Vertical and horizontal fractures
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Vertical fracture occurs in deep reservoirs when fracture gradient is less than overburden gradient
Horizontal fracture occurs in shallow reservoirs
Fracture occurs n a direction vertical to the smallest principal stress i.e. the minimum work
Two basic methods of hydraulic fracturing
1-Proppant fracturing
Proppant fracturing requires that small particles be pumped with the fluid into the fracture. An effort is made to pack the fracture with a bed of these particles in order to support the walls of the fracture to form a conductive path to the wellbore.
PROPPANT FRACTURING
The objective of proppant fracturing: is to pack the dynamic fracture with proppant (small particles) so that when the fracture treatment has terminated and production commences, the fracture will remain conductive. The need to pack the fracture with some form of propping agent was recognized early in the development of the process.
Production from fractured wells not propped declined rapidly.
One of the important design considerations will be to select a fluid capable of transporting and holding the proppant particles in suspension until the fracture has closed.
Proppant Types:
· Sand.
· Sintered bauxite
· Ceramics.
1-Sand
Sand has proven to be successful as a proppant for all types of reservoirs, and it is less expensive than other types of proppant.
Sand for use as proppant should not contain more than 5 wt% fines which, if present in excessive quantities, reduce the fracture conductivity.
Advantage:
When crushed, it breaks into smaller fragments, rather than being powdered. This particular advantage helps to maintain high fracture conductivities even when the closure stresses supported by the proppant are large
2-Sintered bauxite
A high-strength proppant (compressive strength in excess of 1 x 105 kPa), which does not crush as readily as sand under high closure stresses.
Bauxite is denser (pp - 3400-3800 kg'm3) than sand (2650 kg/m3
The fracture fluid designed to transport bauxite will have to be more viscous and hence more expensive than a fluid that will transport sand.
3-Ceramics
Other high-strength proppants have been developed which appear to have advantages with respect to sintered bauxite: however, these are not yet widely applied.
Propped Fracture Conductivity
FC = wf kf
Wf is the final average fracture width
Kf permeability of proppant-packed fracture
FC has the dimensions of length cubed; it may be reported as darcy-fcet, darcy-inches, or even millidarcy-feet.
Fracture permeability.
Final fracture permeability is strictly a function of the diameter of the proppant particles used in the treatment. According to the Blake-Kozeny equation
dp is the diameter of the proppant particle
is the porosity of the packed, multilayer bed of proppant particles.The fracture permeability increases with the square of the proppant particle diameter.
Therefore, it is desirable to use large proppant particles. Actually, the size of the proppant is an optimization problem that must always be settled on economic grounds. Larger particles will require more expensive fluids to transport them. The optimum will depend on a large number of factors, all of which will be discussed later.
Fracture width.
The final fracture width is strictly related to the concentration of proppant in the fracture when it closes.
For a well-designed fracture fluid, proppant settling is minimal.
The final fracture width is strictly related to the concentration of proppant in the fracture when it closes.
For a well-designed fracture fluid, proppant settling is minimal.
is the average dynamic fracture width at the end of pumpingmi is the mass of proppant per unit volume of fluid
is the density of proppant
is the mass of proppant per total volume, including both proppant and fluid.
The effect of closure stresses.
The fracture conductivity can be calculated by the multiple of wf times kf.
This calculated conductivity will exceed the field value when the closure stresses exerted by the overburden become large. In this case, the proppant will embed into the formation causing the actual fracture width to be less than that calculated and also proppant crushing may cause the effective proppant radius to be reduced, thereby reducing the permeability of the fracture.
Closure stress = PBISIP - Pwf by maintaining the bottomhole welt flowing pressure at a high level, part of the overburden stresses can be supported by the fluid in the fracture. Generally, however, to produce the well at an economic rate, pwf is much less than the reservoir pressure (large drawdown) and the proppant must support nearly the entire overburden.
The fracture conductivity can be calculated by the multiple of wf times kf.
This calculated conductivity will exceed the field value when the closure stresses exerted by the overburden become large. In this case, the proppant will embed into the formation causing the actual fracture width to be less than that calculated and also proppant crushing may cause the effective proppant radius to be reduced, thereby reducing the permeability of the fracture.
Closure stress = PBISIP - Pwf by maintaining the bottomhole welt flowing pressure at a high level, part of the overburden stresses can be supported by the fluid in the fracture. Generally, however, to produce the well at an economic rate, pwf is much less than the reservoir pressure (large drawdown) and the proppant must support nearly the entire overburden.
Graph showing the permeability of a propped fracture as a function of the closure stress.
Proppant Settling Velocities
The selection of a fluid is one of the critical steps in the design of a fracture treatment. One of the important properties required of the fluid is an ability to transport and hold the proppant in suspension. It is important to be able to calculate the rate at which particles settle under the influence of gravity.
Two different types of fluids will be considered here
1- Non-Newtonian polymer solutions
2- Foams
1-Non-Newtonian fluids
When a particle settles in a fluid under the influence of gravity, it reaches a constant velocity so that the frictional forces are in balance with the gravitational forces. For Reynold's numbers less than about 2, that is, for
The selection of a fluid is one of the critical steps in the design of a fracture treatment. One of the important properties required of the fluid is an ability to transport and hold the proppant in suspension. It is important to be able to calculate the rate at which particles settle under the influence of gravity.
Two different types of fluids will be considered here
1- Non-Newtonian polymer solutions
2- Foams
1-Non-Newtonian fluids
When a particle settles in a fluid under the influence of gravity, it reaches a constant velocity so that the frictional forces are in balance with the gravitational forces. For Reynold's numbers less than about 2, that is, for
The settling velocity (vs)
The apparent viscosity depends on shear rate and is therefore not a constant. Slattery and Bird have shown that for particles settling in a quiescent non-Newtonian fluid.
2-Foams
The settling of proppant particles in foams must be a complex function of the wettability of the particles, the quality of the foam, and its stability.
No general theory has been presented which shows the relationship of these factors.
Design and optimization of fracture processes
The final design will be best in some economic sense and requires different considerations:
2-Foams
The settling of proppant particles in foams must be a complex function of the wettability of the particles, the quality of the foam, and its stability.
No general theory has been presented which shows the relationship of these factors.
Design and optimization of fracture processes
The final design will be best in some economic sense and requires different considerations:
1-Proppant fracture
1-Selection of fracture fluid and additives
2-Design of proppant fracturing treatments
3-Practical considerations in designing fracture
2-Selection of fracture fluid and additives fluid properties:
1-Low fluid loss
2-Ability to carry and suspend the proppant
3-Low friction loss
4-Easy to recover from the formation
5-Compatible with formation fluids and nondamaging
6-Reasonable cost
1-Selection of fracture fluid and additives
2-Design of proppant fracturing treatments
3-Practical considerations in designing fracture
2-Selection of fracture fluid and additives fluid properties:
1-Low fluid loss
2-Ability to carry and suspend the proppant
3-Low friction loss
4-Easy to recover from the formation
5-Compatible with formation fluids and nondamaging
6-Reasonable cost
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