Technology advances for horizontal drilling, fracturing and multistage completions have contributed to the 65% growth in U.S. oil production in the last five years, with a significant portion of this increase coming from shale plays. However, operators in shale plays often struggle to sustain IP rates, which can decline by up to 80% within the first year. Operators have traditionally resorted to drilling more wells to offset the drop in production. As the current economic climate forces the industry to identify more time- and cost-effective methods for prolonging the productive life of shale wells, refracturing is quickly becoming the preferred alternative, especially for wells with completion inefficiencies that hinder production or leave significant untapped hydrocarbon reserves.

Limitations of traditional refracturing techniques
There are three approaches for refracturing wells: treating existing unproductive perforations, sealing existing perforations and targeting new or previously bypassed pay zones, or using a combination of these approaches. There are several methods to do this, but some have drawbacks, particularly in multistage fracturing operations. Sliding-sleeve completion systems are highly efficient and accurate but costly. Although sophisticated zonal-isolation tools can be installed using coiled tubing (CT), the small inside diameter of CT results in low rates and proppant concentrations and limits the treatment design. Additionally, CT can be costly and may have availability issues. A perforation squeeze can be performed during which cement is used to seal existing perforations, but this can eliminate all existing perforations and can damage producing fractures.

Alternatively, degradable mechanical diverters can be used to isolate zones and to direct stimulation fluids on a temporary basis at a lower cost without logistical limitations and with minimal formation damage. Mechanical diversion is a well-known and increasingly popular method of refracturing wells because of its simplicity. This approach involves pumping a diverter downhole in fracturing fluid, where it then forms a seal over existing perforations or directs fracturing fluid to new zones. By dissolving downhole, degradable diverters eliminate milling, reduce wellbore cleanup and save time, all of which helps to decrease operational costs.

However, traditional degradable diverters—such as balls, degradable ball sealers, rock salt, dissolvable flakes and high concentrations of sand—are unreliable for multistage fracturing operations because they do not provide the optimal level of zonal isolation, nor do they maintain an intact seal for the periods of time necessary for treatment. Balls or degradable ball sealers unseat from perforations without a constant application of pressure, rendering them ineffective if a refracturing operation needs to be halted. Rock salt and flakes have a short lifespan—as low as a few hours—before dissolution, which is usually not enough time to refracture an entire lateral.

Evolution of degradable diverters
As horizontal multistage fracturing becomes commonplace, there is an increased need for longer lasting, more reliable diverting technologies such as advanced degradable diverters. The Weatherford TBlockSure diverting agent and stimulation enhancer is a solid material—made of a proprietary degradable mesh—that provides superior isolation and diversion compared to traditional diverters.

Degradable diverters can be deployed easily and with minimal equipment requirements, which is beneficial from a cost-reduction standpoint. The only equipment required at the well site is the agent and a dedicated pumping unit that can be easily installed at the wellhead. Although the agent is compatible with standard pumping equipment, using the specialized separate pumping unit prevents the agent from contaminating other equipment in the fracturing fleet by remaining in the valves and seats in the fluid ends of fracturing pumps.

The agent is then simply pumped downhole using either slick water or, more commonly, a viscous fracturing fluid. It can be deployed in any well regardless of borehole size, geometry or the original fracturing method used. Additionally, the TBlockSure agent is available in a high-temperature version for environments between 82 C and 149 C (180 F and 300 F) and a low-temperature version for environments between 54 C and 82 C (130 F and 180 F).

Once in place, the agent creates an enhanced zonal barrier or perforation sealant that withstands high treatment pressures. Only at a predetermined time—depending on the bottomhole temperature, the duration of exposure to the temperature and the pH—the mesh dissolves into liquid form to reopen fractures or perforations and to enable flow. No secondary treatment is required to start the degradation process. However, chemicals such as breakers or pH-adjusting agents can be added to the TBlockSure agent to control how quickly or slowly the material degrades. Although the degradation process requires shut-in time, this time can be used to prepare production facilities and should not cause a significant delay to operations.

The agent has a pellet-like appearance at the surface but, once in place downhole, transforms to create a wall that separates individual fracturing stages or seals existing perforations. This barrier is much less permeable than traditional degradable diverters; withstands higher treatment pressures; and dissolves only at a precise, preplanned time. (Source: Weatherford)

Case study
A Marcellus Shale operator used the agent to refracture a 1,981-m (6,500-ft) horizontal gas well with a measured depth of 2,865 m (9,400 ft) and bottomhole temperature of 49 C (120 F). The operator initially screened out existing perforations with high concentrations of sand and, using the plug-and-perf method, created new perforations between the existing perforations. There were 26 existing clusters with 312 perforations and 24 new clusters with 192 perforations. The operator sought a more robust solution for isolating existing perforations and for diverting stimulation fluid to new perforations more effectively.

Weatherford pumped the agent downhole in six batches at a rate of 25 bbl/min. After the sixth deployment of the agent, a sharp spike in the treating pressure to more than 6,500 psi indicated that the existing perforations were sealed. At this point, the new perforations could be targeted with stimulation fluid and the first fracturing stage begun. The agent withstood treatment pressures needed to break into the zones.

These pressure plots show the results of the agent deployed as part of a Marcellus Shale refracturing operation. The left plot shows the results of pumping and displacing six batches of the TBlockSure agent, which increased pressure to more than 6,500 psi and initiated treatment of new perforations. The right plot provides an overview of first-stage fracturing treatment with the TBlockSure agent as it seals existing perforation clusters. (Source: Weatherford)

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