Self-supporting Fracture Induced By Supercritical Carbon Dioxide Fracturing And Its Permeability-Stability Characteristics

Hydraulic fracturing technology is a most commonly-used method to stimulate shale gas reservoirs.However,more than three fifths of shale resources located in water-deficient area in China.During hydraulic fracturing operations,much water is consumed and the lack of water resources has restricted the exploitation of shale gas.In addition,the shale formation in China always has high content of clay,who may react with water in water-based fracturing fluid and the induced shale swelling damage reservoirs and further decrease the shale gas recovery.In recent years,the supercritical carbon dioxide?Sc-CO₂?has been proposed as alternative fracturing fluid.However,due to the special fluid physical properties of Sc-CO₂,as well as structural and mineralogical difference between shale gas reservoirs and conventional gas reservoirs,basic scientific problems such as shale breakdown mechanism,fracture topography and permeability-stability characteristics have not been solved.Hence,in this research,theoretical analysis,laboratory experiments and numerical simulation methods are used to investigate shale breakdown mechanism,fracture topography and permeability-stability when using Sc-CO₂ as fracturing fluid.The objectives of this research are Longmaxi shale,Green River shale and Marcellus shale and the main research contents are self-supporting fractures induced by Sc-CO₂fracturing,fracture permeability evolution under stress-chemical coupling effect,as well as fracture stability.The main research results are listed as follows:?1?The mechanism of rougher and more complex fracture surface,as well as the self-supporting fracture induced by Sc-CO₂ fracturing are revealed.The low viscosity Sc-CO₂ has a high diffusion capacity and forms a larger diffusion area around injection borehole,combing with capillary force which makes Sc-CO₂ easily penetrate into smaller pore throats.In addition,a smaller fluid lag in fracture tip also contributes to the rougher and more complex fracture surface.During Sc-CO₂ driven fracture propagation,cements between mineral particles are destroyed and mineral particles are removed from fracture surface.Those removed mineral particles work as proppant and support fracture,which results to a larger effective hydraulic aperture and permeability.?2?The controlling mechanism of fluid viscosity on induced fracture topography is revealed.Based on conventional model to simulate fracture propagation,the real state equation of injected fluid is taken into consideration to build a new fluid-rock coupling model for fracture propagation.The software COMSOL is used to run the simulation code.Results validate that fluid viscosity plays a vital role in fracture tortuosity from a micro-scale.The low viscosity Sc-CO₂ enters into small pores and makes the fluid-driven fracture rotate into smaller pore throats direction,which results to a more tortuous main fracture as well as more secondary fractures around the main fracture.?3?A multiple-parameter model is built to describe shale fracture effective hydraulic aperture evolution which considering pressure solution,free-face dissolution and clay swelling effects.The model reveals shale fracture permeability evolution under stress-chemical coupling conditions.Results indicate that mineral free-face dissolution is main reason for fracture permeability increase while pressure solution and clay swell are main reasons for fracture permeability decrease.Those three effects operate in different and complex ways and competition roles among them determine whether fracture permeability increases or decrease.?4?The controlling mechanism of mineral crystal structure on shale fracture frictional strength as well as mineral hardness on shale fracture stability are revealed.Results indicate that:mineral crystal structure is the main factor to control frictional strength.Tectosilicate constitutes the shale skeleton and keeps a higher frictional strength during slip.With phyllosilicate content increases,fracture tends to slip with the direction of phyllosilicate's layer structure and show lower friction strength.The mineral's hardness determines shale stability:tectosilicate has a smaller mineral hardness,which inducd a velocity-waenking phenomenon.When velocity increases suddenly,a seismic slip tends to occur.