Research On Factors Influencing Effective Carbonate Fracture Volume Caused By Acid Fracturing

The complex geological structure and discontinuous reservoir collective of carbonate reservoirs make acid fracturing the core means for such reservoirs to construction,stimulations and stabilize production.With the deepening of reservoir understanding and the development of acid fracturing technology,traditional double-wing fracture can no longer fully communicate reservoir.In addition,the situation of natural fracture development,volume acid fracturing technology has emerged in order to achieve full reservoir stimulation and enhance single well production.Therefore,the evaluation of effective acid fracturing volume is particularly important.At present,the effective acid fracturing volume is mostly calculated by acid wormhole filtration model,but in carbonate reservoir,natural fractures are main seepage channels,and its communication range can better reflect effective acid fracturing volume.Simultaneously,the net pressure in fracture calculated by hydraulic fracturing is not equivalent to net pressure in fracture calculated by acid fracturing.Therefore,based on the expansion of secondary fractures,it is necessary to establish calculation models of effective volume and net pressure in acid fracturing fracture,so as to clarify the influencing factors of effective fracture volume.The calculation model of rock mineral dissolution volume was established based on acid-rock reaction.The calculation method of effective acid action distance was established in combination with acid filtration effect.And the calculation model of net pressure in acid fracturing fracture was established based on dynamic change of dissolution acid fracture width.The results shows that with displacement increasing from 2m~3/min to 6m~3/min,the hydraulic fracture width increases from 9.223mm to16.217mm,and the net pressure in hydraulic fracture increases from 6.081MPa to10.693MPa.The acid fracturing fracture width increases from 10.348mm to 19.592mm,and the net pressure in acid fracturing fracture increases from 5.522MPa to 9.738MPa.On the whole,the net pressure in acid fracturing fracture is less than that in hydraulic fracture.Based on fracture mechanics,a numerical model of secondary fracture propagation was established to analyze the propagation of natural fractures when hydraulic fractures intersected with natural fractures under different fracture intersection angles,different in-situ stress environments,different in-situ stress differences,and different intra-fracture net pressures.The results show that the smaller the intersection angle,the easier the secondary fracture to initiate and extend.The main fracture directly pass through natural fracture under low-level principal stress,while the main fracture extend along natural fracture under high-level principal stress.When in-situ stress difference increases from 0MPa to 8MPa,the spread distance of acid fracturing fracture along natural fracture decreases gradually.The extension length of secondary fracture increases with the increase of net pressure in fracture.By extracting ideal model of effective acid fracturing volume,the calculation method was established to clarify the influencing factors of effective volume.The results show that in the direction of fracture length,it mainly manifests the effect of acid-rock reaction rate,acid filtration rate,acid displacement and dosage on the effective acid action distance.While in the fracture width direction,it mainly manifests rock properties and net pressure in fracture on the propagation of secondary fractures and further on the effective volume of acid fracturing fracture.In general,the calculated volume of secondary fracture propagation is larger than the calculated volume of vermicular filtration loss,and the increase amplitude of the volume decreases with the increase of the reaction rate.The net pressure has a greater influence on the effective volume than the effective acid etching distance.