LNMR analysis of the retention of different guar gum structure in sandstone: Based on a new characterization method

Abstract

The adsorption and retention damage of guar gum to reservoirs are important factors affecting the effectiveness of hydraulic fracturing. Accurately understanding and evaluating the adsorption and retention of guar gum can better support the research of low-damage fracturing fluid. However, previous indoor evaluation methods have some limitations in studying the adsorption and retention characteristics of guar gum. The analysis object of low-field nuclear magnetic resonance (LNMR) is water rather than guar gum, which cannot achieve visualization of guar gum retention. In addition, Ordinary guar gum (GG), hydroxypropyl guar gum (HPG), and carboxymethyl guar gum (CMG) are commonly used in the hydraulic fracturing of sandstone gas reservoirs, but there are very few studies that focus on the differences in the adsorption and retention damage of the three guar gums from a microscopic perspective. In this study, a new characterization method for evaluating the retention of polymer in fracturing fluid based on the principles and properties of LNMR was proposed. This method can quantitatively evaluate the amount of guar gum adsorption retention and achieve a visual evaluation of the adsorption and retention of guar gum in cores. The aggregation characteristics of guar gum in porous media can be reflected. The dynamic retention process of GG, HPG, and CMG in sandstone cores was directly characterized through this method. The aggregation and entanglement characteristics of the three types of guar gum in sandstone were analyzed from the structure of guar gum. The experimental results show that the difference in molecular weight and modified groups of guar gum has an impact on its adsorption and retention. The molecular chain after the gel-breaking of GG was longer due to the large molecular weight. Long chains and intramolecular hydrogen bonds caused a larger curling degree of GG molecules. The entanglement degree of guar gum increased due to the introduction of non-ionic long-chain hydroxypropyl. The introduction of anionic carboxymethyl made CMG molecules more stretched due to electrostatic repulsion. The median particle size of the gel breaking liquid of GG was 10.65 μm, HPG was 8.8 μm, and CMG was 5.88 μm. A larger molecular particle size after guar gum gel breaking would aggravate the retention in sandstone porous media. After 150 min of displacement, the adsorption and retention amount of GG in the core was 4.59 mg/g, HPG was 4.17 mg/g, and CMG was 3.86 mg/g. The compactness of aggregates formed by guar gum in the core was HPG > GG > CMG. The gas flooding can relieve the adsorption and retention of guar gum. The higher the degree of entanglement of guar gum molecules, the more difficult to remove them from the core. The flowback degree of gas flooding of GG was 10.2%, HPG was 6.8%, and CMG was 21.9%. This research provides an experimental strategy for the microscopic evaluation of polymer adsorption and retention in rock porous media. The research results provide a deep understanding of the microscopic characteristics of guar gum adsorption and retention from the perspective of guar gum structure, which has some significance for guiding the optimization of fracturing fluid and the study of low-damage fracturing fluid.