COILED TUBING LOWERS COMPLETION COSTS FOR COALBED METHANE WELLS

Alan Coats
Otis Engineering Corp.
Sandersville, Miss.

Ken Johnson
Otis Engineering Corp.
Dallas

In the coalbed methane wells of Alabama, reeled-tubing technology is proving beneficial for retrieving isolating bridge plugs and cleaning the well bore of frac sand, frac balls, perforating debris, and cement residue.

Five-hundred wells have been cleaned of unwanted cement residue using high-pressure, submerged fluid jets. Also, in excess of two thousand wire line-set retrievable bridge plugs have been retrieved in the Black Warrior basin using reeled tubing.

The large number of wells planned for the Black Warrior basin's coalbed methane industry requires an economical means of preparing wells for production. Completion design dictates that all wells have to be cemented, the well bore scraped clean, and multiple zones selectively stimulated.

Reeled tubing with submerged fluid jets allows an operator to effectively:

• Remove cement residue from the casing wall

• Clean frac sand and frac balls remaining from the stimulation process

• Retrieve bridge plugs that have been used to isolate the various coal seams.

Economics is the most important benefit of using reeled tubing. With reeled tubing, operators can produce coalbed methane wells much earlier than previously possible. Workover rig utilization can be optimized by programming wells ready for installing production tubulars, or to wells that might require maintenance.

WELL CLEANOUT

Submerged fluid jet cleanout of cement/residue has become an everyday operation in the Black Warrior basin. The cleaning procedure calls for a pre-frac cleanout of the well and offers the following benefits:

• The cleanout effectively removes contaminants such as cement, pipe dope, scale, and mill varnish that might otherwise reduce the well's productivity (i.e., entering the fractures with the frac sand).

• Following the pre-frac cleanout, contaminated fluids are easily replaced by clean, filtered completion fluids.

• The gauge ring, incorporated as part of the downhole cleaning assembly, gauges the hole to ensure proper well bore ID.

• The removal of cement and other harmful residues prepares the way for downhole logging tools and bridge plugs.

The jet cleaning assembly is shown in Fig. 1.

DESIGN

A computer program is used to calculate the jet size, number of jets, pump rate, vertical pipe travel rate, job time, total fluid, and mixed polymer (friction reducer) required to perform the jet cleanout operation.

The following information is required to run the job calculations: 1

• Available pump pressure

• Well depth

• Fluid density

• Target ID

• Cleaning energy for material

• Indexing tool OD

With the results of the computer program, costs are estimated and jobs prepared for each well. The computer input and output data for a typical Black Warrior basin cement/residue cleanout job are shown in Table 1.

On the average, two to three wells have been cleaned per day utilizing this technology. Cost savings, on a per well basis, are listed in Table 2.

JOB OUTLINE

The typical steps in a cleanout job are:

1. Secure accurate well information, 2 3 such as casing size and weight, wellhead connector size at surface, plug-back tubing depth, cement used.

2. Run the computer program before the job (planning). Use the program during the job (comparison/control).

3. Check all filters and downhole assemblies. Clean them as required.

4. Rig up the pump unit to the reeled-tubing unit with pipe sized for the job. Use filtered water/polymer mixture. Pump through an in-line surface filter (10-gauge screen) and reeled tubing to flush any debris from the pipe. Over flush until the return fluid is clean.

5. Rig up the reeled-tubing unit with an indexing tool assembly consisting of a gauge ring, in-line 20-gauge filter screen, indexing tool, and the orificed jet nozzles. The reeled-tubing unit is rigged up to a circulating "T" which allows returns to pass to a holding tank. There the return fluid is filtered. The fluid is then returned to the pumping unit for recirculation through the reeled tubing as the cleaning fluid.

6. Pump polymer fluid at rates and pressures as prescribed by the computer program. Run the tubing into the hole at the speed dictated by the program. Check fluid returns and cuttings throughout the job to ensure proper results. Once the orificed nozzle has reached TD, continue circulating (approximately 20 bbl). Retrieve tubing at the prescribed speed, checking returns for "bottoms up."

   Note: The fluid on top of the wiper plug at total depth usually contains several barrels of badly contaminated fluid that needs to be diverted to a pit (if available) to prevent plugging the freshwater filters.

7. Rig down all equipment.

   BRIDGE PLUG RETRIEVAL

   To minimize downtime in reeled-tubing bridge plug retrieval, tools have been developed for the conventional oil and gas industry that enable the operator to circulate (i.e., wash) while running into or out of the well bore.

   Because of the work involved, these components make up what is commonly referred to as a heavy-duty workstring. 4 These tools include a new generation of pump-through hydraulic jars, accelerators, knuckle joints, emergency release subs, and tubing connectors.

   The bridge-plug retrieving tool (either overshot or hydraulic type) is made up of 1-1/4 in. reeled tubing and an hydraulically activated emergency release sub. The emergency release sub is recommended to facilitate retrieving the reeled tubing in case the bridge plug does not release. The mechanical arrangement of the heavy-duty workstring equipped with an overshot-type bridge plug retrieving tool is shown in Figs. 2 and 3.

   Two methods of washing over and retrieving bridge plugs with reeled tubing have been developed. The first method uses overshots and the heavy-duty workstring described previously. The second method employs the same heavy-duty workstring and an hydraulically activated retrieving tool.

   Both methods allow the operator to maintain circulation while washing over the plugs and coming out of the well bore.

   With the tools made up to the reeled tubing, the workstring is run into the well bore and the cleanout/ retrieval process completed.

