Often during the drilling and completion of oil and/or gas wells, it is necessary to install and/or remove a variety of downhole tools into or out of the well. Examples of these tools include isolation packers, bridge plugs, wellhead plugs, cement retainers and sump packers, and other similar devices.

Typically, these tools are set and retrieved using various methods such as a wireline, slickline or coiled tubing (CT).  These methods offer distinct advantages and disadvantages, therefore operators are constantly challenged with the dilemma of which method will best maintain a balance between efficient operations and operational costs. 

A conventional wireline with explosive setting tools has been the traditional method of choice for operators because it is less costly and faster than other methods such as CT or workstrings.  Also, its portability enables it to be cost-efficient when performing these services in remote locations and on offshore platforms.

However, conventional wireline services can be inadequate in certain instances because of their limited pulling force due to the finite strength limits of the wire. Also, personnel safety becomes an issue when this method is used because explosives must be transported and handled by specially trained personnel during operations.

DPU® Downhole Power Unit
Several years ago, Halliburton introduced the DPU® Downhole Power Unit (Fig. 1), an electro-mechanical tool designed primarily for setting and retrieving wellbore sealing devices.  Since then, this tool has evolved into an instrument capable of performing numerous downhole tasks in addition to its primary design function.  Over the years these tools have set thousands of plugs, set sump packers at depths up to 27,000 feet, punched holes in casing and pulled subsea wellhead plugs.

Today, the DPU offers a viable, field-proven option for operators who are constantly faced with choosing between methodologies that are either cost-effective but intrinsically unsafe, or are safe, but not cost-effective.

The DPU setting tool can be powered by batteries for slickline operations or powered by a surface DC power supply for electric line operations.  It can be run on slickline, braided line, electric wireline or even coiled tubing if desired and, unlike a conventional wireline, the DPU can apply a controlled linear force of up to 60,000 pounds at the end of the wire.  Using the DPU with a slickline can reduce costs for setting and retrieving packers and bridge plugs by 15-50% when compared to the use of wireline with explosive setting tools for the same purpose.

Figure 1 - Downhole Power Unit

The DPU has several characteristics that enhance the setting/pulling process and safety of its operation. Among these characteristics are:

• Non-explosive activation.  Since the DPU is electro-mechanical, no special transportation issues are required and no radio silence is required at the well site during operations.  Also, it does not require specially trained explosives personnel.
  
• The setting speed of the DPU is approximately .5 in/min.  This slow positive setting speed enhances the setting process by allowing the slips on wellbore sealing devices to slowly latch into the tubing during setting.  The sealing element on the wellbore sealing device also gradually expands during the setting process allowing a positive full 360º form fit to the inside diameter of the tubing/casing.
  
• Offers proven, dependable operation.  The system has been used worldwide for nearly ten years and has set thousands of wellbore sealing devices.
  
• The setting force is enhanced with the hydrostatic pressure thus allowing operation in HP/HT wells.  Common explosive-type setting tools have a maximum hydrostatic pressure rating of 15,000 psi and the explosive setting tool’s output setting force is decreased when the hydrostatic pressure rating is exceeded.
  
• Maintains full power throughout the entire stroke.  Explosive setting tools have a peak setting force that occurs at a particular location during the setting process.  The DPU can produce the maximum rated setting force throughout its entire stroke.
  
• The DPU’s power can be applied while the tool strokes inward or can be configured to apply power while the tool strokes outward as well.
  

DPU Design and Operation
Functionally, the DPU consists of three sections. The top section encloses a pressure sensing actuator, the middle portion is the power source, and the lower end is the linear drive section.  Also, the design allows for two separate DPU models--the DPU25 and the DPU27 which features new Hybrid Circuitry. The models differ in their timer setting and operational logic.

When necessary, the units can operate independently of a surface energy source. In the independent mode, it is powered by either Alkaline or Lithium batteries.  When using C-cell Alkaline batteries, the DPU has a maximum operating temperature of 125°C (250°F). However, by using Lithium Battery Sticks, it can increase its maximum operating temperature to 165°C (329°F).

In operation, the DPU and the subsurface device attached to it are run into the well on a slickline or braided line to the desired setting depth where a surface selected logic initiates its operation. The setting motion is very gradual and controlled (approximately .5 in/min), allowing the sealing element to conform against the casing/tubing wall and the slips to fully engage. When the sealing element is sufficiently compressed and the setting force is reached, the DPU detaches itself by shearing loose from the subsurface device.  After the setting process is complete, the DPU can be removed from the well.

Case Histories

Case History No. 1

Customer Challenge:  
This Gulf of Mexico operator wanted the subsea wellhead SSP Crown Plugs pulled from a GOM Green Canyon well so that the well could be serviced.  SSP wellhead plugs have been used for several years in horizontal subsea trees in the Gulf of Mexico; however, the pulling force needed on subsea plugs for workovers has proven to be a challenge for conventional wireline equipment. 

