The Development and Application of a Recyclable Clear Fracturing Fluid

Stimulated reservoir volume fracturing operation in Changqing oilfeld requires large amount of fracturing fluids. To save water, fracturing fluids are required to be recycled. However, the recycled hydroxypropyl fracturing fluid presently commonly used in Changqingdoes not have the ability to carry sands, and the technique for recycling low molecular weight guar gum fracturing fluid is not feasible for feld use. A recyclable clear fracturing fluid was developed recently in an effort to solve these problems. This fracturing fluid was formulated with 3% XYCQ-1 (a thickening agent), 0.05% XYPJ-2 (gel breaker), and 0.01%-0.1% XYTJ-1 (a water quality conditioner). XYCQ-1 is a polysaccharide thickening agent made by microbial cultivation and fermentation, and has the ability to render fracturing fluid stable viscosity in only 10 sec. XYPJ-2 is a mixture of a natural enzyme and a molecularly modifed enzyme, composed of the same polysaccharide as that in XYCQ-1. Site-specifc mutagenesis was used on the molecular structure of the thickening agent to promote specifc reaction to the enzymes to form non-natural new disulfde bond, thereby ensuring the gel resuming and recycling of gel-broken fracturing fluid. XYTJ-1, by reaction with Ca²⁺ and Mg²⁺in the recycled fracturing fluid to form watersoluble complexes or chelates, eliminates the negative effects of high valence metal ions on gel formation. Laboratory experiments indicated that the properties of the fracturing fluid remained stable at 80℃, and had good sand carrying capacity, frictionreduction and cleanup capacity. Almost no sedimentation was found of the fracturing fluid kept quiescence at room temperature for 24 hours, or stayed in 80℃ water bath for 15 minutes. At injection rate of 64 L/min, the percentage of friction reduction was 67%, and percentage of core damage only 6.70%. In the stimulated reservoir volume fracturing operation in 21 horizontal wells in the Changqing oilfeld, 10.46×10⁴ m³ fracturing fluid was consumed, and the recycled fracturing fluid, after simple treatment (liquid-sand separation by settling), was reused for making fracturing fluid. The fracturing fluid has been recycled for 10 times, and the clear fracturing fluid made from the recycled fracturing fluid has good sand carrying capacity, indicating that the recyclable clear fracturing fluid has satisfed the needs of multi-stage fracturing operation.