Geothermal Wells - Health and Safety Guidelines for Shallow
Part 1: General
101 Scope
101.1 These Guidelines cover the drilling, operation, maintenance and abandonment of shallow geothermal wells. Sections of the Guidelines cover well design, well site preparation, drilling practice, well operations, maintenance, and abandonment. The scope of the Guidelines also includes all subsurface work, subsurface casing, plus the wellhead components to the top flange of the master valve.
Note: These Guidelines do not consider equipment or operations downstream of the master valve. (Or wellhead isolation valves where applicable.)
101.2 These Guidelines are intended to provide sufficient mandatory rules, recommendations and commentary to enable shallow geothermal wells to be drilled and operated safely. However, the presence of any highly acidic waters or other exceptional circumstances may compel consideration of requirements beyond the scope of these Guidelines.
101.3
(1) These Guidelines shall apply to all wells drilled to a depth not exceeding 150 metres where steam or hot water exceeding 70 degrees Celsius is or is likely to be encountered.
(2) The authority having jurisdiction for these Guidelines may also allow compliance with these Guidelines for wells in the depth range of 150 m to 250 m where subsurface geothermal conditions are 20 degrees Celsius or more below boiling point for depth at ambient conditions, and the wellhead pressure is less than 5 berg. (Refer Table 3).
Note: Wells drilled in conditions more severe than those outlined in 101.3(2) shall comply with the Code of Practice for Deep Geothermal Wells NZS 2403: 1991.
101.4 These Guidelines are directed towards the elimination, minimisation, or isolation of hazards associated with:
(1) The installation and operation of drilling machinery and equipment; and
(2) The design, operation, repair, maintenance, or abandonment of geothermal wells.
101.5 Wells being planned, drilled, worked-over or operated within the scope of this document may be specified by reference to existing national and international Standards. Standards have been listed under Related Documents and form part of the requirements of these Guidelines which concentrates on the effects peculiar to geothermal situations.
101.6 These Guidelines are not intended to preclude the adoption of alternative techniques which are based on sound data and engineering, or on the use of alternative recognized Standards. Documented justification shall be maintained wherever a variation from any mandatory provision of these Guidelines is desired.
102 Definitions
102.1 For the purpose of these Guidelines the following definitions shall apply:
Airlift wells: Wells which discharge with the continual or near continual aid of an air compressor.
Blowout: An uncontrolled flow of steam, water, gas or rock material at the ground surface either inside the well or escaping from the well at depth.
BOP: Blowout preventer.
Bore: See Well.
BPD (Boiling point for depth conditions): Representing a column of pure water at its boiling (saturation) temperature corresponding to the pressure at every depth.
Note: The values may be interpolated from published tables of the thermodynamic properties of water, i.e. steam tables. Table 3 shows BPD pressures and temperatures for standard ambient conditions at the surface and depths down to 300 m.
Cellar: An excavation around the top of the well to accommodate part of the wellhead.
Conductor pipe: The large diameter, very shallow pipe sometimes installed before drilling commences, used to retain surface material against collapse or washout, and to elevate returning drilling fluid to above ground level.
Diverter: A well control device consisting of sealing elements compressed in a cylindrical body mounted on a well and operated by hydraulically or air activated cylinders, with piping to direct the discharge from a well at a safe distance during drilling operations.
Downhole heat exchanger: Pipework installed in a well for the purpose of extracting heat. Circulation may be maintained by thermosyphon, mains pressure or circulation pump.
Drilling: Includes workovers and all well site activities associated with rigging up and rigging down.
Geothermal: Associated with heat derived from the earth.
Geothermal energy: Energy derived or derivable from and produced within the earth by natural heat phenomenon.
Geothermal fluid: Includes all steam, water, and water vapour, and every mixture of all or any of them that has been heated by geothermal energy, and every kind of matter derived from a well and for the time being with or in any such steam, water, water vapour, or mixture (but does not include water that has been heated by such energy to a temperature not exceeding 70 degrees Celsius).
Intermediate casing: Casing installed where required by subsurface conditions to enable target depth to be reached for that stage of the well.
LCM: Lost circulation material.
Liner: A casing having openings for the production or injection of fluids, and installed in the drilled hole to prevent collapse of the formation or entry of debris into the well.
