Downhole Screw Pump for Low Pressure Gas Wells
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Solution Overview
Problem
Low pressure gas wells often face insufficient pressure at the producing zone to overcome frictional pressure drops and achieve the desired wellhead pressure, leading to inefficiencies in gas flow, especially in partially depleted wells.
Innovation Solution
A method involving a bottom hole pressure versus flow rate test to determine the required compressor capacity and speed, using a multi-phase screw pump with a downhole motor and variable frequency drive to achieve the desired wellhead pressure without the need for surface compressors or downhole liquid/gas separation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stress or pressure
If a surface compressor is used to compress gas at the wellhead, then the desired wellhead pressure is achieved, but the system requires additional equipment and complexity at the surface
Solution Approach 1:
The compressor is extracted from the surface location and moved downhole into the wellbore. The downhole compressor compresses the gas at the producing zone level, eliminating the need for surface compressor equipment while still achieving the desired wellhead pressure through the compressed gas flow up the tubing.
Solution Approach 2:
A downhole motor serves as an intermediary power transmission device, converting electrical energy from the surface to mechanical energy downhole to drive the compressor. This intermediary enables remote operation of the compressor without requiring surface-mounted compression equipment.
2Power
If a turbine type compressor is used, then gas compression is achieved, but liquid separation is required before compression
Solution Approach 1:
The liquid separation function is extracted from the surface equipment and integrated into the downhole environment. The downhole compressor is designed to handle multi-phase flow directly, allowing liquids to remain in the wellbore while compression occurs, thereby eliminating the need for separate liquid separation equipment.
Solution Approach 2:
The downhole compressor is designed with multi-functionality to handle both gas and liquid phases simultaneously. This universal design allows the single device to perform both compression and multi-phase handling functions, eliminating the need for separate liquid separation equipment that would be required with turbine type compressors.
3Productivity
If the compressor is oversized to handle maximum flow rates, then sufficient capacity is provided, but energy efficiency decreases at lower flow rates
Solution Approach 1:
The compressor speed is made dynamically adjustable through variable frequency drive controlled by a downhole motor. This allows the compressor to operate at optimal speeds for different flow rate conditions, maintaining high energy efficiency across varying production rates rather than being fixed at a single operating point.
Solution Approach 2:
The compressor operating parameters (speed, pressure) are dynamically changed based on actual well conditions and flow rates. By adjusting these parameters in real-time, the system achieves both sufficient capacity for maximum flow rates and high energy efficiency at lower flow rates, resolving the trade-off between productivity and energy use.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enhances efficiency by selecting the appropriate compressor capacity and speed based on the well's specific characteristics, reducing the likelihood of over-sizing and eliminating the need for downhole separation, thereby optimizing gas flow and pressure management.
Implementation Method 1
A downhole motor with a variable speed power supply at the surface may be used to vary the speed of the motor
Implementation Method 2
A screw pump has at least two rotors with helical profiles formed thereon. The helical profiles interleave each other.
Implementation Method 3
A variable speed power supply may be used at the surface to vary the speed of the motor
Implementation Method 4
A pressure drop due to frictional losses occurs as the gas flows from the perforations up the tubing
Data Source
AI summary
A method for producing gas from a well with low pressure involves running a bottom hole pressure test to graph a P-Q curve. The operator computes a frictional pressure drop due to friction of the gas flowing through the production tubing to the surface. A packer is set above perforations in the well. A screw pump is selected that has a capacity equal to the sum of the frictional pressure drop plus a desired wellhead pressure. The screw pump has a flow rate capacity determined from the P-Q curve. The operator may vary the frequency of a downhole motor to achieve the desired wellhead pressure.