Water pressure testing, also known as injection or packer testing, is the core field method for measuring the permeability and hydraulic integrity of geothermal reservoir formations and completed well sections. By injecting water at controlled staged pressures and analyzing the resulting pressure–flow behavior, engineers obtain Lugeon values, hydraulic conductivity, and injectivity data that underpin reservoir assessment, well completion design, and reinjection planning. This article explains the test objectives, tested intervals and components, measured parameters, standard procedures, data interpretation, and frequently asked questions for geothermal well water pressure testing.
Technical Overview
Water pressure testing of a geothermal well is a controlled hydraulic injection test in which water is pumped into a selected borehole interval—or into the completed well itself—under staged pressures while flow rates and pressures are recorded. The primary purpose in geothermal engineering is to quantify how readily the reservoir rock accepts and transmits water, expressed as the Lugeon value, hydraulic conductivity, or transmissivity, and to verify the sealing integrity of casing, cement, and wellhead components. Because geothermal productivity and reinjectivity depend directly on fracture and pore permeability, this test is one of the most decisive measurements in the exploration, completion, and evaluation of a geothermal well.
The classical downhole test isolates a borehole interval with inflatable packers and applies a five-stage pressure cycle (for example, 0.3, 0.6, 1.0, 0.6, 0.3 MPa) with each stage held until steady flow is achieved. The flow-versus-pressure response is then classified into flow regimes—laminar, turbulent, dilation, wash-out, or void filling—which reveal both the permeability magnitude and the mechanical behavior of the fractures. One Lugeon (Lu) is defined as a water take of 1 L/min per meter of tested interval under a net pressure of 1 MPa, and as an order-of-magnitude approximation a hydraulic conductivity of about 1 × 10⁻⁷ m/s corresponds to 1 Lu under laminar conditions, corrected for water viscosity and temperature.
The work is typically performed in accordance with national and international standards such as GB/T 11615 (specification for geothermal resources geological exploration in China), GB 50027 (code for hydrogeological investigation of water supply, which covers injection test procedures), the Chinese water conservancy industry standard SL 31 for borehole water pressure tests, and ASTM D4630 (constant-head injection test) and ASTM D4631 (pressure-pulse test) for determining transmissivity of low-permeability rocks. Field programs are usually executed by accredited third-party laboratories or specialist well-testing crews using calibrated pumps, packers, flowmeters, and electronic pressure transducers.
- Determines formation permeability and transmissivity of the target geothermal reservoir
- Identifies permeable fracture zones, karst conduits, and loss zones along the borehole
- Verifies pressure-tightness of casing strings, cement sheath, and wellhead equipment
- Provides injectivity data for designing reinjection wells and hydraulic stimulation
Testing Samples
Open-Hole Reservoir Intervals
The principal test object is a section of the reservoir rock exposed in the open hole, typically 5 m in length but adjustable to the geological structure. Common geothermal reservoir types tested include fractured and karstified carbonates (limestone and dolomite), fractured granite and volcanic rocks, and consolidated sandstone aquifers. Each interval is isolated with single or double inflatable packers so that the hydraulic response is attributable to a known depth zone.
Cased and Cemented Well Sections
Completed sections—including casing strings, liner, screens or slotted pipes, cement sheath, and wellhead assemblies—are pressure-tested with water to confirm that they can hold design pressure without leakage. This verifies wellbore integrity before stimulation, reinjection, or long-term production, and is a routine acceptance check after cementing operations.
Loss Zones and Aquifer Intersections
Intervals where drilling fluid was lost or where distinct aquifers were intersected are tested individually, either after drilling or as short dynamic injection tests during drilling operations, to characterize their permeability and to assess the risk of cross-flow between aquifers.
Injection Water and Returned Fluid
The water used for injection is itself a controlled input: it must be clean, sediment-free, and chemically compatible with the formation to avoid clogging fractures. Temperature, pH, electrical conductivity, and suspended solids of the injected water and of any returned fluid are checked so that permeability results are not biased by particulate plugging or gas release.
Testing Items
Depending on the test configuration and project objectives, the following quantities are measured or derived:
- Lugeon value (specific water absorption, q), reported in Lu
- Hydraulic conductivity (permeability coefficient, k), in m/s or m/d, viscosity-corrected to a reference temperature
- Transmissivity (T = k × interval thickness), in m²/s or m²/d
- Injectivity index (flow rate per unit pressure increase, Q/ΔP) for the tested interval or whole well
- Static water level and formation (reservoir) pressure at test depth
- Breakdown or fracture-opening pressure where the test is deliberately extended to fracturing
- Flow regime classification from the pressure–flow (P–Q) curve: laminar, turbulent, dilation, wash-out, or void filling
- Pressure-hold performance for casing tests: applied pressure, holding time, and pressure decay or leakage rate
- Water temperature, pH, TDS, and suspended solids of injected and returned fluids
Testing Methods
Downhole Packer (Lugeon-Type) Injection Test
The interval is isolated with packers, the pipeline is filled, and three ascending and two descending pressure stages (for example 0.3–1.0 MPa, adjusted to depth and formation strength) are applied. Each stage is held for 10–20 minutes until flow stabilizes, and the average steady injection rate is recorded. The Lugeon value is calculated as q = Q / (L × P), where Q is the injected flow rate in L/min, L is the tested length in meters, and P is the net applied pressure in MPa; results from the five stages are compared, and the value representative of laminar conditions is adopted.
