Feasibility Study On Fracture-network Acid Fracturing In Offshore Tight Sandstone Reservoir

The tight sandstone gas reservoirs in A block in offshore gas field has the characteristics of deep reservoir buried,high temperature and pressure,low porosity and low permeability,which limit the natural productivity of single well.The reservoir reconstruction is the key technology for effective and economical exploitation of the gas reservoir.The conventional hydraulic fracturing technology is difficult to implement in the offshore oil field,and the fracture morphology is single.The matrix reserves can not be used effectively,and the productivity of single well is still low,which can not meet the requirements of economic development.Based on this situation,the feasibility study of acid fracturing technology which can cause complex cracks is carried out,and a new effective transformation technology is explored for the reconstruction of the tight sandstone gas reservoir in offshore.(1)This paper analyzed the lithological and physical properties,rock mechanical properties and in-situ stress characteristics of the target reservoir.Feasibility analysis of the fracture-network acid fracturing technology in the target reservoir is analyzed in three aspects which are reservoir brittleness characteristics,horizontal stress difference coefficient,and natural fracture characteristics.(2)Considering the factors such as high temperature,string safety,reservoir damage and acid pressure effect,the acid solution system of 8%HCl+ 5%SA601 + 4%SA702 +10%ethanol + 60%NaBr + 3.5%corrosion inhibitor B + 1%SA1-1 + 1.5%SAl-7 + 1%SA-18+1%SA5-5 is formed by prefering the type of acid and acid concentration and acid additives.The system has the properties of high-temperature retarding and corrosion,anti-injury,high-efficiency etching and other properties.(3)The mechanism of the acid pretreatment to reducing the rock strength is analyzed by measuring the concentration of silicon and aluminum ions in the solution after the reaction between the single mineral and the acid solution.The effects of acid concentrations on rock strength are analyzed by triaxial rock mechanical experiments.It is recommended to use 10%HCI+8%HBF4 as the pre-acicd.(4)For the horizontal well length and the heterogeneity of physical properties and difference of horizontal stress,the temporary plugging acid pressure technology is used to achieve uniform reservoir reformation.1.5%fiber is selected as temporary plugging agent and 1.5%fiber(6mm)+ 1.2%large particle(3-4mm)+ 0.6%small particle(0.8～1.2mm)were used as the seam diverting agents.The fracturing software has been used to optimize the process parameters.Through the research of this paper,a set of acid fracturing fluid system suitable for offs －hore tight sandstone reservoirs and a pre-acid solution for reducing the fracturing pressure are developed.The temporary plugging agents are selected to improving the degree of transform －ation.The research results in this paper provide the key technical support for the realization of acid fracturing in the tight sandstone gas reservoir in offshore.