   SAND/GUN DEBRIS

   It is critical that the top of the bridge plugs be as clean as possible before attempting retrieval. Experience has shown that it is necessary to remove well debris using gelled carrier fluids and foaming agents.

   Typical operations are performed in 5-1/2 in. casing with 1-1/4 in. or 1-1/2 in. reeled tubing at depths ranging from 2,000 to 5,000 ft.

   A typical procedure is:

   Mix 15 bbl of gel with filtered freshwater. The viscosity should be between 80 and 100 cp. The viscosity has been determined using both Fig. 4 and actual field experience.

   Average frac sand size, used in the Black Warrior basin, is 12/20 mesh, with average settling velocities ranging from 0.05 to 0.06 fps. 5 Frac fluids are generally freshwater.

   To effectively lift frac sand and other debris from the well bore, the annular lifting velocity, VEL, is directly related to the injection pump rate, as shown in the formula below.

   VEL (fpm) = 24.51 x gpm/Hole ID2 - Pipe OD2

   For Black Warrior basin wells, lifting velocities normally are 10-15 fpm. When comparing this to settling velocities of 3-4 fpm, it can be concluded that the wells may be cleaned of debris using the following method.

   Run into hole with reeled tubing while pumping 7 bbl of gelled fluid to fill the reeled tubing. Once the tubing is filled, shut down the pumps and continue to trip pipe into the hole until tagging the top of the sand/gun debris and record this level. If there is more than 150 ft of debris above the bridge plug, mix an additional 15 bbl of gelled fluid.

   Pump gelled fluid at 1-1/4 bbl/min and wash down to 25 ft above the bridge plug. At this point, switch over to a foaming agent, pumping at 0.25 bbl/min with 1,000 scfm nitrogen.

   Wash down to within 5 ft of the plug before the gel leaves the reeled tubing. The gelled fluid will have a velocity (exiting the reeled tubing) of approximately 2.5 fps.

   This will allow the operator to wash through any perforating-gun debris. Circulate the gun debris and sand out of the well. Reciprocate the reeled tubing (approximately 3 ft) to minimize the possibility of flow cutting the tubing at the surface.

   Once gun debris has reached the surface, wash down to 1 ft above the plug. Circulate the well clean while manipulating the tubing.

   RETRIEVAL PROCESS

   With the well bore clean of all frac sand and perforating-gun debris, the reeled-tubing unit operator can proceed with engaging the bridge plug. Here are the three types of bridge plugs currently available:

   1. Nonequalizing/shear up to release

   2. Shear down to equalize and/or shear up to release

   3. Shear up to equalize and release.

   Experience has shown that the most reliable tool is one that must be equalized and released by upward mechanical jarring action.

   Note that shear down equalization induces damaging compressive loads across the reeled tubing, while nonequalization allows for the possibility of reeled tubing corkscrew damage caused by the bridge plug being blown up the hole.

   On reaching the bridge plug, the operator continues circulation, while slowly setting down 1,000-2,000 lb and engaging the plug. To confirm engagement, the operator will slowly pick up 1,000-2,000 lb over tubing weight. The plugs have normally been pinned for shear release between 4,000 and 6,000 lb.

   With an overshot and the heavy-duty workstring, normal jar action will usually release the plug. Experience has shown that a typical job will require 1-5 jar strokes for release to occur.

   Using the hydraulically activated retrieving tool illustrated in Fig. 5, a 7/16-in. ball is introduced and pumped through the reeled tubing to a pressure seat. Pressure is then applied and release is initiated. Once the plug has released, the ball is then expended to a secondary seat and circulation is re-established.

   On the average, two to three plugs have been retrieved per day using this technology.

   ECONOMICS

   Using reeled-tubing units to retrieve bridge plugs has proven to be much faster than the conventional workover rig. The ability of the reeled-tubing unit to handle pipe at speeds of over 100 fpm translates into substantial cost savings over a 10-12 hr work day.

   On average, a reeled-tubing unit can rig up, wash sand to TD, and retrieve three bridge plugs in 1 day. Performing a like procedure with a workover rig has taken 2-7 days.

   A major consideration when deciding between a reeled-tubing unit and a workover rig is the amount of well control afforded by the reeled-tubing unit.

   With trapped pressure below the bridge plug, retrieval by the rig can be hampered due to the open tubing string.

   However, with reeled tubing, the well is under control at all times, facilitating bridge plug retrieval even under pressure. With this control comes the ability to minimize fluid losses into the formation by aerating the pumped fluid medium with nitrogen.

   Aeration accomplishes two things:

   1. Reduces the weight of the fluid column and accompanying fluid loss.

   2. Increases the washing efficiency of the pumped fluid medium by increasing the vertical velocity of the fluid.

   REFERENCES

   1. Cobb, C.C., and Zublin, C.W., "New Coil Tubing Jet Cleaning System Reduces Cost," Petroleum Engineer International, October 1985.

   2. "Otis HYDRA-BLAST Operational Manual," Otis Engineering Corp., 1986.

   3. "Otis Special Services Operations Manual," Otis Engineering Corp., 1986.

   4. Fowler, S.H. Jr., and Pleasants, C.W., "Operation and Utilization for Hydraulic-Actuated Service Tools for Reeled Tubing," Paper No. SPE 20678, Technical Conference and Exhibition of SPE, New Orleans, Sept. 23-26, 1990.

   5. "Frac Book 11, Design/Data Manual," Halliburton Services, 1986.

   Copyright 1991 Oil & Gas Journal. All Rights Reserved.