Mechanism and Technology of Fracture Height Control in Low Stress Reservoir of Carbonate Rocks
Author:Mi Qiang Bo
Supervisor:yi xiang yi
At present,deep marine carbonate reservoir mainly distributed in Tarim Basin,Bohai Gulf Basin,Ordos basin and Sichuan basin,which has the characteristics of deep burial,various reservoir types and complex distribution of oil and water.Acid fracturing is one of the main methods of carbonate reservoir stimulation.After years of development,a mature acid fracturing technology has been developed,but there are still some technical problems.Taking Tahe oilfield as an example,the carbonate reservoir is thick,and there is no obvious stress shielding layer,so the artificial fractures propagate easily in height,and we can not get an ideal artificial fracture length and satisfactory simulation effect.At the same time,excessive extending of the fracture height will make the fracture connect with the water at the bottom of the reservoir,which caused water flooding and lead to the failure of the stimulation treatment.Therefore,it is necessary to carry out the research on the theory and technology of fracture height control,develop a targeted technology which applies to small / no stress shielding reservoir,and improve the stimulation effect.At present,the technology of fracture height control in carbonate reservoir draws on the experience of theories,models and methods of sandstone reservoir,the fracture height is controlled by optimizing parameters of the treatment,building artificial barrier and so on.Deep carbonate reservoir contains lots of natural fractures(caves)and has small vertical stress difference,and the conventional artificial barrier using fine quartz sand or ceramsite can not meet the demand of height control in carbonate reservoir because of the good permeability and poor stability of the barrier.In this paper,a new method of fracture height control was developed based on the characteristics of the deep carbonate rock reservoir,analysis of rock mechanics and stress distribution.Considering the influence of natural fractures and caves on hydraulic fracture propagation,and the effect of acid solution on the fracture height extension,a precoated diverting agent applied to deep carbonate reservoir was developed.The main achievements of this paper are as follows:(1)Considering the influence of natural fractures and karst caves,the finite element numerical stress model is established.The influence of the combination of fractures and caves and their occurrence on the stress distribution is analyzed,which provides a basis for the study of fracture height propagation.The simulation results show that the angle between the natural fractures and the direction of the maximum principal stress,cavern size and horizontal stress difference will influence the distribution of the Local stress field.The stress filed in fracture tips changes greatest,and the closer to the fractures and caves,the greater the stress changes.(2)Physical simulation experiments of three-dimensional fracture propagation considering the influence of natural fractures and caves were carried out,and the influence of natural fractures occurrence and fracture length to the propagation of hydraulic fractures was evaluated.In case of caverns,the propagation of hydraulic fractures in defferent displacement and viscosity of fracturing fuild considering horizontal stress difference was simulated.(3)Considering the influence of acid to fracture progation in vertical,a novel viscoelastic surfactant-based self-diverting acid was developed,experiments reveal that the novel VDA system has notable reaction retardation,better acid diversion,and tremendous potential of field applications.(4)A settlement model of artificial barrier in hydraulic fracture is set up.A new type of fracture height control agent is developed based on the characteristic of fracture propagation in carbonate reservoir and the damand of fracture height control.The diverting agent becomes impermeable artificial barrier at the formation temperature so as to increase the vertical stress difference.(5)The fracture height controlling treatment optimization was carried out,the influence of thickness and strength of artificial barrier to fracture height control was analysed,the process parameters were optimized.The above mentioned research has proposed a new technology of fracture height control aimed at deep carbonate reservoir with low stress difference.Simulation results show that it has a good effect on carbonate reservoir when the stress difference is less than 2MPa in vertical.The new technology provides technical support for this type of reservoir to improve the effect of acid fracturing.