Effect Of PH On The Performance Of Hydroxypropyl Guar Fracturing Fluid

With increasing exploitation of low-permeable oil and gas resources, hydraulic fracturing was the necessary way to improve its production. Meanwhile, faracturing fluid was the key factor to decide that whether the fracturing work was successful or not. The pH agent was an important component of fracturing fluid, and it played an important role in the different stages of hydraulic fracturing. However, some theoretical issues about the pH have not yet been understood clearly in domestic and overseas. According to the rheology and microstructure theory, this paper focused on the role of pH in different stages of fracturing to determine the best pH range of different construction stages and optimize the best pH agent.The effect of pH on the swelling performance of guar gum was studied. Over the pH range from 9 to 11, the swelling rate decreased gradually. The OH- anions adsorbed onto guar gum molecule chains, which led to the change of guar gum swelling law. The Na+ cations reduced the hydrodynamics volume of guar gum, which decreased the viscosity of the base fluid. The molecular diameter and microscopic conformation of HPG were analyzed by scanning electron microscope and laser scattering instrument.By employing viscosity and rheological method, the effect of pH on the molecular structure and rheology property was studied. At pH 7-12.5, the guar gum fluid possessed lower critical overlap concentration and higher intrinsic viscosity. According to the Zeta potential test, at pH 7-14, the zeta potential decreased with the increase in pH values and then increased, which affected the dispersion state of guar gum. The zero-shear rate viscosity was calculated according to the Cross model. According to the Ostwald-Dewaele equation, the n, K values were calculated, and the K values were relatively higher at pH 8.5-12. At pH 7-14, the viscosity of the base fluid increased firstly and then decreased. The change of the viscosity was related to guar gum molecular conformation.The effect of pH on the delayed crosslinking time and gel viscosity was studied. The effect of pH on the shear behavior of gel was studied by using variable shear tests. At pH 8.5-11.5, there were strong hydrogen bondings between guar gum molecules, which led them posseess outstanding shear resistance. The maximum elastic modulus range and the optimum range of thixotropy were determined according to the rheological experiments. The effect of pH on the static sand carrying performance and gel breaking performance were determined. The best pH range of this stage was 7-12.According to the experimental results and field practice, the best pH range was 9.5-11.5 for the entire construction process.