from 238 b/d (37.9 m3/d) to 776 b/d (123.4 m3/d), a 325% increase in production. Only one trip into the well was required and it was not necessary to pull the production tubing.