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Shallow shale (<1,000m): Use light rigs. They move easily, operate simply, cost less, and finish shallow coring fast.
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Medium-depth shale (1,000-3,000m): Medium rigs work better. Their power and drilling capacity balance quality and efficiency.
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Deep shale (>3,000m): Heavy rigs are needed. They have strong power and pressure resistance, handling deep complexity for quality cores.