Conventional slickline services have limited pulling forces due to the finite strength limits of the wire.  On this well, deepwater and debris above the wellhead plugs compound the required pulling forces of the subsea plugs because the hydrostatic pressures created extra retrieval forces across the subsea wellhead.  Prior experience has shown in these cases that the fishneck can be freed using conventional wireline tools, but the plug cannot be pulled from the profile due to the overbalance forces. 

Halliburton Solution:
At the request of the operator, Halliburton developed an alternative solution that employed a DPU to overcome the downward hydrostatic pressure overbalance force and enable the SSP Crown Plugs to be pulled. The DPU was chosen because of its ability to create a greater pulling force than can be exerted by conventional slickline pulling tools.

In this well, the upper plug was located 5,339 feet below the rig floor.  Due to the fact that there the pressure below the lower plug was 4,820 psi, an 11.6 lb zinc bromide fluid was chosen to be used.  Communication to the surface existed through a vent line from the middle zone between the two plugs.  Therefore, with 11.6 lb/gal fluid at 5,339 feet, the hydrostatic pressure was calculated to be 3,220 psi. The vent (upper plug seal test) line was filled with a combination of oil and seawater requiring an additional 850 psi to pressure-balance it with the upper plug.

With 4,820 psi below the lower plug, approximately 1,800 psi was applied for a slight (200 psi) overbalance to keep the lower plug from moving uphole once its seal was released.  Then the operation to pull the plugs commenced.

Result:
Both plugs were successfully removed with less effort than would have been required if coiled tubing has been used.

Customer Added Value:
The entire job was completed in less than a day thereby producing a significant cost saving for the customer as compared to the expense of mobilizing a coiled tubing unit to do the job.  Also, utilizing the DPU to pull the crown plugs also saved time and money because a slickline can install a lubricator much more quickly than coiled tubing (including the lift frame) can be rigged up.

Case History No. 2

Customer Challenge:
This Gulf of Mexico operator needed a permanent sump packer installed at a depth of 27,006 ft in a deepwater well with a bottomhole pressure of 19,700 psi.

Halliburton Solution:
After rigging up, a Halliburton crew utilized a DPU® Downhole Power Unit in order to deploy the 9-7/8-in. permanent sump packer to a measured depth of 27,006 ft during the completion phase of the well.

Result:
The permanent sump packer was successfully installed at the required depth and in the process, the crew established a new deepwater world depth record for the electro-mechanical deployment of a sump packer. 

Customer Added Value:
Use of the DPU to install the packer saved the operator approximately $600,000 in operating costs on the well by eliminating a pipe trip and the trip’s associated maintenance time.  In addition, the deepwater development project called for sump packers to be set in four other wells in the field.  Upon completion of these wells, the operator realized a total of $3.0 million in operating costs savings on the five wells.

Case History No. 3

Customer Challenge: 
This operator of a Middle East well requested that a 4-1/2-in. monolock be installed at a depth in excess of 6,300 ft and then tested to 2,000 psi.  Afterwards, a hole was required to be opened in the K-55 tubing at a depth of 6, 290 ft. 

Halliburton Solution:
Halliburton rigged up a wireline and ran a 3.813-in. gauge cutter to 6,302 ft.  Then, the
4-1/2-in. monolock was run to 6,295 ft and tested to 2,000 psi. Test results were good.
Next, a slickline-deployed DPU was run to 6,290 ft where it punched a hole in the 4-1/2-in., 12.6 lb, K-55 tubing.

Result:
The slickline-deployed electromechanical tubing punch successfully performed the desired operation for the client.

Summary
Today’s industry requirements to install and/or remove a variety of downhole tools and equipment in and out of the well have created new challenges for the industry.  For example, pulling a subsea wellhead plug can require an extreme pulling force. The difficulties experienced can be compounded by conditions that occur when the plug has been in place for extended periods of time as well as by the more demanding deepwater conditions, such as high overbalanced downhole forces and settlement on top of the plug, that contribute to the need for the higher pulling forces.  These conditions often exceed the capabilities of conventional slickline services.

A newly developed option for operators that has met other challenges in difficult scenarios is the Downhole Power Unit--an electro-mechanical device that produces a high linear pulling force directly at the wellhead plug without jarring.  This tool does not require explosives or hydrostatic setting tools as it is operated by batteries.   Since first being introduced, the DPU has proven to be efficient and cost-effective in the field in applications involving the setting/retrieving of isolation packers, bridge plugs, wellhead plugs, cement retainers and sump packers and other similar devices.