A True Triaxial Physical Simulation Experimental Study To Improve The Scale Of Crack Propagation

The exploration and development of shale gas in the south of Sichuan Basin is in the direction of deep shale gas field,and the heterogeneity of shale reservoir is getting more distinct.For the development of old wells and the implementation of refracturing technology to achieve SRV fracturing for increasing the production,yet the conventional refracturing hydraulic fractures generally initiate and extend along the old fractures without forming a new complex fracture network.Thus,adding temporary plugging agent to it to realize the inside fracture steering or connecting the natural fractures,which is called temporary plugging and diverting fracturing technique has been brought into operation.But there are still questions and unknown about the capability of fracture diverting,the chance of fracture,fracture initiation and fracture propagation mechanism are in the exploration stage.Therefore,the influence of formation conditions,construction parameters and temporary plugging agent concentration on the fracture geometry is studied and summarized by the experiment of temporary plugging and fracturing on the shale outcrop from Longmaxi formation.The coal reservoir of Taiyuan Formation in Shanxi Province was deposited in the early age,and the lithologic combination is a fractured reservoir with a large number of cleats between the black mudstone,sandy mudstone and thin fine sandstone.Hydraulic fracturing technology has been proposed by former researchers as the key technology to increase coalbed methane exploitation and production,but the fracture propagation is extremely complicated due to serious filtration during fracturing.According to the field and laboratory tests,the researchers concluded that the fracture morphology generally includes single-wing vertical fracture,two-wing asymmetric fracture and vertical fracture.Still,there is no complete evaluation system for construction parameters,formation conditions and other factors.Hence,in this paper,the outcrop samples of coal seam,sandstone and shale of Shanxi formation are artificially processed into different combinations to simulate coal bearing strata,and the real in-situ stress field is imitated to carry out indoor physical experiment of hydraulic fracturing to explore the mechanism of fracture extension and penetration.Carbonate reservoirs in central and Western China are characterized by deep burial heterogeneity,and the formation of carbonate reservoirs is naturally fractured with many fissures and caves.These types of reservoir conditions and construction techniques have certain effect on the initiation and propagation path of acid fracturing fractures and whether hydraulic fractures can communicate with natural weakness and form more complex fracture networks.In this paper,the carbonate outcrops from Dengsi and Qixia formation in southern Sichuan were processed and prepared into rock samples,and the laboratory physical simulation experiment of acidizing and fracturing was carried out to study and analyze the fracture initiation and expansion patterns of reservoir characteristics with different construction parameters.(1)True triaxial physical simulation study on fracture initiation and propagation mechanism of shale temporary plugging and diverting fracture.The temporary plugging agent can effectively block the initial fracture,and the pressure boosting could promote the hydraulic fracture to divert in the old fracture or activate the original natural fracture system,so as to improve the complexity of shale fracture network.The final hydraulic fracture form shows three basic types,namely,activate the cross cut fracture of weak plane,steering in the fracture and activate the weakness and simple multiple fractures.When the horizontal stress difference is less than 12 MPa,the temporary plugging agent could bridge the near wellbore or affected by the natural fracture leading to oblique angel fracture.When the stress difference is higher than 15 MPa,it is difficult to activate the natural weak plane near the wellbore,temporary plugging agent can block the initial crosscut fracture of the vertical wellbore and induce the formation of parallel multi-crosscut fractures.Increasing the injection rate can improve the activation probability of the primary weak plane and increase the fracture complexity.While the horizontal stress difference is 12 MPa,it is difficult to activate the natural weak plane near the wellbore.When it reaches 15 MPa,the influence of horizontal stress difference on natural fracture propagation becomes smaller,and complex fracture network structure can be formed when temporary plugging fracturing is carried out.(2)True triaxial physical simulation study on fracture initiation,propagation and penetration mechanism of coal bearing strata.The propagation path of the natural weak plane hydraulic fractures,such as coal cleat,has some influence.The fluid flow along the cleat system reduces the probability of the formation penetration.High displacement ensures that the hydraulic fracture penetrates the interface.When entering the coal and rock with small displacement(10ml/min)injection rate,it can fully filter and flow along the micro fissure when encountering the rock interface,resulting in low net pressure in the main hydraulic fracture.The cementation strength of interlayer is the main factor to determine the feasibility of multi-layer fracturing.(3)Physical simulation experimental acidizing fracturing of artificial fracture initiation and propagation in carbonateWhen the natural fracture or karst cave is not abundant in carbonate,gelling acid is more suitable to ensure the fracture growth,and obtain more complex fracture network as far as possible to increase the SRV acid fracturing.When the in-situ stress difference is low,the use of diverting acid as fracturing fluid can obtain complex fracture network;when the formation with natural fracture or karst cave is well developed,the use of diverting acid could make the main fracture communicate with the far-off fracture or karst cave.