Geothermal well yield testing is a controlled hydraulic evaluation that quantifies the discharge capacity, specific yield, wellhead temperature, and sustainable production potential of a geothermal well. Performed in accordance with recognized standards such as GB/T 11615, GB 50027, and ASTM D4043, this testing is essential for geothermal resource assessment, well completion acceptance, and the design of reliable heating and reinjection systems. Accredited third-party laboratories and qualified hydrogeological engineering teams use multi-level pumping tests, artesian discharge tests, and recovery tests to deliver defensible yield data for project owners and regulators.
Technical Overview
Geothermal well yield testing, also referred to as discharge capacity or inflow testing, is the systematic measurement of the volume of thermal water a geothermal well can deliver under controlled drawdown conditions. The primary output is the well discharge rate Q, typically expressed in cubic meters per hour (m³/h) or cubic meters per day (m³/d), together with the corresponding dynamic water level, wellhead temperature, and the derived specific yield q = Q/s, where s is the drawdown. Because a geothermal well is both a hydraulic and a thermal production asset, yield testing integrates flow measurement, water level monitoring, and temperature logging into a single coordinated procedure.
The test is normally conducted after well completion, cleaning, and development, and before the well is placed into commercial production. Results feed directly into geothermal resource classification, reserve estimation, pump selection, surface facility design, and reinjection planning. The work is governed by standards including GB/T 11615 (geothermal resources geological exploration), GB 50027 (hydrogeological investigation of water supply), GB 50296 (technical code for water supply wells), ASTM D4043 (selection of aquifer test methods), and ASTM D5737 (measurement of well discharge), with flow measurement performed per ISO 1438, ISO 5167, or ISO 9104 depending on the device used. Accredited third-party laboratories are frequently engaged to provide independent, traceable test reports for regulatory submission.
Testing Samples
Newly Completed Geothermal Exploration Wells
Deep exploration wells completed into sandstone, carbonate, or fractured granite and volcanic reservoirs are tested at the completion stage to establish the proven yield, static water level, wellhead temperature, and reservoir hydraulic parameters. These wells form the primary evidence base for geothermal resource evaluation reports.
Production and Development Wells
Wells drilled for district heating, industrial process heat, greenhouse, balneological, or aquaculture applications are tested before handover to verify that contractual yield, temperature, and water quality targets are met and to size the production pump and surface piping.
Reinjection (Injection) Wells
Injection wells undergo the complementary injection capacity test, in which measured volumes of water are injected at stepped rates while wellhead injection pressure and water level are recorded, confirming the reinjection capacity required for pressure maintenance and reservoir sustainability.
Existing and Aging Wells
Wells that have been in service for several years are re-tested periodically to detect yield decline, temperature drift, scaling, or clogging, providing the basis for well rehabilitation, pump resetting, or revised abstraction permits.
Water Samples Collected During Testing
Representative geothermal water samples are captured once discharge has stabilized at each drawdown level. Field parameters (temperature, pH, electrical conductivity, TDS) are logged on site, and preserved samples are delivered to the laboratory for full hydrochemical analysis, including major ions, silica, and trace components that support geothermometry and corrosion-scaling assessment.
Testing Items
A complete geothermal well yield test campaign measures and reports the following parameters:
- Static water level, recorded after an adequate rest period following well development.
- Dynamic water level during stabilized pumping at each discharge stage.
- Drawdown (s) corresponding to each tested discharge rate.
- Well discharge rate Q at each level, and the maximum tested yield Qmax.
- Specific yield q = Q/s (yield per unit drawdown) and the q–s relationship.
- Wellhead water temperature at each stage, and downhole temperature profiles where required.
- Wellhead shut-in pressure and artesian flowing pressure for self-flowing wells.
- Sand (sediment) content of the discharge water, indicating adequate well development.
- Recovery water level curve after pump shutdown, used for unsteady-state analysis.
- Field hydrochemical parameters (pH, EC, TDS) and laboratory water chemistry from stabilized samples.
- Non-condensable gas content, where two-phase or gas-rich production is anticipated.
- Derived hydraulic parameters: transmissivity (T), hydraulic conductivity (K), and where observation wells exist, storativity (S).
- Indicative thermal power output, calculated from discharge rate and temperature difference.
Testing Methods
Multi-Level Steady-State Pumping Test
The standard approach for a production-grade evaluation is a stepped or multi-level pumping test, generally with three descending water levels. The first drawdown is typically set at approximately one third of the design maximum, the second at two thirds, and the third at or near the design drawdown. At each level, pumping continues until quasi-steady conditions are achieved, commonly defined as water level fluctuation within 1 percent (or a few centimeters) and discharge fluctuation within 3 percent, sustained for at least 4 to 8 hours; geothermal wells often require longer stabilization because of thermal equilibration of the wellbore and surrounding formation. Recording frequency is high at the start of each step (every 2 to 5 minutes) and relaxed to every 30 minutes once stable.
