Hierarchical Well Testing Approach for Carbonate Gas Field Appraisal: A Case Study from Uzbekistan

Abstract This paper presents a systematic hierarchical well testing approach for appraising a naturally fractured carbonate gas field in southern Uzbekistan. The Jurassic carbonate reservoirs' complex dual-porosity nature posed significant challenges, necessitating a multi-scale testing framework to accurately assess reservoir flow and storage capacity. The appraisal program involved drilling a sparse network of wells covering the entire reservoir area. This well network served as the foundation for an intensive testing campaign. The hierarchical testing approach integrated multiple spatial and temporal scales: wireline formation tests during drilling (centimeter to meter scale, minutes to hours), drill-stem tests in open-hole sections (hundred-meter scale, days), selective perforation interval tests with both standard and extended durations (10-meter scale, hours to weeks), injectivity tests in low-productivity intervals, commingled perforation tests, and well interference testing (kilometer scale, days). All tests employed high-resolution downhole and surface gauges for comprehensive data acquisition. Drill-stem tests in extensive open-hole sections enabled assessment of integrated reservoir properties and overall productivity. Selective interval testing characterized vertical and lateral reservoir compartmentalization, with pressure gradients across different zones enabling gas-water contact determination. These tests evaluated reservoir properties for various rock types, differentiating fractured from matrix-dominated intervals. Interference testing involving two wells simultaneously facilitated evaluation of reservoir storage capacity in the interwell space, characterization of dual-media structure with permeability distribution between fractures and matrix, and determination of critical fracture compressibility. Integration of pressure depletion analysis with interference-test-derived storage capacity provided calibration for the geological model, yielding reliable movable gas reserves estimates. The methodology mobilized reservoir fluids throughout practically the entire reservoir volume, enabling comprehensive characterization of both volumetric and productive properties. This study presents the first comprehensive hierarchical well testing methodology specifically designed for naturally fractured carbonate reservoirs in the Afghan-Tajik Basin, systematically integrating meter-scale to field-scale testing to enable accurate simulation model development during the early appraisal phase—critical for project success.

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