- Email: [email protected]
Chapter 11 Mechanics of hydraulic fracturing
369
Chapter 11
Mechanics of hydraulic fracturing Hydraulic fracturing in rocks takes place when the fluid pressure within the rock exceeds the smallest principal stress plus the tensile strength of the rock. This results in tensile failure or splitting of the rock. A hydraulic fracture may be initiated by natural, geological processes in the earth whereby the fluid pressure increases and/or the smallest principal stress decreases. Artificial or man-made hydraulic fractures in petroleum activities are normally initiated by increasing the fluid pressure in the borehole to the point where the smallest principal stress at the borehole becomes tensile. Continued pumping at an elevated pressure causes the formation to split and the fracture will grow in the direction of least resistance. Some distance away from the borehole the fracture will always propagate in the direction normal to the smallest principal stress in that specific formation. As the least principal stress often is in a horizontal direction, the resulting fractures will be vertical. If we consider a vertical open hole, the picture will be as indicated in Fig. 11.1. As the figure shows, two symmetric fracture wings develop perpendicularly to the least principal stress. Hydraulic fracturing has been used commercially as a stimulation technique in the petroleum industry since the early fifties. Such fracturing jobs are designed to stimulate production from reservoirs with low permeability. This often involves pumping large amounts of fluid and solids (proppants), thus creating long fractures filled with proppants. A massive hydraulic fracturing (MHF) job may exceed one thousand cubic metres of fluid and one million kilograms of proppant. The fracture thus creates a high-permeability flow channel towards the wellbore which has a large drainage area towards the low-permeability formation. If the fracture was not filled with solid material, it would close when the fluid pressure drops. Unintentional fracturing may occur during drilling operations, often referred to as lost circulation, see also Section 9.1.2. This is a potentially hazardous situation, since the hydraulic support of the fluid column in the wellbore is reduced. The wellbore pressure may drop below the pore pressure in the formation and if the formation is permeable, formation
Fig. 11.1. Vertical fracture around a vertical well.
Chapter 11
Mechanics of hydraulic fracturing Hydraulic fracturing in rocks takes place when the fluid pressure within the rock exceeds the smallest principal stress plus the tensile strength of the rock. This results in tensile failure or splitting of the rock. A hydraulic fracture may be initiated by natural, geological processes in the earth whereby the fluid pressure increases and/or the smallest principal stress decreases. Artificial or man-made hydraulic fractures in petroleum activities are normally initiated by increasing the fluid pressure in the borehole to the point where the smallest principal stress at the borehole becomes tensile. Continued pumping at an elevated pressure causes the formation to split and the fracture will grow in the direction of least resistance. Some distance away from the borehole the fracture will always propagate in the direction normal to the smallest principal stress in that specific formation. As the least principal stress often is in a horizontal direction, the resulting fractures will be vertical. If we consider a vertical open hole, the picture will be as indicated in Fig. 11.1. As the figure shows, two symmetric fracture wings develop perpendicularly to the least principal stress. Hydraulic fracturing has been used commercially as a stimulation technique in the petroleum industry since the early fifties. Such fracturing jobs are designed to stimulate production from reservoirs with low permeability. This often involves pumping large amounts of fluid and solids (proppants), thus creating long fractures filled with proppants. A massive hydraulic fracturing (MHF) job may exceed one thousand cubic metres of fluid and one million kilograms of proppant. The fracture thus creates a high-permeability flow channel towards the wellbore which has a large drainage area towards the low-permeability formation. If the fracture was not filled with solid material, it would close when the fluid pressure drops. Unintentional fracturing may occur during drilling operations, often referred to as lost circulation, see also Section 9.1.2. This is a potentially hazardous situation, since the hydraulic support of the fluid column in the wellbore is reduced. The wellbore pressure may drop below the pore pressure in the formation and if the formation is permeable, formation
Fig. 11.1. Vertical fracture around a vertical well.