February 22, 2017
Incompatibility Between Frac Fluids and Resin-Coated Proppant Can Cost You During Your Well Completion
Understanding how resin-coated proppant interacts with frac fluid can mean the difference between an optimized completion or one that requires expensive remediation.
Frac fluid typically comprises water, proppant, and less than 1% additives, including breaker-oxidizer. The additives vary the hydraulic fracturing fluid consistency and pH. If pH < 7, it is thenacidic; If pH > 7 it is alkaline. Neutral pH is 7.0. Distilled water at RT (77oF) has pH of 7.
The primary resin proppant systems are phenolics Novolac, phenolic polyurethane, polyurethanes or phenolic resoles. A proppant manufacturer coats a proppant, mostly raw sand with a specific resin type for varying performance enhancements: preventing proppant flowback, increasing propped fracture conductivity, increasing particle crush strength, and preventing embedment.
Generally, phenolics Novolac- and resol-based resins are stable in acidic pH although the resin may soften at pH >10.5. Polyurethane resins are acid sensitive and more stable at basic (alkaline) pH conditions while significantly degrading above 175°F. On the other hand, phenolics Novolac and resol resins are stable up to 400° F and closure stress up to 16,000 psi.
Frac fluid additives and resin-coated proppant interaction
The objective is to achieve minimum chemical interaction between the resin of the proppant with the frac fluids and additives. A reputable proppant manufacturer continuously tests resin-coated proppants in the presence of frac fluids and additives to prevent unwanted reactions that cause fluctuation in viscosity, or spike the consumption of the breaker-oxidizer by the resin, which may lead to premature proppant consolidation or a screenout.
First, let’s look at curable resin-coated proppant in linear gel or crosslinked gel; typical completion designs. Correctly engineered phenolics curable resin-coated sand in an frac fluid— pH < 10.5 — prevents proppant flowback because the proppant effectively consolidates in the fracture. However, at pH > 10.5 conditions in combination with high bottom hole temperature levels, the phenolic resin of the coated proppant can soften, causing the proppant pack to potentially consolidate prematurely in the wellbore rather than in the fracture. An operator would be delayed by expensive wellbore clean up costs and downtime to drill out the proppant pack.
Next, let’s talk about precured resin-coated proppant in gel. Note that phenolics precured resin-coated proppants are used to increase propped fracture conductivity and crush strength. These resin-coated proppants are inert in acidic, neutral, low and medium level alkaline fluids because the resin is completely cured during manufacturing. At medium level alkalinity—about pH 11—the proppant is also nonreactive. (Frac sand is nonreactive.) But, at strong alkaline environments with pH > 11, the wrong combination of frac fluid with precured resin-coated proppant would destabilize the gel, which in turn would not effectively transport the proppant into the fractures. The decreased viscosity would cause a screenout and associated downtime to clean out the well.
Breaker and resin-coated proppant
Whether you’re working with curable resin-coated proppant or precured resin-coated proppant, breaker and resin compatibility are vital for efficient proppant transport and completion of the well. Incompatibility will cause the resin to consume large amounts of the breaker, instead of the breaker breaking the gel. If the gel is not completely broken down by the breaker, then the gel viscosity will not decrease, which would lead to filter cake type deposits of gelled guar inside the fractures. That is a major challenge since such gel deposits can plug pore spaces, reduce fracture conductivity, and decrease hydrocarbon production. Formation damage is also very likely.
Fracturing fluid and resin-coated proppant can complement one another for an optimized well completion if there is good chemical compatibility between the frac fluids and the resin coated proppant. A low quality polymeric (chemical) coating on resin-coated proppant can cause curable resin-coated proppant to set up prematurely and precured resin-coated proppant to screen out, both undesired cases for a Completions Engineer. Keeping these dynamics in mind when you’re designing a well completion will produce a better well and lower your cost per BOE.