Development and Performance Evaluation of a Novel Nano-Composite Crosslinked Fracturing Fluid for Ultra-Deep Reservoir in Tarim Basin

Abstract

Abstract For major ultra-deep oilwells in Tarim basin, hydraulic fracturing operation are challenged by the actual vertical depth in excess of 6000 m and the temperature at bottom hole over 160 °C. The long injection path from ground to target formation generates considerable friction, resulting in extremely high ground pressure, which usually exceed the limit of operation equipment. Moreover, traditional crosslinked fracture fluids always lost their viscosity and sand-carrying ability at high temperatures. Consequently, it is meaningful to develop and research novel fracturing fluids for hydraulic stimulation in high-temperature, ultra-deep reservoirs. Nano-composite technology has been proven to be a potential solution to some challenges associated with crosslinked fracture fluid systems. In this work, a kind of low-cost molybdenum disulfide (MoS2) nanosheet was firstly synthesized by hydrothermal chemical method. Afterwards, the MoS2 nanosheets were mixed with polyacrylamide solution with specific molecular weight by ultrasonic dispersion, and a certain amount of organic zirconium crosslinker was added to prepare the nano-composite crosslinked fracturing fluid. Finally, a series of indoor evaluation tests were performed to compare the performance of the nano-composite crosslinked system with the similar crosslinked fluid without adding nanosheets, such as rheology properties, drag reduction efficiency, proppant suspension and retained conductivity. The obtained experimental results have shown that the thermal stability of the nano-composite crosslinked system is much superior to that of a comparable fluid lacking the MoS2 nanosheets. The introduction of nanomaterial allows the novel fracturing fluid to operate at greatly lower polymer concentrations (0.2%-0.3%) compared with current commercial fluid systems (0.4%-0.5%) designed for 180 ℃ reservoirs. The retained apparent viscosity can be maintained above 75 mPa·s after shearing 120 mins at 180 ℃. Rheological studies have shown that the novel system has superior crosslinking properties, and the crosslinking time can be controlled within 4-10 minutes. In addition, this novel nano-composite crosslinked fracturing fluid has enough sand-carrying viscosity under high-temperature conditions, and allows for efficient cleanup by use of an oxidizer-type breaker. Little or no polymer residue and efficient cleanup are contributing to lower reservoir damage, better fluid conductivity, and improved well production. Newly proposed nano-composite crosslinked fracturing fluid provides a new solution for fracturing stimulation of ultra-deep high-temperature reservoirs in the Tarim Basin, and hence improving the oil recovery.