Constant-Head and Pressure-Pulse Injection Tests
For very low-permeability intervals where steady injection cannot be sustained, constant-head injection (ASTM D4630) or transient pressure-pulse methods (ASTM D4631) are used. These techniques track the decay of pressure or flow with time and yield transmissivity through type-curve or numerical matching, and are particularly useful in tight basement rocks considered for enhanced geothermal systems.
Whole-Well Hydrostatic Pressure Test
For casing and completion integrity, the well is filled with water and pressurized—commonly to about 1.5 times the maximum anticipated operating pressure and always within the casing rating—and held for a specified period, typically 10 to 30 minutes. Acceptance is judged by a pressure drop within the design limit (for example, no more than 5% decay or the value stipulated in the well program), supplemented by monitoring for visible leakage at the wellhead.
Data Interpretation and Reporting
Field records are corrected for pipeline friction losses, hydrostatic head differences, and water viscosity at formation temperature before permeability is calculated. The P–Q curves are plotted for each interval, the flow regime is classified, and the report presents depth-referenced profiles of Lugeon values, hydraulic conductivity, transmissivity, and water loss zones, together with equipment calibration records and test-time histories, providing the basis for completion and stimulation decisions.
Applications
Water pressure testing results directly support the full life cycle of a geothermal project:
- Geothermal resource assessment, mapping the permeability structure of the reservoir and ranking target horizons
- Well completion design, including selection of casing seats, cementing and grouting requirements, and placement of screens or slotted liners
- Reinjection system design, where injectivity data determine whether a well can accept returned fluids at sustainable pressures
- Stimulation planning for enhanced geothermal systems, using measured breakdown pressures and pre-stimulation permeability as baseline values
- Aquifer protection, identifying loss zones and cross-formation flow paths that must be isolated during drilling and completion
- Wellbore integrity verification for new completions, workovers, and rehabilitation of aging geothermal wells
- Ground-source heat pump and hydrothermal field development in sedimentary basins, karst carbonate terrains, and fractured crystalline rocks
FAQ
What is the difference between a water pressure (injection) test and a pumping test?
An injection test pushes water into the formation under pressure and is suited to deep, artesian, or low-yield intervals where pumping is impractical; a pumping test withdraws water and observes drawdown. The two methods yield comparable transmissivity values for the tested interval, and injection testing is often preferred in geothermal wells because it can be run selectively with packers at reservoir depth and at reservoir temperature.
What Lugeon value indicates a productive geothermal reservoir?
As general guidance, values below 1 Lu indicate very low permeability typical of intact rock, 1–5 Lu is low, 5–25 Lu is moderate and often viable for fractured reservoirs, 25–100 Lu is high, and above 100 Lu indicates very high permeability usually associated with open karst conduits or major fracture zones. Productivity ultimately depends on transmissivity of the whole producing section and on temperature, so interval values are interpreted together with well test data.
How is the maximum test pressure chosen?
Staged pressures are selected so that the interval is characterized without unintentionally fracturing the rock, normally a fraction of the estimated overburden and breakdown pressure, unless the program deliberately measures fracture-opening pressure for stimulation design. Deep geothermal wells commonly use higher stage pressures than shallow boreholes, always within the pressure ratings of the packers, tubing, and wellhead.
Can the test be performed in high-temperature wells?
Yes, provided the downhole equipment is rated for the bottomhole temperature. Standard elastomer packers have temperature limits, so high-temperature wells require specially rated packers, heat-stable pressure transducers, and procedures that account for thermal expansion of the fluid and viscosity reduction of hot water when converting flow data to permeability.
How long does a water pressure testing program take?
A single 5 m interval with a five-stage cycle typically requires about one to two hours including rig-up. A complete program logging multiple reservoir intervals, or a combined formation and casing integrity test sequence, may extend from several hours to several days depending on well depth, the number of intervals, and stabilization times.
How are the results used in well design?
Permeability profiles determine where to set casing to shut off unproductive or unstable zones, which intervals to leave open or perforated for production and reinjection, whether grouting is needed to seal excessive losses, and what injectivity can be expected—inputs that together define the economic potential and operating strategy of the geothermal well.
