Jumat, 01 April 2011
Fracturing Horizontal and Deviated Wells
It is now commonly accepted that good completion and stimulation practices are as essential to the success of a horizontal (deviated) well as they are to that of a vertical well. It is therefore no longer unusual to see hydraulic fracturing treatments performed on horizontal wells. Also, it has become clearly understood that the relative positions of the fracture and the well have a profound effect on the performance.
For vertical wells, there are only two basic well-fracture configurations: a horizontal (pancake) or a vertical fracture. Horizontal wells, on the other hand, have three basic configurations. In a shallow formation, a horizontal well can also be intersected by a horizontal fracture. If a horizontal well is drilled in a deeper formation and subsequently fracture simulated, a vertical fracture will be created. Depending on the well’s relative orientation to the minimum horizontal stress direction, this fracture will be either longitudinal (lateral axis of the fracture coinciding with the wellbore axis) or transverse (lateral axis of the fracture perpendicular to the wellbore axis). The number of transverse fractures intersecting a well can be substantial; three to five transverse fractures are not exceptional). ( Figure, Possible well-fracture configurations). A longitudinal fracture provides much more fracture-well communication than a transverse fracture. Multiple transverse fractures, however, may provide more fracture surface.
(When we list these basic fracture-well configurations, we understand that these are idealized limiting cases. In practice, intermediate configurations may also occur.)
Once a well is drilled, there is little or no way to influence the well-fracture configuration. Therefore, knowing the minimum horizontal stress direction and directing the well according to it should be of major concern prior to drilling.
To understand the performance of these well-fracture configurations, one has to keep in mind that the horizontal anisotropy of the permeability field usually follows the stress anisotropy. The created fracture will be not only perpendicular to the minimum horizontal stress but also perpendicular to the minimum horizontal permeability. Therefore, hydraulic fracturing cannot do too much to overcome the limitations posed by a small horizontal permeability. On the other hand, hydraulic fractures are extremely useful in overcoming limitations posed by small vertical permeability or unusually large pay thickness.
The optimal configuration depends on the permeability contrast (both horizontal and vertical), and on the pay thickness to well length ratio. For high permeability, the longitudinal configuration is advantageous, especially because there is no need to create large fracture width (in contrast to high-permeability fracturing of vertical wells). For low permeability formations, multiple transverse fractures are likely to perform better.
Turns and Twists
One of the main problems with a transverse fracture is that the fracture initiates in parallel to the well axis, and then turns to be aligned to the far field stress. The turning and twisting near- wellbore part of the fracture may cause excessive fracturing pressures during job execution; even worse, it may cause a choking effect during production. The phenomenon is often called near- well tortuosity.
Fracturing deviated wells is a great challenge because it involves several unresolved issues and a variety of possible scenarios. A careful perforation strategy favoring the far field stress direction might be useful if fracturing a deviated section is unavoidable.
The uninvited effects of tortuosity can be partly cured by using a special technique known as breakdown with proppant slugs. The technique involves injecting limited volumes of proppant slugs during pumping of the pad. The proppant slugs act similar to abrasive drilling, preparing a wider channel for the subsequent slurry.
Another suggestion for avoiding tortuosity problems for transverse fractures is to use very short perforation intervals (1.5 times the well diameter.)
Multiple transverse fractures need zonal isolation and several sub-treatments. The creation of a longitudinal fracture can be also accomplished in several isolated steps.
(sources from many litterateurs)