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Water-soluble polymers have been extensively used in all sections of the oil and gas upstream industry, but their inherent thermothinning behaviour has limited their applications in harsh environments. To address this issue, thermoviscosifying (or "thermothickening") polymers (TVPs) whose aqueous solution viscosity automatically increases upon increasing the temperature were introduced in the early 1990s. This review first recalls the background for developing such smart materials, followed by demonstrating the mechanism of thermothickening. Next, three major TVPs including N-alkyl substituted acrylamide copolymers, grafted polyethers, and cellulose derivatives are summarized with respect to their structure-property relationship, then their practical trials or potential uses in oil and gas drilling fluids, cementing slurries, hydraulic fracturing, steam flooding, and enhanced oil recovery are discussed. Finally, the advantages and disadvantages of the current TVPs are commented and future prospects are discussed to close this review.
Water-soluble polymers have been extensively used in all sections of the oil and gas upstream industry, but their inherent thermothinning behaviour has limited their applications in harsh environments. To address this issue, thermoviscosifying (or "thermothickening") polymers (TVPs) whose aqueous solution viscosity automatically increases upon increasing the temperature were introduced in the early 1990s. This review first recalls the background for developing such smart materials, followed by demonstrating the mechanism of thermothickening. Next, three major TVPs including N-alkyl substituted acrylamide copolymers, grafted polyethers, and cellulose derivatives are summarized with respect to their structure-property relationship, then their practical trials or potential uses in oil and gas drilling fluids, cementing slurries, hydraulic fracturing, steam flooding, and enhanced oil recovery are discussed. Finally, the advantages and disadvantages of the current TVPs are commented and future prospects are discussed to close this review.
Hydrophobically modified polyelectrolytes (HMPEs) have been widely used in numerous industries as rheology modifiers because of their unique solution microstructures and fascinating viscosifying ability; however, the net contribution of hydrophobic association to the thickening power of these structured fluids remains ambiguous due to the lack of comparable HMPEs and unmodified parent polyelectrolytes (PEs). In this work, a series of model HMPEs with a different hydrophobe content and length of the hydrophobic blocks in the copolymer (N H ) were prepared by the micellar polymerization of acrylic acid and stearyl methacrylate, and the obtained HMPEs were further posthydrolyzed to obtain structurally similar, same-molecular-weight normal PEs by cleaving the hydrophobic pendant group. The difference in zero-shear-rate viscosity (η 0,H −η 0,h ) between the HMPEs and PEs can be scaled versus N H and the concentration of the hydrophobic group in solution (C H ) as η 0,H −η 0,h ∼ N H 3 C H 3.29 when the polymer concentration exceeds the critical entanglement concentration in deionized water and as η 0,H −η 0,h ∼ N H 3 C H 3.72 in a 0.02 M NaCl solution when the polymer concentration is higher than the overlap concentration, indicating the power-law contribution of the hydrophobic clustering in the HMPEs.
Multi-pad hydraulic fracturing is believed a cost-effective procedure to unlock the tight oil from low-porosity, low-permeability reservoirs. However, the inconvenience of difficult-dissolving process at surface and crosslinking of the conventional guar-based fracturing fluid systems cannot satisfy such fracking jobs because of the massive proppant loading, high flow rate and large volume of the fluids used. To address these issues, a crosslinking-free and rapid-dissolution fracturing fluid system based on synthetic hydrophobically associating polymer (HAP) "water-in-oil" emulsion was developed. The HAPs are derived from classical water-soluble polymers by incorporating small amount of long hydrophobic side chains onto the polymer backbone. When above a critical associating concentration, these polymers can automatically form a three-dimensional transient network by intermolecular association, reminiscent of cross-linked structures, offering the suspending capacity for proppants. With inverse emulsion polymerization, the obtained HAP emulsions can not only get high molecular weight, but also be rapidly dispersed and finally dissolved within 5 minutes. It was found concentrated HAP polymer emulsions can be dispersed online with surface water or even produced fluids to get final designed concentration. Laboratory rheological study shows that 1% of the as-prepared fracturing fluid can reach more than 50 mPa%s at 150 0C. Compared with guar-based fluid, the HAP fracturing fluid can be completely broken, and the viscosity, surface tension, skin damage of the residual fluid on the permeability are all smaller, while the fluid loss is comparable, proppant-carrying ability is even better. Most importantly, no further surfactant was needed to assist the flowback the fluid. Since September 2013, such associative polymer fracturing fluids were successively used in 29 wells of 3 well pads, Yan-227, Yan-22 and Bin-37 blocks in Shengli Oilfield, Sinopec, where the temperature ranges from 110 to 145 cC. Totally 60,000 m3 fluids were consumed in these fracking jobs, and 87, 9, and 45 stages were successively fractured in the horizontal sections, respectively.
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