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Laboratory Investigation of Fracture Propagation in Multiple Fracturing with Complex Well Interference


 Fracture propagation is a complicated process in multiple fracturing treatment, especially when initial stress field altered after long-term producing. It has been verified that injection and withdrawal of underground liquid alters stress field then leads to fracture reorientation, which benefits wells by stimulating areas with more residual oil and less depleted pressure. To investigate fracture propagation with altered stresses, multiple fracturing treatment are simulated by inducing stresses change with complicated well patterns under true-triaxial condition.
 Multiple fracturing treatment are performed on cubic samples with 30cm side length in laboratory on a self-assembled true-triaxial fracturing system. System is composed of five parts, true-triaxial apparatus, hydraulic injectors, digital data acquisition, drilling and completion, acoustic emission. Samples are loaded with independent confining stresses and then wells are drilled and completed with stresses applied. Well patterns of three-spot and five-spot are located on the samples to simulate field well patterns. Then multiple fracturing treatments are performed with fracture propagation monitored using AE by changing injecting pressure of each well and independent confining stresses applied on the samples.
 Process of fracturing and refracturing treatment is successfully simulated with two ways of altering initial stress field, changing the confining stresses and inducing stresses change by injecting from different located wells. Poroelastic theory is used to illustrate two kinds of reorientation. It is shown that fracture initiates and extends perpendicular to the minimum horizontal principal stress under initial confining stresses during the initial fracturing. By changing the confining stress, increasing the injecting flow rate and using high viscosity of injecting fluid, new fracture initiates and reorientates to a new direction which is perpendicular to the former fracture in refracturing. In the five-spot wells pattern, initial fracturing is performed on the middle well with same low backpressure applied on the other four edge wells and fracture obtained is just perpendicular to the minimum horizontal principal stress as it should be. However, by changing the injecting pressure of different located edge wells which are supposed to provide porous pressure from different directions to the middle well, fracture orientation in multiple fracturing can be changed to over 30° in angle. All fractures initiated and propagated are successfully monitored by acoustic emission and later verified by slicing the samples.
 Large blocks of samples are used to eliminate the scaling problem as much as possible, besides, simulations of fracturing and refracturing are all monitored by acoustic emission to depict the propagation of fracture. Angles of fracture reoriented are obtained and evaluated in the complicated fracturing process which can be used to quantitatively study the effect of stresses change to fracture reorientation based on poroelastic theory. Complicated well patterns are applied to reveal well interference.