Photomicrograph (50X) of 20/40-mesh Brady sand that has been coated with LRS. (Images courtesy of Halliburton)

Proppant flowback in the Permian Basin has been a problem in hydraulic fracturing treatments for many years. As proppant produces out of the fractures along with the produced fluids, fracture conductivity diminishes with time as the fracture width decreases. This choking effect causes the potential production of the well to decline. If the produced proppant remains in the well bore, it may cover the perforation interval, limiting the production flowpath into the well bore. A well cleanup is often required to remove the unwanted proppant from the well bore to reestablish production from the entire perforated interval.

If the proppant flows back to the surface, it can cause severe damage to downhole equipment (e.g., bottomhole rod pumps or electrical submersible pumps) and surface equipment (e.g., chokes, pipelines, and storage facilities).

Most operators have tried to minimize proppant flowback by pumping resin-coated proppant (RCP) during the tail-in of the fracturing treatment. In late 2004 and early 2005, the suppliers of RCP struggled to meet demands for these precoated materials. The operators decided to use an on-the-fly liquid-resin system (LRS) as an alternative to conventional RCP. Because LRS can be coated on any natural sands, either white or low-grade, or any types of man-made proppants at the wellsite, regardless of their mesh sizes, Permian Basin operators continued to select LRS as the preferred flowback control solution in subsequent years.

Liquid-resin systems

The LRS comprises a liquid resin and a hardening agent, which are preblended and delivered to the wellsite in separate containers (Nguyen et al. 2003). If these two components remain separate and uncontaminated, they have a shelf life of several months.

The LRS coating process involves direct coating of proppant with the mixed resin during the transfer of the proppant from its storage container into the carrier fluid. Coating of the mixed resin onto the proppant is completed before the addition of the proppant to the gel carrier fluid. Therefore, the gel cannot coat the proppant because the resin coating is already in place. This condition helps enable more consistent properties in the coating agent and helps eliminate the possibility that an excess amount of coating agent is carried into the well. While this LRS can still affect breaker and crosslinker performance, the effects are far less severe than those of the “wet-coating” processes.

The LRS was designed to handle the wide range of bottomhole static temperatures (BHST) in the wells of the Permian Basin. It was formulated with a proprietary additive to help remove the crosslinked-gel coating from the proppant to enhance contact between proppant grains, thus increasing the consolidation of the proppant pack even without applied closure stress. As a result, even under low or no closure-stress conditions, high consolidation strength of the coated proppant pack can still be developed. In addition to the capability to provide consolidation strength, this resin is also formulated to provide elasticity, which is beneficial to withstanding repeated stress-strain cycles that occur during normal production operations.

Figure 1 shows the coating of LRS on 20/40-mesh Brady sand after the coated proppant was cured and removed from a laboratory pack chamber used for consolidation measurement. The capillary pressure between grains pulls the liquid resin to the contact points, thus helping prevent resin from occupying the pore spaces.

Bonding between grains, illustrated by the footprints at the contact points, helps establish the consolidation strength of the proppant pack to withstand stress load or high shear. This consolidation strength (i.e., unconfined compressive strength) is proportional to the concentration of LRS coated on the proppant.

Consolidation strength of the proppant pack is dependent on the fluid/proppant system, proppant size, curing temperature, curing time, and resin concentration coated onto the proppant. Flow experiments have shown that this coating concentration of LRS provides sufficiently high consolidation strength to survive exposure to high flow rates.
Conductivity testing performed in the laboratory using modified API linear flow cells shows that coating LRS on 20/40-mesh, high-strength ceramic proppant actually improves the conductivity of the proppant pack. The tackiness of LRS coating alters the proppant pack density by increasing inter-grain friction, thus providing higher porosity within the pack. LRS not only bonds the proppant grains to each other, but also bonds to the fracture faces. This bonding distributes the point-source load of the proppant across the formation face, thus reducing the spalling effect and the formation fines intruding into the proppant pack.

Field application

The wells are drilled in more than 40 formations ranging in depth from 1,400 to 16,000 ft (427 to 4,880 m), with BHSTs from 90 to 250°F (32 to 121ºC). Depending on the operator and formations, well drilling spacing ranges from 160-acre spacing down to 10-acre spacing for infill drilling. Approximately 95% of the well bores at perforated intervals of the fracture-treated wells are vertical or slightly deviated, and approximately 5% are horizontal.

The objectives of the fracturing treatments performed in the Permian Basin include bypassing near-wellbore damage, enhancing wellbore communication with as many pay intervals as possible, and increasing or maintaining well productivity. Because of the possibility of proppant flowback in the area and marginal reserves, most operators decided to complete the wells using conventional hydraulic fracturing treatments in which the on-the-fly coating of curable liquid resin on the proppant is applied during the tail-in proppant stages of the treatment to help lock the proppant in place.

Most fracturing treatments are performed in either sandstone or dolomite formations, but other formations such as limestone and shale are also included. The main fracturing treatments are preceded by a breakdown test and an acid ballout. Fracture initiation pressure, fluid efficiency, and closure stress information are often obtained from wells that have been previously treated in the same field.

Fracturing fluid is generally pumped down casing or tubing. The average pad size for the wells is between 15 and 50% of the total fluid volume. Pump rates applied in the fracturing treatments range from 15 bbl/min. to more than 50 bbl/min.; however, more than half of the fracturing treatments are performed with pump rates of 30 to 40 bbl/min.

Various proppant materials are pumped, including mostly natural sands (such as Brady and Ottawa) and sometimes man-made proppants (intermediate- or high-strength ceramics), with mesh sizes ranging from 20/40 to 8/16.

Except for a few premature screenouts, more than 95% of the fracturing treatments were successfully performed as per design. Figure 3 provides a summary of the number of fracturing treatments and amounts of LRS-coated proppant applied per year in the Permian Basin since 2005.

Conclusions

Based on the lessons learned and results obtained from the fracturing treatments performed since late 2004, the following conclusions have been made:

• With careful planning, LRS can be efficiently combined with hydraulic fracturing treatment to coat any proppant during any of the proppant stages to transform this proppant into a consolidated, permeable, in-situ screen for controlling proppant flowback.
• Application of LRS provides an effective method of controlling flowback of proppant and formation particulate to maintain well production without disruption (i.e., workovers) caused by solid particulate production.
• Properly coating and curing of LRS onto proppant allows the consolidated proppant to withstand the high drawdown and effects of stress-strain cycles during well shut-in and production.
• LRS has been an economic alternative to RCP with the added benefit of enhanced conductivity, proppant consolidation, and flowback control.

Acknowledgement
This article is a summary of the SPE paper, Controlling Proppant Flow Back to Maintain Fracture Conductivity and Minimize workovers: Lessons Learned From 1,500 (Note: now 2,500) Fracturing Treatments. Trela, J.M., Nguyen, P.D., Smith, B.R. 2008. Paper SPE 112461 presented at the SPE International Symposium and Exhibition on Formation Damage Control. Lafayette, Louisiana. 13-15 February.