A compression-set straddle packer with fluid pressure-boosted packer set has a two-part multicomponent mandrel and a multicomponent sliding sleeve that reciprocates within a limited range on an upper part of the two-part multicomponent mandrel. A two-part lower packer element compression sleeve in a lower part of the two-part multicomponent mandrel reciprocates on a packer element piston mandrel component and pressure-boosts compression of a lower packer element when high-pressure fluid is pumped into the straddle packer. A bias element constantly resists relative movement between the multicomponent mandrel and the multicomponent sliding sleeve and balances pressure-boost between the lower packer element and an upper packer element supported by the upper part of the two-part multicomponent mandrel.