Deep formations of the Midland Basin, from the top of the Pennsylvanian-age Wolfcamp D interval (∼8000 ft [∼2400 m] in depth) to the Ordovician-age Ellenburger Group (>10,000 ft [>3000 m] in depth), have had >9.4 billion bbl of wastewater injected into them since the mid-1980s, accounting for more than half of the overall volume injected in the basin from unconventional operations. Ellenburger, Silurian–Devonian, and Pennsylvanian units, in decreasing order, receive most of the wastewater volume. To investigate pore pressure evolution from salt-water disposal, a full geocellular model was developed, with all available well log data, stratigraphic data, petrophysical analysis, and core data. The model was then upscaled to 14 layers; the average cell thickness, not including the basement, is ∼260 ft (∼80 m). A hydrogeological model was then constructed (north-south grid orientation and cell size set at 1 × 1 mi2 [1609 × 1609 m2] with a total of 396,340 active cells). The model covers an area of ∼149 × 190 mi2 (∼240 × 306 km2). Porosity varies from 0.1 to 19 vol. % based on core and well log analyses. Permeability, which is interpreted and modeled from both injection and well log data, ranges from 0.01 to 1800 md. Model calibration is constrained by historical injection wellhead pressure of a subset of 247 wells while maintaining an injection rate history of 980 wells. Pressure buildup by January 1, 2023 ranges from 200 to 500 psi (1.38 to 3.45 MPa) around the main injection centers, with a relative uncertainty of ±25%, but it can be as high as >700 psi (>4.83 MPa) locally near injection well sites. A sensitivity analysis explores the impact of permeability anisotropy and of historical oil and gas production. The results of the hydrogeological model provide the basis for further assessment of injection-induced earthquake hazard assessment as published as companion papers in this AAPG Bulletin special issue.