The pressure environment of oil extraction varies greatly from the bottom of the well to the surface, and the problems and requirements for pressure gauges at each stage are also completely different. What problems usually occur in the three stages of oil extraction, namely the low-pressure end (initial treatment of wellhead produced fluid and subsequent), the commonly used range (core production process section), and the high-pressure end (formation energy drive or artificial pressurization section)? Why has choosing a work pressure gauge become the key to avoiding problems? Let’s take a look at the specific content below.
1. Firstly, understand the low-pressure end (initial treatment of wellhead produced fluid and subsequent)
Generally, problems are prone to occur in scenarios such as oil gas water separator outlet, electric dehydrator, storage tank, low-pressure conveying pipeline, and water injection system return water. This stage is usually when the working low pressure is less than 1.6 MPa (about 250 psi) or close to the pressure commonly used.
However, due to the complexity and strong corrosiveness of the medium, the extracted fluid still contains residual H ₂ S, CO ₂, Cl ⁻ ions, formation water (high mineralization), as well as residual crude oil and solid particles after three-phase separation after preliminary dehydration and degassing. Therefore, the risks of corrosion and scaling still exist.
Let’s talk about the pulsation and fluctuation issues that arise, as pressure pulsations caused by upstream pumps (such as delivery pumps and injection pumps) may be transmitted to the low-pressure system.
It is also because under low pressure, the flow rate of the medium may be slower, and wax, asphalt, scale, etc. are more likely to deposit at the instrument pressure inlet, causing blockage of the pressure tapping tube or diaphragm and measurement failure. The range of a low voltage gauge is small, and a slight swing of the pointer may represent a large percentage change, which can easily cause misjudgment.
How to choose a pressure gauge for this stage of Low pressure end (initial treatment of wellhead produced fluid and subsequent)？
1. Choose corrosion-resistant materials: As there is a liquid receiving part, the body of the pressure gauge must be made of corrosion-resistant materials, such as 316L stainless steel. For high sulfur conditions, consideration should be given to using Monel alloy, Hastelloy alloy, or PTFE lining.
2. Equipped with diaphragm isolation: It is strongly recommended to use diaphragm sealed pressure gauges (chemically sealed), which completely isolate the medium from the Bourdon tube of the instrument core through a metal diaphragm. The material of the membrane should be selected according to the characteristics of the medium (such as excellent resistance to Cl ⁻ corrosion of tantalum membrane).
3. The body structure should have anti blocking design: for example, choose an open flange diaphragm or a seal with a flushing interface to facilitate online flushing and blockage removal.
4. Accurate range selection: The working pressure should be between 1/3 and 2/3 of the range to ensure optimal accuracy and resolution. Avoid using instruments with large measuring ranges to measure small pressures.
5. Accuracy and readability: You can choose a slightly higher accuracy meter (such as ± 1.0%), and ensure that the dial scale is clear and has sufficient resolution in the low voltage area.
The second is the commonly used scope (core production process section)
The general workflow includes wellhead (self injection well), three-phase separator, inlet and outlet of pumps (oil pump, water injection pump), pre-treatment of compressors, and main process pipelines. The pressure range used in these stages is 1.6 MPa to 25 MPa (approximately 250 psi to 3600 psi), which is the most concentrated and typical working pressure stage in petroleum extraction surface engineering.
The main issues that arise are extreme pressure shocks and pulsations. The start stop and operation of plunger pumps and compressors can generate severe pressure pulsations and “water hammer” effects. During pigging operations, the passage of the pig can also cause an instantaneous pressure surge.
The instruments near large rotating equipment (pumps, compressors) need to withstand continuous structural vibrations, resulting in the problem of sustained and severe vibrations.
Some process sections may have high medium temperatures (such as after being heated in a furnace) or extreme environmental temperatures due to environmental factors (such as the Arctic and desert). There may also be a significant difference between high and low temperatures.
