Fracture Initiation And Propagation Induced By Water Imbibition In Shale

Horizontal well and multi-stage hydraulic fracturing are the general mode for the commercial development of shale gas plays,and a wide range of contacting and interactions between shale and the water-based working fluids always exist during the drilling-completion,stimulation and prodution of shale gas wells.The shale-water interactions have invevitably become the key for water-based drilling fluid improvement,well instability and fracturing fluid filtration control,flowback operation optimization and nanoscale gas-water flow mechanism exploration.For the stimulated shale gas wells,field statistical analysis shows that wells with low fluid recoveries usually have high producticities.Fracture extension and micro-scale stimulations induced by shale-water interactions is considered as one of the main reasons,and this also become the basis of well shut-in operation after stimulation.However,with respect to the water imbibition mechanisms and the mechanical process of fracture initiation and growth caused by shale-water interactions,it still requies systematic,modeled and quantitative researches further,so that the potential impact of water retention on well production could be clearly answered,and the well shut-in time could be well-founded.Taking the Longmaxi shale in Sichuan basin as subject,this paper carries out a series of researches focusing on the shale fluid imbibition and fracturing initiation.Detailedly,this paper researches the water imbibition process and the imbibied fluid distribution in shale,and then investigates mechanisms about how the shale-water interactions cause the reduction of shale strength and induce fracture initiation and propagation.Based on the case studies of fracture initiation and propagation under in-situ conditions,we also assess its influence on the final well producingn rate.The main research results included the followings.This paper proposed a new pore-fracture fluid imbibition model in nanoporous shale and researched the fluid imbibition process,imbibied fluid distribution and ranges.Considering capillary force,oismotic pressure,pore structure parameters,nanoscale flow effects in shale matrix and the displacing efficiency of imbibition fluid,a new pore-fracture fluid imbibition model was proposed.Shale imbibition experiments verified that this model had an accuracy of more than 90%.According to the model predictions,the imbibied fluid stored in shale fractures and pores accounted for 22.3%and 77.7%.During shut-in period the fluid imbibition distance in shale fractures and matrix would be no more than 231 cm and 17 cm.Experiments also showed that water imbibition can induce fracture initiation.Shale core obervation,CT scan and NMR also indicated that after water imbibition the fractures in shale samples had an obvious extention and some new cracks may emerge,and the corresponding sample porosity and permeability also had a different degrees of increase.A quantitative model was put forward to evaluate the rock strength reduction caused by shale water imbibition.Based on the Young equation and Gibbs-BET equations,the paper computed the surfce free energy changes caused by liquid water and vapour adsorption,and then by relating the energy release with the rock strengh parameters on the basis of Griffth Theories,we quantitatively evaluate the rock strength weakening induced by shale-water interactions.Also,with respect to the shale hydration phenomenon,this paper illustrated the micro forces during shale clay mineral hydration process and emphasize the critical role of hydration force in the damages of clay mineral layers,which effectively expained the features of huge hydration swelling pressure and small hydration strain in shale hydration process.The subcritical fracture growth in shale was first investigated by considering shale-water interactions.Limited by the currently testing methods of rock fracture subcritical growth velocity,this paper proposed a new method which based on the rock uniaxial/triaxial stress and strain curves to calculate the subcritical fracture growth velocity in shale.Results of 18 shale samples showed the velocity was about 1.738×10-6～20.795×10-6m/s,and samples imbibied with fluid usually had a higher velocity although this value was affected by fluid types,imbibition time and sample confining pressure.Then by computing the mechanical energy release rate of the shale fracture tips,the mechanisms of fracture subcritical growth induced by capillary pressure was well illustrated.According to the mechanism studies,the positive effects of shale-water interactions on fracture fracture initiation and propagation in shale was elaborated.Case studies showed that during shut-in period of shale gas wells,if the rock strength weakening caused by shale-water interactions was involeved in models,the down threshold pressure that induced fracture initiation would be lower than the situation which did not consider the shale-water interactions,and if the subcritical fracture growth was also involved,the down threshold pressure would get a further decrease,approaching the minimum horizontal principal stress.Finally,this paper evaluated the influence of shut-in operation on well producivity.Well production simulation showed that the shut-in operation in shale gas well had an obvious promotion on gas cumulative amount,which may benefit from the continuous micro stimulation caused by shale-water interactions,even thouth there may have some additional formation damages.Field production data also confirmed that proper shut-in operation can promote gas production,and a proper shut-in time for a stimulated shale gas well can be determined by referring to the fracture fluid pressure under in-situ conditions.When it decreases to the minimum horizontal principal stress,the shut-in operation should be stopped.