Study On Conductivity Of Acid Etched Fractures In Deep Carbonate Reservoirs Of Shunbei Area

Acid fracturing is the main transformation technology of carbonate reservoirs in Shunbei area of Tarim Basin.Shunbei reservoirs are characterized by deep burial and high temperature.Acid fracturing is often faced with the difficulties of fast acid-rock reaction,limited acid action distance,and difficult to form acid etched fracture network with satisfactory overall conductivity.However,the overall conductivity law of acid etched fractures in Shunbei reservoirs has not been understood yet.This thesis studies this problem.The experimental test method is mainly used in this thesis.On the basis of fully considering the actual formation temperature in Shunbei area,reasonably selecting the width of the experimental fractures,and combining with the typical acid-rock contact time in the field,a series of acid displacement and conductivity test experiments are carried out to deeply investigate the variation law of the conductivity of the main and secondary acid etched fractures formed during the acid fracturing in Shunbei,and the specific influencing factors of the conductivity of the acid etched fractures in Shunbei are clarified.Finally,based on the previous experimental test results,the transformation idea of improving the conductivity of acid etched fractures in Shunbei reservoir is proposed,and the calculation empirical models of conductivity of different types of acid etched fractures are established.Studies have shown that the acid-rock contact time and the type of acid have significant effects on the conductivity of main fractures.Extending the acid-rock contact time and using low viscosity acid can improve the karst quantity of main fractures and increase the conductivity of main fractures.Under the typical effective closure stress of Shunbei reservoir,the conductivity of gelled acid,cross-linked acid and in-situ generated acid is up to 28D·cm,16D·cm and 23D·cm respectively.However,when the acid-rock contact time of gel acid and in-situ generated acid increases to more than 60 and 70 minutes,there is no significant increase in the conductivity of main fractures with increasing the acid-rock contact time.The surface morphologies of secondary fractures are obviously different from main fractures,the width of secondary fractures is narrow,channel formed on the surface of secondary fractures due to acid competitive dissolution.In addition to acid rock contact time and acid type,karst quantity and conductivity of secondary fractures are also related to acid concentration at the entrance.Increasing acid-rock contact time,lower acid viscosity and higher acid concentration at the entrance can increase the karst quantity of secondary fractures,form deeper channels and obtain higher conductivity of secondary fractures;due to the formation of high conductivity channels,the conductivity of secondary fractures are generally higher than main fractures when the karst quantity is lower than main fractures.The established calculation model of the conductivity of the main and secondary fractures provides a basis for the prediction of the conductivity.This thesis provides guidance for acid fracturing process design and acid system optimization in Shunbei Oilfield.