Study On The Conductivity Of Acid Etching Fractures In Low Young's Modulus Carbonate Rocks

Carbonate reservoirs in the Middle East have huge oil and gas resources and a certain scale of difficult-to-use reserves,so reservoir stimulation is needed.Acid fracturing is one of the key technologies of carbonate reservoir stimulation.The acid etching conductivity is one of the most important indexes to influence the effect of acid fracturing.At present,the Middle East carbonate reservoir has the problem of low conductivity after acid fracturing.In this paper,the factors affecting the conductivity of acid etching fractures in carbonate reservoir and the methods to improve the conductivity have been studied,and the following understanding has been obtained:(1)Based on extensive and in-depth investigation of domestic and foreign experimental simulation of acid fracturing of carbonate rocks,research on the influencing factors of conductivity and acid fracturing technology,this paper defines the difficulties and challenges of acid fracturing of carbonate reservoirs in the Middle East based on their physical properties,mineral composition and rock mechanics characteristics.That is to say,carbonate reservoirs in the Middle East are characterized by strong mineral homogeneity and low Young's modulus,and it is difficult to achieve non-uniform etching of fracture walls and maintain conductivity for a long time during acid-fracturing process.The high porosity in the Middle East is the key factor for the formation of the low Young's modulus.(2)In this paper,a large number of acid fracturing model experiments of carbonate rocks have been carried out by means of acid etching test equipment and conductivity test equipment.Through the experimental data obtained,the law and degree of influence of various factors on the conductivity of acid etching fractures have been revealed,Including the influence of Young's modulus,closure pressure,the influence of rock mineral composition,the influence of acid etched fracture surface morphology,the influence of acid system and the influence of acid injection rate and acid concentration.It is found that the low Young's modulus of carbonate rocks in the Middle East has a great influence on the conductivity of acid etched fractures.For the reservoir with Young's modulus greater than 40GPa,the conductivity can be stabilized at about100?m~2·cm under high closing stress,and the conductivity can be stabilized at10?m~2·cm with Young's modulus being less than 40GPa and more than 20GPa,the conductivity can be stabilized at 10?m~2·cm,while in the reservoir with Young's modulus less than 20GPa,the conductivity basically disappears in the later stage.The conductivity of low modulus reservoir is more sensitive to closure stress.With the increasing of closure stress,the conductivity decreased by more than 98%.The carbonate rocks in the Middle East,which are extremely homogeneous in minerals(calcite content of more than 90%and in some blocks as high as 98%to 100%),are less prone to forming fracture and acid etching patterns that are conducive to flow conductivity,such as grooves,compared with rocks with more diverse mineral content.Therefore,it has been confirmed that the main controlling factors affecting the acid fracturing conductivity of the Middle Eastern carbonate rocks,namely,the low compressive strength under the low Young's modulus and the strong mineral homogeneity.Combined with the closure model of acid etching fracture,the prediction formula of conductivity considering the influence of Young's modulus is derived.And the results fit well with the experimental data.(3)Based on the comprehensive understanding of the characteristics of carbonate reservoirs in the Middle East and the main controlling factors of the conductivity of acid etched fractures,two ideas for improving the conductivity of fractures are formed:the final fracture width can be increased by increasing fracture propping strength and the permeability can be increased by improving fracture etching pattern.Then three technological strategies are put forward:carbonate fracture sanding process and non-uniform distribution of acid on fracture surface and closed acidification process.And the corresponding theoretical and experimental evaluation is completed.All the three processes can effectively improve the acid fracturing conductivity of carbonate rocks in the Middle East,and the best conductivity is when the ideal groove is formed by non-uniform acid distribution on the fracture surface.Therefore,for carbonate rocks with low modulus and strong mineral homogeneity,it is an important direction to greatly improve the conductivity of this kind of reservoir to realize the ideal seepage groove formed by non-uniform acid distribution on fracture surface.