The higher the pressure of this workflow, the stronger the corrosive media (H ₂ S, CO ₂) have on the material, and there may be a risk of stress corrosion cracking (SCC). Regarding the issue of corrosive environments.
How to choose a pressure gauge for this stage of commonly pressure end (core production process section)？
1. Seismic and impact resistant design is necessary:
Liquid filled pressure gauge: The case is filled with glycerin or silicone oil, which can greatly buffer pointer shaking and protect the internal mechanism from mechanical wear. This is the standard configuration for this stage.
Reinforced structure: Choose products with all stainless steel casing, thick front cover, and reinforced movement.
Overvoltage protection: The instrument should have an overvoltage capability of ≥ 1.5 times the full range, and a red no entry zone warning should be set on the dial.
2. Adapt to temperature changes:
Choose a wide temperature range filling fluid (such as silicone oil, working temperature range -40 ℃~+150 ℃) to avoid high temperature vaporization or low temperature solidification. Consider compensating for environmental temperature differences.
3. High security and reliability:
For key monitoring points (such as before the separator safety valve), high-precision and high stability instruments or transmitters should be selected.
The rated pressure of interfaces and connectors (such as needle valves and pipe cables) must be matched with the system, and reliable connection methods such as threaded encryption sealing and welding must be used.
4. Maintenance convenience: Equipped with isolation valves and pressure relief valves to achieve non-stop maintenance and calibration.
The third stage is the high-pressure end (driven by formation energy or artificially pressurized section)
This stage includes deep/ultra deep well wellheads, high-pressure water/gas injection wellheads, fracturing (acidification) pump trucks, high-pressure manifolds, subsea Christmas trees, and high-pressure gas storage and transportation.
The commonly used pressure range is>25 MPa, up to 100 MPa (approximately 15000 psi) or even higher.
The main problems that arise are extremely high static pressure and potential overpressure risk: the system itself stores enormous energy, and any small leakage or failure can evolve into a catastrophic accident.
Extreme pressure shock: During hydraulic fracturing operations, the pressure rapidly rises from zero to tens of thousands of psi in a short period of time, and this shock is the ultimate test for any instrument.
Hydrogen embrittlement and hydrogen sulfide stress corrosion cracking: In high-pressure H ₂ S environments, steel is prone to hydrogen induced cracking (HIC) and sulfide stress corrosion cracking (SSCC), leading to brittle fracture of the material.
The risk of seal failure doubles: Under high pressure, the requirements for thread seals, flange seals, and diaphragm seals increase exponentially.
The measurement accuracy is difficult to maintain: the drift of sensors under high pressure and the creep problem of elastic components are more prominent.
How to choose a pressure gauge for this stage of high pressure end ( driven by formation energy or artificially pressurized section)
1. Specialized design and highest level certification:
Special pressure gauges or sensors designed for ultra-high pressure must be selected, and medium and low pressure gauges must not be used for modification.
All pressure bearing components (joints, valve bodies, diaphragms) must have clear pressure rating markings (such as 10K, 15K psi).
2. Strict material and process requirements:
The main material needs to be high-strength alloy steel (such as AISI 4340, 17-4PH) and undergo special heat treatment.
For acidic environments, materials must be certified according to NACE MR0175/ISO 15156 standards to demonstrate their resistance to SSC.
3. Multiple security designs:
Pressure relief back cover: When the internal pressure unexpectedly rises, it can release pressure in a targeted manner to protect the operator.
Flow limiting hole/bursting disc: Installed on the pressure tapping pipeline to prevent a large amount of medium leakage when the instrument completely fails.
Integrated heavy-duty casing: with a protection level of at least IP65, capable of withstanding accidental impacts.
4. Connection reliability:
API 6A/17D standard high-pressure threaded connections (such as ACME threads), flange connections, or high-pressure clamp connections are commonly used.
Metal to metal sealing or advanced elastomer sealing rings are commonly used as sealing forms.
5. Conservative range selection: The maximum working pressure should not exceed 50% of the range (usually recommended between 25% -75%), leaving sufficient margin for peak pressure.
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