Premature screenouts have detrimental effects on pumping operations and zonal isolation procedures during fracturing jobs, and well productivity after fracturing jobs. The premature screenout is caused by insufficient fracture growth and inadequate fracture aperture to accept certain amount of proppant concentration. The causes are roughly classified into three types. The first one is near fracture plane twisting or discontinuity which is known as nearwellbore tortuosity. The second one is simultaneous propagation of multiple fractures from wellbore. The third one is unexpected excessive fluid leak-off from a primary fracture to natural fracture system. Among them, the third one is recognized as the most difficult one to be prevented or overcame especially for naturally fractured reservoirs. This is because the huge fluid leak off may happen not only in near wellbore regions but also in far fields away from the wellbore. In this case, a fracture design may have to be completely modified from the original one.
Various types of procedures for preventing or overcoming premature screenouts are proposed in the fracturing industry. But, judgements for effectiveness of these procedures are based on qualitative discussions through limited field experiences. To propose confident procedures responding to specific field characteristics, quantitative evaluation is required using a geomechanical model incorporating proper physics.
We try to construct a geomechanical model to explain a premature screenout occurred in an actual fracturing job, which aims to develop a domestic tight oil reservoir. The screenout is interpreted to be caused by excessive fluid leak-off from a primary fracture to natural fracture system. For the construction of a quantitative geomechanical model, a hydraulic fracturing simulator, which can model a primary tensile fracture propagation interacting with a natural fracture system, is used. The most difficult parameters to estimate are properties of a natural fracture system comprising of fracture porosity, initial fracture aperture and shear dilation effects. These parameters are constrained by extensions and shapes of observed microseismic clouds. The other parameters are estimated by integration of core test, log and injection test data. The evaluated in-situ stress state shows weak strike-slip faulting stress regime, which implies a certain degree of facilitation of shear slip on natural fractures by raising pore pressure at fracturing jobs.
By using the constructed geomechanical model, sensitivity analyses are conducted to evaluate growth of a primary tensile fracture and width of its aperture in accordance with shear dilation on natural fractures and viscosity of injected fracturing fluid. Consequently, it is revealed that the primary fracture propagation is limited to around 10m away from the wellbore and its aperture becomes less than 2mm if a degree of shear dilation on natural fractures shows stronger than those expected, or an original high viscosity attained by x-linked gel system decreases. The fracture aperture of 2mm width is close to the expected proppant admittance criteria described in the past studies. It means a potential to occur screenouts. According to the post-job laboratory test, a shortage of hydration time to make a high viscous fluid by mixing gelling agents and water is found to lower fluid viscosity. Taking into account the low viscos fracturing fluid, the constructed geomechanical model can explain the actual premature screenout. It means the model can be utilized to improve fracturing design in the future.
