Slimhole well design can reduce well–construction cost through a reduction in steel, fluids, and disposal costs. In the industry, there has been a misconception that slimhole size involves the tradeoff of slower rate of penetration (ROP) and less–efficient fracture treatment. Improvements in downhole tools, drillstrings, rig capability, and drilling–fluid design have been implemented to improve ROP in slimholes. Completion designs were refined for slimmer holes to avoid any significant loss in stimulation effectiveness and maintain well value. Through systematic replication of learnings and designs across basins, slimhole well design has advanced. Slimhole well design (transition from 8.5/7.875–in. hole and 5.5–in. production casing to 6.75/6.25–in. hole and 5.5/4.5–in. production casing) can reduce unconventional well cost, which reduces well unit-development cost (UDC).