TECHNOLOGY NORM SCALE SAFELY DISPOSED OF BY FRACTURING WELL

Nicholas H. Gardiner
Halliburton Energy Services
New Orleans

Fracturing technology has helped operators, offshore Louisiana, to pump naturally occurring radioactive material (NORM) contaminated wastes into nonproducing wells.

Louisiana is one of the few states that currently have specific regulations that govern disposal of NORM.

A clearly designed disposal method is required because NORM contamination is a potential health hazard, a waste management problem, and a possible public relations problem for the industry.1

NORM CONTAMINATION

NORM is widespread in the environment, and many oil and gas formations contain uranium, radium, radon, and other radioactive elements.

When gas and oil are produced, NORM traces are also produced. Over time, NORM is deposited in scale and sludges, which then contaminate equipment, production tubulars, production vessels, and other industry facilities.

The contaminated waste is usually collected in drums and stored for future disposal.

One operator had collected more than 2,300 55 gal drums of NORM contaminated solids.

NORM in the scale originated principally from radium, which coprecipitated with barium and strontium sulfate from produced water.

This type of scale can be found in downhole tubing and in aboveground processing and transport equipment.

Piping, sludge pits, filters, brine disposal/injection wells, and associated equipment can also be contaminated with NORM, as well as equipment associated with well workovers.

MMS REQUIREMENTS

A planning team of operating and service company engineers was formed to plan and carry out a disposal operation in an offshore well. The team planned to fracture a formation and then pump NORM into it, Before start up, the operator had to secure approval from the Mineral Management Service (MMS). MMS approved the procedure, but only after the team demonstrated that:

The disposal fracture would be short
NORM contaminated particles would not migrate within the disposal sand
Below ground accumulation of NORM contaminated solids would not pose a health hazard at the platform.

To demonstrate the soundness of its proposal, the team focused on fracture length predictions, particle retention and bridging, formation damage predictions relating to formation sand pore throat plugging, bonding of radioactive radium to sulfate scales, and the distance and shielding of radioactive isotopes from the surface.

SHORT FRACTURE

Because long fractures could reach a nearby working well, fracture length was a major concern.

Based on the sand permeability as determined by a sidewall sample analysis of the target formation and the Nordgren's rectangular fracture theory,2 the fracture length of the designated well was expected to be very short.

This eliminated the possibility of reaching a neighboring well bore.

PARTICLE MIGRATION

Next, the team demonstrated that NORM material would not migrate from the deposition point after completing the disposal.

Well bore integrity was established by the casing bond log showing adequate cement bonding above and below the disposal sand and the open hole logs showing adequate bounding layers of shale above and below the disposal sand.

The team validated its claims to MMS by using a sample of screened NORM-contaminated solids to determine particle size distribution and Saucier's criteria for particle retention3 along with Abram's rule governing particle bridging.4

Saucier's criteria showed that NORM contaminated solids would be retained within the formation without any migration. Abram's rule illustrated that NORM-contaminated solids would not migrate through the formation because of particle bridging.

Additionally, Herzig's work relating to pore plugging confirmed that particles moving through the pores will eventually bottleneck, causing formation plugging.5

The bonding qualities of radium further demonstrate the safety of NORM disposal in fractures of the designated well. When produced waters are supersaturated with barium sulfate, precipitation can occur in the form of scale. If radium is present, radium can co precipitate because radium ions are the correct size and charge to substitute for barium in the precipitation process. As a result, radium forms an ionic bond within the crystal lattice of the scale.

The only way to release the radium is by dissolving the scale material, and based on the solubility constants of barium and radium sulfate, release of radium is unlikely. Therefore, the radium will stay bonded with the scale and will not pose a migration risk.6

SURFACE EXPOSURE

In the designated well, the NORM contaminated material would be pumped into a formation about 1 mile below the sea floor. At this depth, the accumulation of large volumes of barium sulfate scale in the formation would not pose a health problem at the surface.7

DISPOSAL PROCESS

After meeting the criteria established by MMS, the team initiated the NORM-disposal process.

About 2,300 drums of accumulated NORM contaminated waste were processed onshore. The NORM contaminated solids were removed from the drums, passed through a temporary processing facility, and suspended in a bentonite based gel to form a uniform slurry. The NORM slurry was transported offshore by a 180 ft stimulation vessel equipped with two 500 bbl, Coast Guard-approved storage tanks, and various pieces of pumping equipment (Fig. 2).

To contain all fluids in the event of spills, the area surrounding the pump skids was enclosed by an 8 in. pollution rail. The two 500 bbl tanks were placed on a plastic sheeting to help cleanup small spills. All lines to the platform were tied into a storage tank to bleed off line pressure and to backsurge the tubing if necessary.

The NORM material was pumped from the marine vessel to the platform through a 3 in., 12,000 psi working pressure flexible hose.

PREDISPOSAL ACID JOB

After lines were tested at 10,000 psi, an injection rate was established with seawater. The formation took fluids at a rate of 5.5 bbl/min and 2.200 psi. Next, 2,000 gal of 15% hydrochloric acid were pumped to open the perforations. When the acid reached the perforations, pressure decreased from 1,100 to 580 psi. Seawater was then pumped as a displacement fluid.

PUMP IN/SHUT IN TEST

A pump in/shut in test was performed to determine closure pressure and leakoff rate. A 100 bbl volume of crosslinked hydroxypropyl guar (HPG) fracturing fluid was pumped at an average rate of 15.9 bbl/min to initiate fracturing. Tubing pressure fluctuated from 2,400 to 1,500 psi during the test.