Local authority: Regional council or territorial authority.
Low pressure steam wells: Steam-producing wells with a measured shut-in wellhead pressure of less than 35 kPa.
Master valve: The primary containment valve on the well.
NRV: Non-return valve.
Pressure: Gauge pressure (that is, pressure above ambient) unless specified otherwise.
Note: As steam tables normally use absolute values, add atmospheric pressure to gauge pressure before using the tables.
Production casing: The deepest cemented casing extending to the surface.
Production liner: A casing string installed to protect the hole or other casings from the corrosive or erosive effects of fluid flow.
Note: Depending on its purpose, this liner may be cemented, or free to expand with increase in temperature.
Pumped wells: Wells that are fitted with artificial lift devices, which may be surface- or downhole-mounted, to enable geothermal fluid to be extracted.
Quench: The injection of cold liquid into a well to condense or prevent the formation of steam, or to reduce temperatures for other purposes.
Reinjection well: A well that is drilled for the purpose of re-injecting geothermal fluids into the ground.
Self-discharging wells: Wells which discharge geothermal fluids without the aid of continued artificial lift.
Surface casing: The first casing installed in the well which supports a drilling wellhead.
Well: A fully- or partially-lined hole in the ground.
Wellhead: A set of valves and other pressure-rated components, connected to the top of the well and used to contain the well fluids.
Workover: Maintenance or repair in an existing well.
102.2 The word "shall" refers to requirements which are essential for compliance with these Guidelines except where alternative methods are adopted in accordance with Section 101.6.
The word "should" refers to options which are strongly recommended.
103 Units of Measurement
With the exception of diameter, SI units of measurement and abbreviations are used in these Guidelines. Where Imperial units are preferred, the SI equivalent is quoted in brackets immediately after. Table 1 below lists the principal units used and their conversions.
Note: Because most of the rotary drilling Standards were developed in the United States of America, there is still widespread use of the US version of Imperial units of measurement (for example, feet and psi), together with others adapted in the petroleum industry e.g. barrels). Although New Zealand officially adopted SI units in a planned conversion during the period 1971-76, the drilling industry has had to maintain familiarity with the units still used by equipment suppliers, technical literature, etc.
104 Nomenclature and Abbreviations
| Abbreviation | Nomenclature |
|---|---|
| API | American Petroleum Institute |
| BPD | Boiling point for depth |
| STD | Standard |
| C02 | Carbon dioxide |
| H2S | Hydrogen sulphide |
| NB | Nominal bore |
| m | metres |
| mm | millimetres |
| C | Celsius |
| MPa | megapascals |
| lb/ft | pound per foot |
| kg/m | kilograms per metre |
| kPa | kilopascals |
| kPag | kilopascals (gauge) |
| kg/l | kilograms per litre |
| ID | Internal diameter |
| F | Fahrenheit |
Table 1: Unit Conversion Factors
| Imperial/US | Multiply by/Divide by | SI/Metric (to 4 significant figures) |
|---|---|---|
| inch | 25.40 | mm |
| feet | 0.3048 | m |
| sq. inch | 645.2 | mm2 |
| Imp. gallon | 4.546 | litre |
| U.S. gallon | 3.785 | litre |
| barrel | 159.0 | litre |
| pound (lb) | 0.4536 | kg |
| short (US) ton | 0.9072 | tonne |
| long (Imp) ton | 1.016 | tonne |
| kip (103 lbf) | 4.448 | kN |
| psi (lbf/in2) | 0.006895 | MPa |
| psi | 0.06895 | bar |
| psi | 0.07031 | kgf/cm2 |
| Fahrenheit - 32 | 5/9 | C |
Table 2: Mean Boiling Point Values
| Elevation (m) |
Ambient Pressure (kPag) |
Boiling Temperature (deg. C) |
|---|---|---|
| Sea level | 101.3 | 100.0 |
| + 500 | 95.5 | 98.3 |
| + 1000 | 89.9 | 96.7 |
| + 2000 | 79.5 | 93.3 |
| + 3000 | 70.1 | 90.0 |
Note: The ambient air pressure varies with the local weather and notably with the elevation, while the boiling temperature of water is also affected by the dissolved solids and gas present. In the absence of better data, mean values may be interpolated from the Table 2 above.
