Approach by tracking the sign of the effect on flow resistance:
Instead of writing out the full productivity index formula first, it helps to think of the well plus near wellbore region as a resistance to flow, similar to resistance in an electrical circuit. The productivity index behaves like a conductance, it is high when the total flow resistance is low, and it drops when the flow resistance rises.
Effect of hydraulic fracturing on flow resistance:
A hydraulic fracture is essentially a very high permeability, low resistance channel connecting deep into the reservoir and back to the wellbore, bypassing the low permeability rock right around the well. This sharply cuts the total flow resistance the oil has to overcome, so the productivity index goes up.
Effect of longer perforations on flow resistance:
With short perforations, flow lines have to converge sharply into a small number of small tunnels near the well, similar to many lanes of traffic merging into a narrow gate, which adds resistance. Lengthening the perforated interval opens up more flow paths and eases this convergence, lowering the resistance and raising the productivity index.
Effect of a higher skin factor on flow resistance:
Skin factor is literally defined as extra resistance added near the wellbore, on top of the resistance of the undamaged formation. Increasing the skin factor means deliberately adding more resistance, for example through worse formation damage or poor perforation design, which can only lower the productivity index, never raise it.
Effect of viscosity reduction from steam on flow resistance:
Oil viscosity directly controls how much resistance the fluid itself offers as it moves through the pore network, a thicker fluid resists flow more. Steam injection heats the oil and thins it out, lowering this fluid resistance and therefore raising the productivity index for the same reservoir rock properties.
Final Answer:
\[ \boxed{\text{Hydraulic fracturing, increasing perforation length, and reducing viscosity by steam injection}} \]