Tubing pressure was then monitored for instantaneous shut in pressure (ISIP), and a closure pressure of 330 psi was graphically determined from the smoothed pressure-vs. square root of shut in time plot.

Analysis of this portion of the pump in/shut in test with a proprietary analysis program indicated a fluid efficiency of 7.6%.

FRAC JOB

NORM injection consisted of a 500 bbl pad of fracturing fluid, followed by 975 bbl of the bentonite gel based NORM slurry. After injection, 600 bbl of seawater were used to flush the NORM slurry from the tubing, equipment, and immediate well bore.

Disposal was carried out at an average rate of 17.8 bbl/min, and tubing pressure varied between 2,100 and 4,300 psi. The entire injection sequence was also analyzed as a pump-in/shut in test.

Pressure falloff was again recorded to determine fluid efficiency and closure pressure. A closure pressure of 475 psi was graphically determined from the smoothed pressure vs. square root of shut in time plot. Analysis of this injection with the proprietary analysis program indicated a fluid efficiency of 25.5%.

The fluid loss efficiency of the fracturing fluid pad in the pump in/shut in test strongly indicated the fracturing fluid pad in the NORM injection leaked off well before the end of the job. This left the NORM slurry at the leading edge of the fracture for a significant portion of the job. The higher fluid efficiency of the NORM slurry is a result of the solids in the NORM slurry acting as an efficient fluid loss additive.

The particle size distribution of the NORM solids was very close to that of silica flour typical]y used as a fluid loss additive in fracturing fluids (Fig. 3). The NORM slurry showed no tendency to sand out during the fracture.

ADDITIONAL DISPOSALS

Five more NORM disposal fractures were required to complete the disposal process. Pressure was held on the casing during all disposal cycles to monitor for tubing to casing communication. Before each NORM slurry was pumped, casing pressure was increased to 2,000 psi with seawater.

A pop off valve ensured a maximum casing pressure of 3,000 psi. During most pumping stages, because of temperature shrinking of the annulus fluid, 1,000 to 1,500 psi drops in pressure were observed in the casing.

The total volume of fluids pumped for all six disposals was:

NORM slurry 4,682 bbl
Fracturing fluid (pad)-1,765 bbl
Decontamination water 400 bbl
Decontamination gel-423 bbl
Seawater 6,020 bbl.

The stimulation vessel supplied 4,000 hhp of pumping capability for the NORM disposal fractures. The jobs used a maximum of 2,380 hhp.

Because of high friction pressure, the maximum horsepower was reached while pumping NORM slurry. An initial value of 3,660 psi was used as a bottom hole treating pressure. During the course of the disposals, this value proved to be a reasonable assumption. Friction pressure drops for the NORM slurry were accurately modeled by a proprietary software program using viscometer data.

Fig. 4 shows the relationship between the tubing pressure, casing pressure, pump rate, and time. A job log for a typical NORM disposal fracture is shown in Table 1.

SAFETY CONSIDERATIONS

Safety was a primary concern throughout the operations.8 During the onshore NORM processing phase, safety procedures were strictly enforced. The processing equipment was in a restricted area covered with plastic sheeting. Radiation levels were monitored before, during, and after operations. Personnel were trained in handling, emergency procedures, and other specialized techniques. All equipment was pressure tested to ensure that no leaks existed.

At the NORM disposal site, personnel were briefed again, equipment and flow lines were checked, and catch basins and absorbent materials were placed under all connections in case of a leak. The lines were then pressure tested to 5,000 psi.

After the NORM slurry was pumped, each tank was rinsed with seawater, which was then pumped behind the slurry. Each tank was surveyed to determine residual contamination levels. The pumping iron, triplex pump, and centrifugal pump were also surveyed.

Any remaining NORM contamination was collected with a wet/dry vacuum and placed in a storage capsule, which was later dropped down the hole before plugging the well. A final survey was then conducted of all equipment and lines to ensure that no NORM contamination was present.

After the job, all restricted areas were riven a final survey, and all personnel were surveyed for NORM contamination with a hand held survey meter. All personnel surveys were within acceptable limits.

Fracturing disposal of NORM contaminated solids into Gulf Coast formations is a viable disposal alternative. Knowing particle size distribution of NORM contaminated solids is important for job planning and design. Also, when properly selected, the disposal site can be reused for future disposals with this method.

ACKNOWLEDGMENTS

The author would like to thank the management of Halliburton Energy Services for permission to publish this article.

REFERENCES

Gray, P., "Radioactive materials could pose problems for the gas industry," OGJ, June 25, 1990, pp. 45 48.
Nordgren, R.P.,"Propagation of a Vertical Hydraulic Fracture," SPEJ, August 1972, pp. 306-14.
Saucier, R.J., "Considerations in Gravel Pack Design," JPT, February 1974, pp. 205 12.
Abrams, A., "Mud Design to Minimize Rock Impairment Due to Particle Invasion," Paper No. SPE 5713, Annual Technical Conference and Exhibition, New Orleans, Oct. 5 8, 1976.
Herzig, J.P., Flow of Suspensions Through Porous Media Application to Deep Filtration, Industrial and Engineering Chemistry (1970) 62, No. 5.
CRC Handbook of Chemistry and Physics, R. C. Weast (ed.), CRC Press Inc., Boca Raton, Fla., 1982.
Toohey, R.E., Natural Radiation Environment, Annual Health Physics Society Meeting, Columbus, Ohio, June 21, 1992.
Woods, S.E., Abernathy, S.E., Chambers, D.G., and Hebert, R., "Processing and Disposal of Scales Containing Naturally Occurring Radioactive Materials," 4th International Symposium on Oil Field Chemicals, Geilo, Norway, Apr. 18 21, 1993.