Limestone reservoirs are of low permeability and high heterogeneity,and acid fracturing is often employed for reservoir stimulation in the development of such oil and gas reservoirs.After being acid etched,however,limestone changes significantly in mechanical properties.Under high closure pressure,the steady-state creep rate of acid-etched fractures increases greatly,making these fractures more susceptible to creep closure,which can lead to a decline or even a complete loss of conductivity.Therefore,revealing the creep mechanism of acid-etched limestone is crucial for the stimulation and efficient development of deep oil and gas reservoirs.In this paper,a creep pattern and a model of acid-etched limestone are proposed by investigating the creep characteristics of acid-etched limestone.Then,a triaxial creep experiment of acid-etched limestone is carried out,and the influences of creep of acid-etched limestone and the micro-asperity on the wall surface of acid-etched fractures on conductivity are studied by using the ABAQUS finite element analysis method.And the following research results are obtained.First,acid-etched limestone exhibits pronounced creep characteristics,with a significant reduction in peak strength at low confining pressures,but less influence of acidizing on peak strength at high confining pressures.Second,there is no distinct boundary between the initial creep stage and the steady-state creep stage,nor between the steady-state creep stage and the accelerated creep stage.Third,axial stress is the main factor controlling the closure of acid-etched fractures,the presence of the acid-etching softened layer significantly reduces the strength of the limestone fractures,and the stress applied on the softened layer is 4 to 6 times that within the acid-etched limestone.Fourth,the creep constitutive relationship of acid-etched limestone fractures conforms to a parallel model of Hookean and Maxwell bodies,exhibiting anelastic behaviors.In conclusion,the parallel model of Hookean and Maxwell bodies can effectively predict the creep characteristics of acid-etched limestone,so as to improve the risk prevention and control efficiency in acid fracturing process of deep limestone reservoirs.The research results provide theoretical guidance for the reservoir stimulation and efficient development of deep and ultradeep limestone oil and gas reservoirs.It is recommended to continue researching the mechanical properties of various rock types after acid etching,thereby promoting the innovation and high-quality development of limestone reservoir stimulation technologies.
关键词
石灰岩酸蚀蠕变/裂缝闭合/酸蚀软化层/微凸体/蠕变本构模型/麦克斯韦尔模型/ABAQUS有限元分析
Key words
Acid-etching creep of limestone/Fracture closure/Acid-etching softened layer/Micro-asperity/Creep constitutive model/Maxwell model/ABAQUS finite element analysis
维普
万方数据