| Title | Effect of Natural Fracture on Hydraulic Fracture Propagation in Naturally Fractured Geothermal Reservoirs |
|---|---|
| Authors | Rahman M M, Mohammad Ali Aghighi, Seikh Abdul Ravoof and Altaf Hussain Syed |
| Year | 2010 |
| Conference | World Geothermal Congress |
| Keywords | EGS, Induced hydraulic fracture, Natural fracture, Poroelasticity, Coupled model, Fracture interaction, Angle of approach, differential stress |
| Abstract | This paper investigates interaction of induced and pre-existing fracture by coupling wellbore, induced fracture and natural fracture in a poroelastic geothermal reservoir. Possibilities of fracture crossing, bending, arrest and shear dilation for various angles of approach are investigated under different scenarios: in-situ stress state, reservoir rock and fluid properties and characteristics of natural fracture.The effect of a pre-existing natural fracture on the extension of a hydraulically induced fracture has been modeled based on poro-elastic behavior. The approach of Warpiniski and Teufel was adopted to evaluate the fracture propagation that would occur after an induced fracture intersects an existing natural fracture. This two dimensional numerical model will simulate the interaction between them which includes wellbore, formation and fractures. In a fully coupled manner model considers pore pressure change and stress state as hydraulic fracture approaches natural fracture. Possibilities of fracture crossing, bending, arrest and shear dilation for various angles of approach are investigated under different scenarios: in-situ stress state, reservoir rock and natural fracture properties. Results can be used in enhancing geothermal systems where hydraulic fracture treatments are carried out to achieve an optimal permeability enhancement. The result of this study in particular has a beneficial application in the design and optimization of hydraulic fracture treatments in naturally fractured geothermal reservoirs. The model departs radically from current models in that poro-elastic behavior is not used, uniform pressure inside the natural fracture was used in practice this is not true and effect of fracture toughness ignored. A modified leak-off model for intersecting fracture based on poro-elasticity is introduced as the leak-off is increased in the intersection. In this model a triple system of wellbore-fracture-formation is considered and the fracture will initiate first then propagate some extent then interact with natural fracture. The triple system facilitates investigating the combined impact of fluid flow and geomechanical state of the reservoir on hydraulic fracturing process. Penetration of fracturing fluid into the formation is more realistically modeled by fully coupling and reduced pressure at the fracture tip region is also considered. A poro-elastic solution for the stresses in the interaction zone has been used as a basis for hydraulic/natural fracture interaction criteria. The criteria compares favorably with the experimental results. Comparison of the numerical results with experimental results has shown that the main effect of the natural fractures is the width constriction that occurs when the induced fracture propagates into the natural fracture. Numerical and experimental studies for such propagation indicate a near wellbore width and effective length reduction due to the additional normal stress acting on the plane of the induced fracture. Possibilities of fracture arrest increases with increase in high differential stress state and shear strength of pre-existing fractures. Fracture crossing is another behavior for natural fracture with small aperture while fracture dilating for natural fracture with larger aperture. |