Intervention with gel technology to seal and abandon a non-productive formation in state waters offshore Louisiana saved the operator days of rig time and associated costs. The estimated savings for the operator was approximately US $282,000 compared to conventional methods.

The objective of this job was to seal and isolate an existing set of non-productive

The cement bond log showed the well was capable of accepting the gel treatment. (Log courtesy of Superior Energy Services)
perforations prior to re-completing a higher-pressure zone deeper in the well. Typically, a well is completed in the deepest zone first. When that zone depletes, cement is pumped into the perforations and, normally, a bridge plug is set in the production casing just above the depleted interval. After the perforations are sealed, a new production interval is pursued at a shallower depth in the well bore. This process is repeated until all of the productive intervals are depleted, and the well may eventually be sidetracked, decommissioned or permanently/temporarily abandoned.

In this particular well, instead of completing a zone at a shallower depth, the new proposed production zone was nearly 100 ft (30 m) deeper in the well bore. To complicate the issue even further, the current depleted zone was gravel-packed, which presented the operator with a unique challenge.

The new production zone was deeper than the current non-productive zone, which meant that if cement were used to isolate the depleted zone, it would need to be drilled out with coiled tubing prior to perforating the new zone. Another (more expensive) option would be to move a drilling rig over the well and perform a full workover of the well by pulling the tubing and completion assembly, squeezing the depleted zone and then re-completing the well in the new zone. Both of these processes could take several days and would require a great deal of equipment and manpower, thereby resulting in increased costs.

The solution
Due to the unique challenge presented, the engineering staff at Superior Energy Services recommended using WellPRO HydroFIX Gel in an effort to seal the non-productive zone and complete the well further down the hole. There are several reasons the company recommended this product. First of all, the sealant would penetrate and seal the formation and gravel pack and could then be removed from the tubing and well bore with little effort.

Another reason they chose to use a gel sealant instead of a cement slurry in this situation was that the penetrating properties of the gel sealant are superior to that of cement since the gel does not contain any solids. Instead, it contains a low molecular weight ionic polymer that forms a rubber-like gel when combined with a carboxylate complex crosslinking agent. Before the crosslinking takes place, the gel has a low viscosity, which facilitates injectivity and allows the gel to penetrate and seal the producing formation along with the gravel pack assembly. Once the gel is set, the fluid movement ceases.

Prior to recommending this sealant, an extensive engineering evaluation of the well components and operational parameters was performed. The cement bond log (CBL) was reviewed, and the isolation was analyzed. On this job, it was determined that the cement job was indeed satisfactory and adequate isolation did exist between the two zones. This was a critical step in the engineering evaluation process. If the cement bond was inferior or non-existent, there was a possibility that the gel might divert into the wrong zone.

Once adequate isolation was verified, the sealant formulation and retarder concentration was optimized in the laboratory, and testing was performed under the downhole conditions of 8,000 psi and 220ºF (104ºC). The anticipated job placement techniques were also simulated during the laboratory tests.

Finally, the job procedure was finalized, refined and discussed via conference call with all involved personnel before arriving on location, and a detailed set of contingency plans was defined.

After reviewing the formation properties and well conditions, it was determined that a volume of low molecular weight (LMW) gel followed by a smaller volume of high molecular weight (HMW) gel would be the best possible combination of sealants for this particular situation. With only a small amount of differential pressure, the LMW gel would penetrate the gravel pack and formation, whereas the HMW gel would not. Since the well was on a vacuum, the HMW was incorporated to keep the sealant from being displaced too far into the formation.

Seventeen bbl of the LMW gel sealant was mixed on the surface using a batch mixer and then placed in the well bore via coiled tubing (CT). The LMW was followed by an additional 6 bbl of HMW gel. The HMW was also mixed with a quantity of diatomaceous earth (DE), which was included in an effort to prevent the gel from “running away” and to allow the gel to seal at the formation interface. Both sealants were then displaced down into the well while circulating the CT across the gravel pack. Since the fluid column height was unknown, the sealant was displaced out of the CT reel and then allowed to seep out into the formation. Once in place, the sealants were allowed sufficient time to set up as per the laboratory test results obtained prior to the job.

After allowing the gel to cure, the tubing was circulated clean by jetting through the gel with the CT, and full returns were observed soon after the jetting process began. Before washing through the gravel pack screen, a complete bottoms-up was obtained in an effort to reduce the equivalent circulating density in the tubing and to minimize the chances of getting the CT stuck in the hole. If returns would have been lost once the gravel-packed zone was penetrated, there would not have been a way to circulate the gel out of the tubing. Once clean returns were observed, the CT was lowered to just below the new production zone, and the tubing across the proposed production interval was jetted clean. Finally, the CT was pulled back above the gravel pack, and the tubing was circulated clean one last time.

After washing through to the new production interval the e-line unit was rigged up and the perforating guns were lowered into the well. The initial bottom-hole pressure (BHP) was 4,800 psi and quickly increased to 5,606 psi after applying 800 psi to the tubing string at the surface. The new interval was perforated in the pressurized zone, and the BHP instantly increased to 7,121 psi while the surface tubing pressure increased to 2,315 psi. The depleted zone did not take any fluid, and no pressure losses were observed in the well bore after the new zone was perforated.

The e-line assembly was pulled out of the hole with no drag observed, and the entire bottomhole assembly was retrieved. The well was then ramped up to a production rate of 1.5 MMcf/d of natural gas.

Conclusion
The usage of the specialized gel in conjunction with additional equipment and services provided from the service company demonstrates how using new technology along with traditional remediation equipment can enhance the process of extracting oil and gas from deep below the earth.

As mentioned earlier, the gel sealant used on this job is a low-viscosity liquid and is capable of penetrating several feet into the gravel pack/formation matrix. After being exposed to temperature and pressure for a specific period of time, it forms a rigid gel that locks all of the fluids and remaining gas in place in the formation. As a comparison, cement is only capable of penetrating several inches into the formation, and therefore a complete seal is not easily obtained when a gravel pack is present.

The entire time period to place the gel sealant, clean out the well and perforate the new zone was 36 hours. The operator saved both time and money (when compared to conventional methods), and a shut-in well was reverted back to daily production.