Research On Fracture Propagation Prediction And Oil Enhancement Potential Evaluation Of Refracturing

Refracturing treatment is the main technique to improve production of low permeability and unconventional oil and gas reservoirs.Considering the changing formation conditions due to the initial fracturing and underground liquid withdrawal,it is difficult to describe the fracture propagation prediction accurately after refracturing,and the evaluation method for stimulation potential needs to be improved.Therefore,it is necessary to carry out systematic evaluation and researches on fracture propagation prediction and oil enhancement potential of refracturing.Based on main factors affecting the fracture propagation of refracturing,the fracture propagation is studied by combining laboratory physical experiments and numerical simulation,and then a systematic method for evaluating the production enhancement potential of refracturing is established in this paper.First of all,the true triaxial simulation equipment for multiple fracturing is designed and constructed.The loading of fully enclosed three independent stresses ensures uniform stress distribution,and using the autoclave as a stress support to simulate complex fracturing of multi-well.The drilling,completion and cementing process under stress loading environment is realized to restore the actual reservoir situation.The real-time monitoring of fracture initiation and propagation by acoustic emission is applied to describe fracture propagation without taking out the sample and changing the stress environment in multiple fracturing.Secondly,the fracture propagation of the cubic sample by multi-fracturing was studied on the true triaxial fracturing simulation equipment.The main factors affecting fracture propagation of refracturing,including stress distribution,injection fluid,injection flow rates,sample properties,etc.were analyzed in the design of experimental process.Besides,the existing acoustic emission fracture positioning method was optimized to improve the accuracy of fracture location,and a single-phase fluid-solid coupling model was established to verify the experimental results and used to predict the fracture propagation.Then,the multi-fracturing to initiate fracture and generate new fractures are simulated under pseudo-triaxial conditions by artificially prepared small-scale cylinder cores.The flow capacity of simple two-dimensional fractures is also studied,providing a new way for research of flow capacity of complex fractures.At the same times,the injection-production system was established through multi-well intersected fractures in samples that form complex three-dimensional fractures during multiple triaxial fracturing.Based on this,the flow capacity of multi-fracturing fractures was determined when they were opened,expanded and reorientated,and the figures of flow capacity changing of the fracture under different injection and production conditions for multi-fracturing.Finally,considering the effect of porous elasticity and propped fractures,and on the basis of flow capacity data from experiment,a refracturing stress model considering two-phase fluid-solid coupling with complex well pattern is established and a fully coupled solution is performed.Combined with the experiment data of fracture propagation,the method for predicting fracture propagation of refracturing was determined,and the parameter thresholds of fracture propagation and fracture reorientation were clarified.The numerical simulation model was used to evaluate the refracturing enhancement potential under different reorientated fracture angles.Based on the above researches,a systematic evaluation method of oil enhancement potential before refracturing was established.Combining mathematical statistical methods and neural network algorithms,it can be applied in predicating the oil enhancement potential of candidate refracturing wells.Results show that when using 30×30×30cm~3 artificial casting samples for multi-fracturing simulations,the maximum three independent stresses loading can be reached to 15MPa,and the uniformity of stress distribution of the sample meted the experimental requirements when the wellbore diameter is 10mm and the well spacing is greater than 70mm.Relevant parameter design meets the experimental requirements and fracture propagation can be accurately monitored and obtained in multi-fracturing.By increasing the injection fluid velocity and viscosity,changing the original in-situ stress,real-time multi-well injection to change the local porous elastic stress,it can effectively reopen and extend the initial fracture and induce fracture reorientation.There are significant differences in the fracture curve and the acoustic signal feedback between the reopening and extending fractures and the occurrence of fracture reorientation.The established multi-parameter fracture propagation rule can be used to determine the fracture propagation of refracturing under different conditions,and the reliability has been verified by the theoretical model.The initiation of new fractures and fracture reorientation during multi-fracturing can greatly improve the fracture flow capacity.By adjusting the injection way,the closing time of complex fractures can be effectively relieved,and the time of high fracture conductivity of fractures can be extended.The flow capacity decay rate is expressed as stepped constant pressure<stepped constant gradient flow<constant pressure<constant flow,and for stepped constant pressure injection,the flow capacity decay rate is shown as stepped constant pressure from large to small<stepped constant pressure from small to large.The established flow capacity change figures can provide a reference for the flow capacity prediction of actual fractures in refracturing.Refracturing treatment effect can be improved by established oil enhancement potential evaluation method which combining prediction of fracture propagation,evaluation of oil enhancement potential,and prediction of oil enhancement potential.The research results can provide theoretical guidance for the effective implementation of refracturing in low permeability and unconventional oil and gas reservoirs.