Nanoparticle-coated 20/40 US mesh sand before (on left) and after (on right) formation fines flow through the proppant-pack. Picture on right shows the high affinity formation fines have to fixate onto nanoparticle treated proppant. Photos were taken at 60X magnification. (All figures courtesy of Baker Hughes Inc.)

The migration of formation fines during hydrocarbon production has been a major problem to resolve. Formation fines can migrate from distant sections of a reservoir and concentrate in the near-wellbore region, where they can cause production damage. These small particles can interact to form larger particles in the passages of the near-wellbore region that can plug pores or proppant packs and sand-control screens. If formation fines pass through a sand-control screen, local erosion of the screen is a concern, and downhole pumps can also be damaged. To prevent these potential problems, migratable fines should be kept as far away from the near-wellbore region as possible.

Over the past decade special polymers have been developed as ultra-thin tackifying agents or surface-modification agents to coat on proppant or formation rock to keep formation fines from moving. The tackiness observed between the polymer-coated particles is believed to be the result of the many van der Waals associations between long hydrophobic carbon chains in the polymer structure.

The most recent development is the use of nanoparticle technology to fixate formation fines onto hydraulic fracturing proppant. This mechanism captures formation fines as they enter a hydraulic fracture and keeps them away from the well bore. The nanoparticles employed are unique, inorganic nanocrystals that have very high surface force attractions. These nanoparticles readily attach to the surface of ceramic and silica proppant particles. Laboratory testing of proppant packs treated with low concentrations of nanoparticles demonstrates that the nanoparticles are capable of fixating formation fines such as colloidal silica, charged and non-charged particles, and expandable and non-expandable clays onto proppant particles with remarkable efficiency.

Proppant pack tests

The simulated formation fines used in this study include natural Bentonite, Illite, and lab prepared Rev Dust. Rev Dust is commonly used as simulated drilling debris for drilling mud evaluation and is composed of expandable and non-expandable clay particles, cristobalite, and quartz particles, with a mean particle size of about 20 microns.

The nanoparticles used to treat the proppant surface are inorganic nanocrystals less than 100 nanometers in size, with one select product having a size of 35 nanometers.

The nanoparticles were used in slurry product form. Three types of 20/40 US mesh proppants were used for the tests: high strength ceramic proppant (HSP), intermediate strength ceramic proppant (ISP), and common quart proppant.

The nanoparticle slurry product was added to the water-based fracturing fluid (tests included borate crosslinked polymer fluids and viscoelastic surfactant fluids) and then mixed with the proppants, which simulated on-the-fly addition of nanoparticles and proppants during a fracturing treatment. The nanoparticle loading used for the tests was 1 gram of the nanoparticles per 1,000 grams of proppant, which represents 1 lb of nanoparticles per 1,000 lb of proppant in field operations. After the fracturing fluids were broken, the mixture was poured into a 1-in. internal diameter by 18-in. long acrylic tube with 100 US mesh stainless screen at the bottom. The bottom end cap had a 1?8-in. hole in the center with a valve for controlling fluid discharge. The amount of proppant used for each test filled the lower 12 in. of the acrylic tube once settled. KCL brine at 2% by weight (% bw) was used to wash out the broken fracturing fluid before the simulated formation fines solution was run through the proppant pack. The three formation fines solutions tested were 0.5% bw Bentonite, 0.5% bw mixture of Bentonite and Illite, and 0.25% bw Rev Dust, each in tap water. The excessively high concentration of fines in the test solution was to determine the efficiency of the nanoparticle-treated proppant to attract and remove a large amount of fines from flowing water and to approximate total fines fixation capacity of the nanoparticle-treated proppant.

At the start of each test, the simulated formation fines fluid was syringed into the upper 6-in. blank section of acrylic tube. The bottom valve was then opened, and the fines-laden fluid was allowed to flow through the proppant packs. A hydrostatic head of simulated fine fluid was kept in the blank section above the proppant pack to achieve continuous flow through the untreated and treated proppant packs, and the gravity feed discharge was observed in a collection beaker over time.

The untreated proppant pack tests showed that after approximately one proppant pack pore volume that fines breakthrough was observed in the collection beaker. Within approximately four pore volumes the discharged fluid looked about the same as the fluid feeding into the pack from the top, indicating the type and amount of simulated formation fines in the water had no affinity for the 20/40 US mesh proppant particles in the proppant pack once the 2% bw KCl fluid was removed.

Nanoparticle-treated proppant packs

The tests were repeated with nano- particle-treated proppant packs. Unexpectedly, as the simulated fines water flowed through the proppant pack, no visually noticeable fines broke through after 20 proppant pack pore volumes were collected. Discharging clear water was quite remarkable because of the type and amount of fines within the water, the degree of difficulty of taking such fines out of the water, and the very large pores and channel passages within the 20/40 US mesh proppant pack. The fines-laden water readily flowed through the nanocrystals-treated proppant pack, yet the efficiency of particulate removal was surprising. The first visually noticeable fines breakthrough occurred after about 45 pore volumes, but the amount of fines in the collection fluid remained noticeably low, to at least 60 pore volumes, indicating the relatively low amount of nanoparticles used on the proppant particles were capable of holding a significant amount of fine particles. The photo on the left of Figure 1 is a 60X (60 times) magnification of the 20/40 US mesh quartz sand proppant treated with 1 lb of nanoparticles per 1,000 lb of proppant. The photo shows no detection of the nanoparticles on the surface of the proppant, as expected. The photo on the right of Figure 1 is a 60X magnification of the nanoparticle-treated 20/40 US mesh quartz proppant taken from near the top of the proppant pack in the acrylic tube after exposure to approximately 60 pore volumes of 0.25% bw simulated formation fines fluid. The photo shows a significant amount of fines fixated onto the proppant particles.

Effluent turbidity measurements

A stack of two separate International Standards Organization (ISO) conductivity cells with the same amounts of 20/40 mesh ceramic proppant were prepared and a 2,000 psi load stress was applied for comparison tests. Each test cell had 2 lb/sq ft proppant loading, which represented a fairly thin layer of proppant for the test fluid to flow through. The proppant in one cell was treated with nanoparticles, and the other was untreated. One-quarter percent (by weight) Rev Dust solutions were flowed through the nanoparticle-treated and untreated proppant beds separately at flow rate of 10 ml/min. The effluents were continuously collected in clean glass bottles. Each bottle of effluent represented roughly two pore volumes of the proppant bed. The turbidity of the effluent samples was then measured using a spectrophotometer at different pore volumes.

The turbidity comparison results at ambient temperature are shown in Figure 2. The comparisons show that effluents from the nanoparticle-treated proppant bed had very low initial turbidity and the turbidities of the effluents from the untreated proppant bed were much higher than that from the nanoparticle-treated proppant bed.

Nanoparticle-treated fracture proppant has been found to attract and fixate formation fines in a remarkably efficient manner. The fixation mechanism is based on inorganic nanocrystals that exhibit very high surface force attractions. Laboratory tests show the fixation forces work for both charged and non-charged type formation fines.