Study On Swelling Behavior And Application Performance Of Seawater - Based Fracturing Fluid

With increasing exploitation of offshore oil and gas resources, more and more fine resource was exploited while low-permeable resource account for20%of total unexploited resource, which suggests the good prospects of hydraulic fracturing technology. Because of the limitation of offshore conditions, traditional batch mixing method for preparing fracturing fluids is not applicable to platform and offshore conditions, while on-the-fly mixing method directly with seawater can improve operational efficiency, save cost and freshwater, which will be widely applied in the near future. However seawater contains large amount of metal ions with high concentration of total dissolved solids (TDS) leading to poor solvent structure and hydration properties of seawater-based fracturing fluids, hence on-the-fly mixing method can be hardly achieved. This paper focused on the solution structure and hydration behavior of seawater-based fracturing fluids system to understand the regular pattern of influences of a series of factors to hydration behavior. On the basis of the above, a hydration schedule applicable to on-the-fly mixing method was formed.By employing viscosity and rheological method, intrinsic viscosity and rheological properties of guar gum (GG), hydroxypropyl guar (HPG) and carboxymethyl guar (CMG) in seawater with the effect of shear rate, concentration, temperature and pH were investigated. An intrinsic viscosity-increasing effect was observed on GG and HPG in seawater comparing to deionized water, while intrinsic viscosity of CMG in seawater was much smaller than that in deionized water due to a screening effect reducing segmental repulsions. The rheology of all three samples were well characterized by Cross model that exhibiting the transition from Newtonian to pseudoplastic occured in the low-shear-rate range at the concentrations of interest to industries. The a value in zero-shear viscosity vs concentration power-law equation for three samples gave an order of CMG>HPG>GG, which introduced the strongest viscosifying efficiency of CMG in seawater. Aqueous CMG solution had the lowest viscous flow activation energy and exhibited strong pH-dependent viscosity, which were also affected by shear rate.The static and dynamic characteristics of particle morphology, surface structure, swelling behavior, dissolution behavior and molecular structure were analyzed by light microscopy, scanning electron microscopy, atomic force microscopy and laser particle size analyzer. The influences of metal ions, pH, temperature and shear behavior to hydration behavior of CMG were determined, besides the volume ration of swelling was determined. The results showed that the volume ration of CMG in seawater was11.1, besides the higher the temperature was, the lower the ration became. The lower the pH was, the higher the ratio became. The surface compactness and the number and distribution of hole structure were the main factors affect swelling and dissolution performances, which can be modified by external conditions that the higher the temperature, the smaller the pH and the stronger the shear effect were, the quicker the swelling speed became.By control external conditions, the hydration performance of CMG in seawater was characterized by the relationships between apparent viscosity and hydration time, on the basis of which the first order kinetics model was built with the consideration of stirring speed, pH, temperature, concentration and particle size. By considering η5, t80and k, the influence pattern of each condition to hydration performance was determined, which exhibited good coherence to microcosmic analysis. The optimum swelling condition was obtained on the basis of the microstructure and performance study as follows:600r/min stirring speed,20℃temperature,6pH value and dosage of guar gum within0.5%. The swelling time can be optimized within5min to meet the requirement of continuously preparing seawater-based fracturing fluids.To verify the feasibility of the hydration method, the rheological, proppant carrying and degradation performance of seawater-based fracturing fluids prepared by which were tested. The prepared fracturing fluids showed good stability at140℃,which can be totally, degraded by ammonium persulfate within2h. The results suggested that the hydration method is applicable to field utilization.