Constant-Rate and Unsteady-State Tests
For reserve verification, a constant-rate test of 24 to 72 hours (or longer for large thermal reservoirs) is performed at the recommended production rate. Water level, discharge, and temperature are logged continuously. The drawdown and recovery data are analyzed with unsteady-state methods such as the Theis type-curve match and the Cooper–Jacob straight-line solution to derive transmissivity and hydraulic conductivity, in line with ASTM D4105-type procedures and the requirements of GB 50027.
Artesian (Self-Flowing) Discharge Test
For wells that flow naturally, discharge is measured at a minimum of three flowing stages created by throttling the wellhead valve, while wellhead pressure, flow rate, and temperature are recorded at each stage. The test establishes the artesian yield-pressure relationship, the maximum self-flowing discharge, and the flowing wellhead temperature that governs surface equipment design.
Recovery Test
Immediately after pump shutdown, the rebounding water level is logged until at least 80 to 90 percent recovery or for a duration comparable to the pumping period. Recovery data provide a high-quality basis for transmissivity calculation because they are free of pumping-induced turbulence, and they confirm that the reservoir recovers without significant interference.
Flow and Level Measurement Techniques
Discharge is quantified by one or more of the following: V-notch (triangular) weirs per ISO 1438 and ASTM D5242, rectangular weirs per ASTM D5243, volumetric tank methods for low flows, electromagnetic flowmeters per ISO 9104, clamp-on ultrasonic flowmeters, and differential-pressure orifice devices per ISO 5167. Water levels are measured with graduated electric water-level tapes, pressure transducers with sub-centimeter resolution, or acoustic well sounders for deep hot wells. Temperatures are recorded with calibrated RTD or thermocouple probes at the wellhead and, where required, by downhole temperature logging. All instruments are calibrated against traceable references before mobilization.
Data Interpretation and Yield Determination
The Q–s and q–s curves from the stepped test are classified as linear, parabolic, exponential, or logarithmic, and the fitted equation is used to extrapolate the well yield at the design drawdown without over-pumping the formation. Specific yield q is reported for each level to characterize well productivity, and the maximum recommended yield is selected considering the curve behavior, sand production limits, pump setting depth, thermal breakthrough risk, reservoir pressure support, and the capacity of the reinjection system. The indicative thermal power is computed as the product of mass flow rate, specific heat capacity, and the temperature difference relative to the reference sink temperature, expressed in megawatts thermal (MWt).
Applications
Geothermal well yield testing underpins nearly every stage of geothermal project development and operation:
- Geothermal resource assessment and reserve classification (proven, controlled, and inferred categories) for exploration reports and mining rights applications.
- Well completion acceptance, providing the contractual yield and temperature basis for handover between drilling contractors and project owners.
- Design of production wellfields for district heating networks, including pump sizing, wellhead configuration, and peak-load dispatch strategy.
- Sizing of reinjection wells and tail-water management systems to maintain reservoir pressure and permit compliance.
- Open-loop groundwater source heat pump system design, where sustainable yield and reinjection balance are critical approval conditions.
- Feasibility studies for greenhouse heating, spa and balneology development, aquaculture, and industrial process heat.
- Long-term reservoir monitoring, aging diagnosis, and justification for well rehabilitation or workover programs.
- Regulatory submissions, environmental impact assessments, and water abstraction licensing supported by independent test data from accredited third-party laboratories.
FAQ
What is the difference between well yield and specific yield?
Well yield Q is the actual discharge rate of the well at a given drawdown, expressed in m³/h or m³/d. Specific yield q is the discharge per unit drawdown (q = Q/s), expressed in m³/h per meter of drawdown. Specific yield normalizes productivity between wells and is the key indicator for comparing well quality and extrapolating performance.
How long does a complete yield test take?
A typical three-level steady-state test with recovery occupies 3 to 7 days on site, allowing for static level measurement, step stabilization periods of 4 to 24 hours each, and recovery monitoring. Constant-rate verification tests for reserve reporting may extend the campaign to 1 to 2 weeks, and observation-well monitoring can add preparation time.
Why are three drawdown levels required instead of a single flow measurement?
Three levels define the shape of the Q–s curve, revealing whether additional drawdown continues to gain meaningful flow. This curve is essential for predicting yield at any design drawdown, detecting turbulent or dewatering losses, and selecting a production rate that the reservoir can sustain.
Why is temperature measured throughout the test?
Continuous temperature recording confirms that the produced water is genuine formation fluid rather than cooled wellbore storage, verifies the thermal output that defines the well's commercial value, and detects cold water intrusion. Water samples for chemistry are only valid once temperature has stabilized.
How is the maximum allowable yield determined?
The maximum allowable yield is derived from the fitted Q–s curve at the design drawdown, then constrained by pump setting depth, acceptable sand content, regulatory abstraction limits, reinjection capacity, and long-term reservoir pressure and temperature sustainability. Accredited third-party laboratories and qualified hydrogeologists document the derivation in the final test report.
Can a yield test be performed on an artesian well without a pump?
Yes. Self-flowing wells are tested by controlling the wellhead valve to create three or more flowing stages while measuring discharge with a weir, flowmeter, or volumetric method. The resulting pressure-flow-temperature relationship characterizes the well's natural production capacity and whether pumping would add meaningful yield.
