Typical “gushers” happen when the drill punctures a pocket of oil and gas in an uncontrolled or unexpected fashion. Today, things are much more measured and controlled, and drilling mud is fine-tuned to provide pressure control in the well shaft so that the well doesn’t “gush” (as much, anyway) when the drill punches through the last bit of rock into the reservoir.
This is of course vastly more complicated than my one-paragraph summary, but this is ELI5, after all.
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