Understanding Stress Shadow

In horizontal wells, stress shadow is an important factor to consider. It describes the amplification of stresses in the vicinity of hydraulic fractures. When carrying out a second hydraulic fracture parallel to the existing one, the stress shadow causes the fracture-propagation pressure to be larger than that of the initial stage. As the cluster spacing is reduced, stronger stress shadows are to be expected. This in turn results in a disequilibrium of fluid distribution among all the perforation clusters which leads to smaller propped surface. Stress shadow effect will eventually impact the hydrocarbon recovery as was observed.

Well spacing is of a great importance as it was shown in several studies that selecting the right location of wells can enhance well productivity. Unconventional reservoirs have poor permeability, which is less than 0.1 mD, so they require multiple hydraulically fractured wells in the same geological section to acquire an economic productivity. As hydraulic fractures can extend deeply into the reservoir, interference and fracture hits can occur between different wells. The response of a tubing head pressure of an observation well while nearby wells are being fractured sometimes suddenly show an increase in pressure. This pressure then dissipates back to its regular response once the fracturing operation stops.

"Child" well refers to a newly stimulated well whereas the "parent" well refers to an old well that is already producing hydrocarbons. The production rate of the "parent" well may be affected by the high stresses generated due to stimulating the "child" well. These stresses can lead to the propagation of new fractures into the previously fractured region. This is a result of a decrease in poroelastic stresses due to reservoir depletion near the "parent" well. The outcome interference can be detected when a severe drop in production is observed in the "parent" well. The production rate, however, is often recovered to normal conditions few months after suspending the hydraulic fracturing operations.

It is important to consider the size of completion design prior to selecting the appropriate well spacing. A small-sized completion design refers to a completion with large cluster spacing and small amounts of proppant and fluid. When the "parent" well is of a small size, then it is possible that some parts of the reservoir located between the "parent" well and the new well will remain unstimulated. If the "parent" well is of a large completion size, then the parts between both wells will most probably be overstimulated (depending on the actual distance between the wells). For tight well spacing, overstimulation would not be considered advantageous as it would affect the production rate of both wells with time as discussed previously. In addition, the cost of the well design will be much more expensive where the increase in production might not justify the additional costs. For areas with small completion designs, wide spacing between wells can result in large untapped acreage and hence tighter spacing can overcome this issue and lead to a better estimated ultimate recovery (EUR) for a given region. On the other hand, large completion designs displayed a lot of interference for tight well spacing which lead to lower EUR. Wider well spacing overcames the interference effect and greatly enhanced the EUR. The impact of interference on EUR appears to be more severe for large completion designs.

https://jpt.spe.org/twa/key-design-considerations-maximizing-recovery-rate-unconventional-reservoirs