Research And Application Of Decline Prediction Model For Long-fracture Conductivity

Recent years,deep and low-abundance extra-low-permeability reservoirs were mainly developed by long-fractures waterflooding(half fracture length is more than 200m).However,in these processes,reservoir performance decreases due to fracture conductivity damage caused by proppant embedment,deformation,crush and diagenesis under the effective closure pressure.Now inadequate attention has goes to researches on how fracture conductivity damage is related to proppant behavior,how conductivity loss affects seepage field and well deliverability.Moreover,fracture conductivity damage cannot be really taken into consideration by available numerical reservoir simulators.One of the disadvantages is that no conductivity decline will be addressed when determining operating constraints.Consequently,it is of great necessity to conduct studies on the laws of fracture conductivity decline related to proppant during long-fracture waterflooding and how to determine the operating constraints with respect to the decline.It is supposed that the findings of this dissertation will provide theoretical fundamentals and technical supports for the efficient development of these reservoirs.Based on the principles and theories in rock mechanics,elastic-plastic mechanics,material mechanics,contact mechanics and geometry,a novel model was established to predict fracture conductivity.It is noted that the effects of proppant behavior,such as embedment,deformation,crush and diagenesis,were taken into consideration in this model.Then,the sensitivity analysis was conducted by the developed model to investigate the fracture conductivity damage and laws of fracture conductivity decline along fracture length with time.In addition,numerical reservoir simulation was conducted to study how fracture conductivity loss affects seepage field in reservoirs.Then,two predictive models for long-fracture single-well single-phase steady deliverability were proposed to investigate the fracture conductivity difference along fracture length and used to perform sensitivity analysis.On this basis,a method,called two-way asynchronously coupling analytical solution and numerical simulation at selected time steps,was used to investigate how drawdown pressure affects long-fracture single-well two-phase non-steady deliverability.Finally,the well patterns,the operating constraints and the adjustment approaches at different stages were studied by the above mentioned method for a long-fracture waterflooding reservoir.It is found that the fracture conductivity is subjected to proppant particle size,elastic modulus,Possion's ratio,effective closure pressure,and reservoir temperature.The conductivity of long fractures for either injection or production is found to decline with time.Variations exist in fracture conductivity along with fracture length,and change with time too.The production sub-fractures closer to the wellbore suffer more serious conductivity decline.The injection sub-fractures farther to the wellbore,however,suffer more serious decline.In contrast,the production long fractures suffer conductivity decline more serious than the injection sub-fractures.As the conductivity decline deteriorates,the reservoir development efficiency debases,the average waterflooding swept area diminishes,the flow tends to diverge the linear flow pattern,but the remaining oil increases.The flow in the middle reservoir region shows more similarity to the linear flow pattern,and this similarity attenuates from the middle to the producer or the injector.The larger the drawdown pressure is,the higher the well deliverability,but the more serious the conductivity decline of a long production or injection fracture.As a result,wells suffer larger reductions in deliverability.Therefore,the drawdown pressure needs to be optimized according to the practical waterflooding conditions.