| Carbonate reservoirs are rich in oil and gas resources,which makes a great contribution to the world oil and gas production.Tahe Oilfield is located in the northern Tarim Basin,which is a fractured-vuggy carbonate reservoir with great development potential found in China.However,due to the complex geological conditions of the block,it is a high-temperature,high-pressure and low-permeability reservoir,and oil and gas are mainly rich in trapped pores.Acid fracturing is the main method of carbonate reservoir stimulation.At present,although a large number of experimental studies have been carried out on carbonate rock acid fracturing,there are some problems:(1)The minimum critical displacement of effective fractures formed by acid injection into the bottom hole is not clear;(2)The research model is relatively simple,and the type of acid used is mainly gelling acid with hydrochloric acid as the matrix.Due to the rapid reaction of hydrochloric acid and carbonate rocks,the concentration of the acid solution decreases rapidly during the diffusion process of the fracture surface,and the effect of the acid solution at the end far from the wellbore is not obvious.Therefore,it is necessary to compare and screen the acid fracturing stimulation performance of other acid systems.In this thesis,the gelled acid,solid acid and authigenic acid used in the field acid fracturing operation are applied to the true triaxial acid fracturing experiment in the laboratory environment.By preparing artificial rock samples with different pore distribution ranges,the minimum critical displacement of acid fracturing fracture initiation and the interaction behavior of hydraulic fracture and pore encounter under different acid systems and acid concentrations were explored.The research results are as follows:(1)Minimum critical displacement of gelling acid,solid acid and autogenous acid for acid fracturing:10%,15%,20%gelling acid indoor minimum critical displacement are10-11m L/min,16-18m L/min and 18-20m L/min respectively,corresponding to6.3-6.9m~3/min,10-11.3m~3/min and 11.3-12.6m~3/min on site.10%,15%,20%solid acid indoor minimum critical displacement are 8-9m L/min,9-10m L/min and 10-12m L/min,corresponding to 5-5.6m~3/min,5.6-6.3m~3/min and6.3-7.5m~3/min on site.8%,10%,12%autogenous acid indoor minimum critical displacement are10-11m L/min,11-12m L/min and 12-13m L/min,corresponding to 6.3-6.9m~3/min,6.9-7.5m~3/min and 7.5-8.2m~3/min on site.(2)The interaction behavior of hydraulic fractures and pores of fracture-cavity carbonate rock samples:During the acid fracturing process of the fracture-vuggy sample,the fracture propagation path is controlled by the horizontal stress difference in the near-wellbore area,and is dominated by the attraction of the hole far from the wellbore and close to the holes:increasing the pore-hole distribution angle,the fracture turning angle increases to a certain extent but less than 45°turning angle.when the crack activates the hole,whether the crack will break through the hole wall and continue to expand is related to the acid concentration and injection displacement:the same acid system,the acid concentration increases,the possibility of crossing the activated hole is greater;for the acid-rock reaction rate,gelled acid is greater than solid acid and greater than authigenic acid.Under the same displacement,the gelled acid is more likely to traverse the activated pores,followed by solid acid and authigenic acid has the weakest penetration ability.With the increase of injection flow rate,cracks are easier to cross holes.According to the fracturing curves,fracture expansion area and activated pores of gelled acid,solid acid and authigenic acid samples obtained from experiments,under indoor conditions,gelling acid has the best reformation effect,authigenic acid is the second,and solid acids are less effective. |
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