Rapid Analytical Estimation of Productivity and Injectivity in Horizontal Geothermal Wells

Horizontal wells have become a cornerstone of modern geothermal development because their extended contact with the reservoir markedly increases both production and injection capacity relative to vertical completions. Quantifying this performance in early design phases calls for rapid yet credible estimates of the productivity index (PI) or injectivity index (II)—defined as the volumetric flow rate per unit pressure differential. Although reservoir simulations offer high-fidelity, they are computationally intensive and time-consuming. In contrast, analytical solutions provide quick predictions that are sufficiently accurate for screening studies, sensitivity analyses, and preliminary well‑placement decisions.

Several analytical formulations, originally developed for oil and gas reservoirs, can be transferred to geothermal systems (with fluid properties for brine). This paper presents the most widely used steady-state correlations—Borisov (1984), Giger et al. (1984) , Joshi (1991) , and Renard & Dupuy (1990)—together with the hybrid 3D‑2D method of Egberts & Fokker (2001). The methods are applied to a geothermal doublet; the resulting PI and II estimates are benchmarked against a high‑resolution numerical simulation that explicitly honours permeability heterogeneity and structural features such as faults and boundaries.