Feasibility Demonstration Of Hydraulic Fracturing And Acid Fracturing Composite Stimulation Technique In Carbonate Reservoir

Carbonate is an important type of oil and gas reservoir.The reserves and production of oil&gas in carbonate reservoirs have already accounted for half of global total.Due to the poor permeability and complex reservoir spaces in partial carbonate reservoirs,various reservoir stimulations are applied in oil&gas exploration and exploitation in carbonate reservoirs.The acid fracturing,hydraulic fracturing,acid-carrying sand fracturing and other stimulation technologies of carbonate reservoir which cannot form a fracture with deep penetration and high conductivity,are difficult to meet requirements of production.However,effective penetration distance of living acid restricts the application of acid fracturing in deep,high-temperature carbonate reservoirs.In addition,severe fracturing fluid leakoff in fractured carbonate reservoirs make pump pressure sensitive to Proppant concentration.It is difficult to achieve near-wellbore fracture conductivity by high proppant concentration.In a word,it's difficult to build deep-penetration fracture by a single stimulation method.It is necessary to study the composite stimulation technique that can take advantages of various fracturing technologies and achieve deep penetration and high conductivity of fracture.Based on these issues,aiming at improving penetration distance of stimulation method,this thesis,on the basis of the previous research,put forward the combination of hydraulic fracturing and acid fracturing to avoid weaknesses and achieve deep stimulation of carbonate reservoirs.This thesis takes advantage of rock mechanics,fluid mechanics,acid-rock reaction dynamic mechanism,numerical analysis and computer programming to demonstrate the major theoretical issues which is involved in the new technology.The main works that have been done in this thesis are summarized in four aspects,(1)Proposing a new composite stimulation technique of carbonate reservoir that connected hydraulic fracturing and acid fracturingBased on the analysis of carbonate reservoirs' geological characteristics,and the difficulties in the stimulation of carbonate reservoirs,technique limitation of existing stimulation methods for carbonate reservoirs,hydraulic fracturing and acid fracturing composite stimulation technique has been proposed.Aiming at improving fracture penetration,a principle model about the connection between acid-etched fracture and propped fracture has been established.(2)Analyzing the characteristic of proppant distribution in fracture and establishing a new model for propped fracture conductivityBased on the mass conservation equation and momentum equation,a mathematical model including the flow of sand-carrying fluid,proppant settlement and transport was established,which is coupled with PKN fracture propagation model to be solved.What's more,a new model for propped fracture conductivity,considering multi-scale characteristics of proppant with different sizes,has been established.(3)Analyzing effective penetration distance of living acid and acid etching fracture conductivity affected by multi-factorsA non-steady-state acid flow reaction model,considering the effects of formation heat transfer and the heat of acid-rock reaction,has been established,which is coupled with fracture temperature model to be solved and predict the effective penetration distance of living acid and acid etching fracture conductivity.(4)Revealing the connection characteristic of propped fracture conductivity and acid etching fracture affected by multi-factorsBased on the hydraulic proppant transport model and the acid flow reaction model,the distribution of the acid etching conductivity and the propped fracture conductivity in the direction of the fracture length has been analyzed and discussed.Additionally,the thesis studied the value and position of conductivity connection point under a series of engineering parameters and material parameters included liquid volume,acid concentration,and so on.This study indicates that combination of hydraulic fracturing and acid fracturing could form a longer combination fracture to enhance oil&gas recovery compared with a single stimulation method.Adjustment of construction parameters and fluid characteristic has significant guidance for the connection of conductivity.Feasibility of the composite hydraulic fracturing and acid fracturing has been demonstrated theoretically.