Abstract

Improving the production from unconventional oil and gas reserves requires the right chemicals to not only create fractures, but also to minimize damage in the proppant pack and reservoir. The interactions between polyacrylamides (PAMs), additives, clay minerals and silica are highly complex, and the effects of varying water quality—including changing pH levels and electrolyte concentrations—can change the relative charge of each of those components. Potentially, these shifts in charge can cause the repulsion, absorption, and even flocculation of the various components.

The objective of this study was to develop strategies to minimize the damage caused by clay, quartz and feldspar suspensions that can originate from interactions between wet sand sources and the fluid system. To accomplish this goal, the team monitored fine adhesion to the proppant surface and measured turbidity and zeta potential of the fines suspended in the fluid system to identify flocculation potential. The study compared three PAMs:

1.   Traditional anionic copolymer of sodium acrylate and acrylamide

2.    An acryloyloxy-ethyl-trimethyl-ammonium chloride (AETAC) based cationic polyacrylamide

3.    Terpolymer with partial 2-acrylamido tert-butylsulfonic acid (ATBS) substitution

Testing was performed in multiple water qualities with varying salinities and pH levels. The study revealed that particle bridging of fines by cationic polymers to the proppant pack is more strongly affected by the salinity than anionic polymers. The overall strength of the electrostatic charge on the proppant/fines surface changes with pH. This charge can repel or attract the polymer to the proppant/fine surface and affect the amount of particle bridging that is observed. The team assessed additives such as surfactants, clay controls, and biocides for their ability to improve particle dispersion. The study also revealed that anionic PAMs in lower-salinity brines with total dissolved solids (TDS) lower than 15,000 parts per million (ppm) interacted favorably with the fines in solution. At higher TDS conditions (greater than 30,000 ppm), the particles in the supernatant showed signs of ion-dipole interactions, as shown by increased flocculation of the fines with the fluid system. For cationic PAMs, a sole additive was able to disperse the fines better in the fluid system. These results, combined with the conductivity data, can be used to further optimize formulations, enhance production, and expand the accessibility of local/lower-cost sand mines to reduce supply chain constraints by leveraging wet sand in hydraulic fracturing operations.

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