How do you design hydraulic fracturing for different reservoirs?

There are two antipodes of reservoirs which require a different approach to hydraulic fracturing design and optimization: -conventional reservoirs where the fracture fluid has the ability to hydraulically displace native in the reservoir rock -shales or tight source rocks where applying overwhelming pressure, creates massive shear failure The majority of unconventional development targets reservoirs somewhere in between those two types, extending the scope by using horizontal drilling and multiple transverse fracturing stages.

The most important aspect of fracture geometry is the orientation of the principal stresses. Optimally the axis of a horizontal well should be normal to the maximum stress, making the direction of hydraulic fracturing transverse. Land positioning most often requires North-South or East-West directions especially if very long lateral lengths are desired for lower development cost. One has to keep in mind that extensive development causes stress interference which makes an ideal design irrelevant in the course of time. For those two reasons the optimal direction cannot be always followed, but its understanding will help alleviate well to well interference and the timing of operations.

During conventional hydraulic fracturing it is a must to use designing software like a Fracpro, Gohfer since you need to know your closure pressure and fluid efficency so you calculate your pad volume to avoid a screenout. However, it is very hard to get a fracture closure during minifrac on tight unconvetional reservoirs (shale), so people rarely do minifrac tests before hydraulic fracturing on tight formations.

The selection of fracturing fluid depends on the reservoir conditions and the desired fracture characteristics. Options include water-based, oil-based, or foam-based fluids. Chemical additives ensure the fracturing operation is effective and efficient, and serve many functions from limiting the growth of bacteria to preventing corrosion of the well casing. 25 to 75% of the injected fracturing fluid flows back to the surface when the well is produced, depending on formation properties, the fracturing program design and the type of fracturing fluid used. Flowback water will continue to return to the surface over the producing lifetime of the well but mainly occurs in the early stage of production

In high permeable conventional sandstone reservoirs companies are still using borate crosslinked gels since pumping rates are not that high and need to control leak off. For shale, it is common to use friction reducer with no viscosity however to get a greater length of a fracture it is a trend to use high viscsoity friction reducers (HVFR).

In shear dominated fracture stimulation, the role of the prop pant is diversionary rather than supporting mechanically the fracture width. Therefore the proppant type and concentration may change as a frack stage is pumped. Generally fine sand is targeting the far field, while coarse sand is placed in the near field of the induced fracture network.

Proppants must oppose earth stresses to hold open the fracture after release of the fracturing fluid hydraulic pressure, material strength is of crucial importance. The propping material must be strong enough to bear the closure stress, otherwise the conductivity of the (crushed) proppant bed will be considerably less than the design value (both the width and permeability of the proppant bed decrease). Other factors considered in proppant selection are size, shape, composition, and, to a lesser extent, density.

Massive hydraulic fracturing typical of shale wells uses between 1.2 and 3.5 million US gallons (4,500 and 13,200 m3) of water per well, with large projects using up to 5 million US gallons (19,000 m3). Greater volumes of fracturing fluids are required in Europe, where the shale depths average 1.5 times greater than in the U.S. Whilst the published amounts may seem large, they are small in comparison with the overall water usage in most areas. Some producers have developed hydraulic fracturing techniques that could reduce the need for water.Using carbon dioxide, liquid propane or other gases instead of water have been proposed to reduce water consumption.

We wish to place a certain amount of proppant in the pay interval and to place it in such a way that the maximum possible productivity index is realized. The key to finding the right balance between size and productivity improvement is in the proppant number. We have to calculate maximum possible productivity index and also the optimum dimensionless fracture conductivity from proppant number. Then calculate optimum half-length and averaged propped fracture width.