Study On The Mechanism Of Overflow And Intrusion In Fractured Formations Of Carbonate Rocks

Fractured carbonate reservoirs have always been the focus and difficulty of exploration and development of oil and gas reservoirs.Due to the narrow window of drilling safety density,inaccuracy of prediction of formation pressure in carbonate reservoirs and fluctuation of bottom hole pressure during construction,overflow gas invasion often occurs during drilling,which poses a great threat to well control safety and delays drilling cycle.It is necessary to study the overflow and intrusion mechanism of fractured carbonate formations and explore the characteristics of gas intrusion.In this paper,the phenomena of overflow gas invasion in fractured carbonate formations are studied.The characteristics of overflow gas invasion in fractured carbonate formations are analyzed,and numerical models are established to study the underbalance gas invasion and gravity displacement gas invasion.The main research results are as follows:(1)Through theoretical research,it is found that underbalance gas invasion,gravity displacement gas invasion and leakage gas invasion are the three main gas invasion modes in fractured carbonate formations.The characteristics of overflow invasion are reservoir pressure sensitivity,diversified gas invasion and transformation,and repeated well control operations.(2)The micro-closure model and gas flow model of fracture are established,and the two models are coupled with the fracture pressure as a node.The gas invasion model of fracture considering fracture closure is formed jointly.The dynamic law of gas invasion and dynamic change of fracture are obtained,and the influence of fracture geometry and pressure difference on the overflow law is analyzed The research shows that after the high-pressure fracture is drilled,a short period of underbalance overflow will occur,and the pressure drop and overflow rate of the closed fracture will change obviously.The larger the width,the larger the radius and the pressure difference,the larger the overflow rate.The model considering fracture closure reveals the overflow law under natural conditions.Compared with the case without considering fracture closure,the model has slower pressure drop,faster overflow rate,more overflow volume and longer overflow duration.(3)CFD is used to simulate the gas-liquid two-phase flow law of gravity displacement in fixed-pressure boundary and closed-boundary fracture.A numerical model of gravity displacement is established.The dynamic change of gravity displacement rate of fixed-pressure boundary and closed-boundary fracture is obtained.The influence of different fracture shapes and operation parameters on gravity displacement law is analyzed.The research shows that gravity displacement overflow is inevitable for closed fractures with high dip angle.The larger the fracture opening and dip angle,the longer the fracture penetration length,the smaller the wellhead backpressure(the greater the density of drilling fluid),the larger the leakage rate and overflow rate of fractures,the larger the viscosity of drilling fluid,the smaller the leakage rate,and the longer the fracture extension length at constant pressure boundary,the smaller the displacement rate.The gravity displacement overflow can be effectively alleviated by applying wellhead backpressure+low density and high viscosity drilling fluid in field construction.The research results enrich the basic theory of carbonate formation overflow,and provide theoretical support for the safe and efficient development of carbonate reservoirs.