Research On Fracture Propagation Driven By Supercritical Carbon Dioxide In Tight Sandstone Reservoir

Supercritical carbon dioxide?SC-CO₂?fracturing is a newly developed non-aqueous fracturing technology.Compared with hydraulic fracturing,SC-CO₂fracturing has many advantages,such as friendly to reservoirs,thorough flowback.SC-CO₂ fracturing is the first used in developing tight sandstone reservoirs in North America.The field data shows SC-CO₂ fracturing has a significant effect on enhancing production from tight sandstone reservoirs.The previous researches about SC-CO₂fracturing mainly focus on fields study or relatively simple numerical simulations,which do not consider SC-CO₂ impact on rock mechanics and SC-CO₂ state characteristic.This thesis studies SC-CO₂ fracturing technology and investigates tight sandstone mechanical properties affected by SC-CO_2.Beseides,fracture propagation driven by SC-CO₂ and proppant transport inside a fracture are modeled.The main contributions are as follows:?1?Tight sandstone mechanical properties affected by SC-CO_2.Based on high temperature high pressure?HTHP?triaxial testing system,a SC-CO₂generation device was first designed.Afterwards,experimental tests on mechanical properties of tight sandstone under HTHP conditions were conducted to investigate SC-CO₂ impacts on tight sandstone.Beside,the mechanical properties affected by confining pressure,temperature and pore pressure were studied.The experimental results showed that SC-CO₂ could reduce tight sandstone compressible strength and increase its brittleness.Besides,when the tight sandstone was saturated in SC-CO₂,increasing temperature,confining pressure and pore pressure would reduce brittleness.Otherwise,increasing temperature,confining pressure and decreasing pore pressure would result in higher compressible strength,Young's modulus and residual strength.?2?Fracture propagation driven by SC-CO₂Considering SC-CO₂ phase characteristics in the reservoir conditions and the impact of SC-CO₂ on tight sandstone mechanical properties,fracture propagation driven by SC-CO₂ was modeled.A dimensionless parameter used for judgeing energy dissipation characteristic during a fracture propagation was obtained.Besides,compared with SC-CO₂ phase characteristic and its impacts on tight sandstone mechanical properties,SC-CO₂ viscosity and leakoff were the key factors resulting in SC-CO₂fracturing greatly different from hydraulic fracturing.When the permeability of tight sandstone was super small,SC-CO₂ viscosity dominated fracture propagation.Compared with hydraulic fracturing,in a SC-CO₂ fracturing,fracture propagated longer with narrower width and smaller well bottom hole net pressure.When permeability of tight sandstone increased a little bit,SC-CO₂ viscosity and leakoff coefficient dominated fracture propagation.At this moment,in a SC-CO₂ fracturing,fracture propagated shorter with narrower width and smaller well bottom hole net pressure.?3?Proppant transport in SC-CO₂ fracturingWith proppant concentration increasing inside a fracture,slurry flow would transfer from laminar flow to Darcy flow.Considering this flow characteristic related to proppant concentration,a proppant transport model which could describe proppant horizontal transport and vertical settlement was built.With this model,the transport of proppants which were injected intermittently was studied.Several main parameters affecting proppant transport were studied,such as Young's modulus,SC-CO₂ injection rate,SC-CO₂ viscosity.A damped wave characteristic of proppant concentration inside the fracture was observed.Most important of all,a key characteristic parameter which could uniformly describe proppant damped wave was obtained.With this parameter,proppant transport inside a fracture would be quickly perceived,which would help fracturing engineers to greatly improve a proppant designing.