Flow Assurance Technical Section

Response to various queries are as under:

1. Purpose of Choke is to control the drawdown on the reservoir which is achieved by the choke limiting the quantity of well fluid that can be produced from the well. It is not used to control Flowing well head pressure.
2. Choke has critical flow when Downstream pressure <=0.55 times upstream pressure, for natural gas wells. When choke is having critical flow, for the given upstream pressure and size of choke, a constant and maximum mass flow rate is achieved. Therefore even in the event of pipeline failure and downstream pressure going down to atmospheric, the production mass flow rate from the well remains same and does not increase. Conversely, as downstream pressure increases to >0.55 upstream pressure, the choke is no longer critical and flow reduces and starts mimicing a pipeline flow.
3. Gas being a compressible fluid, a constant mass flow rate from a choke does not imply constant gas velocity in the downstream pipeline(s), as actual gas velocity of gas in the pipeline depends upon the prevailing line pressure. For a constant diameter pipeline, higher line pressures give lower actual flow velocities and lower pressure give higher actual flow velocities. Line pressure reduces along the pipeline as there is pressure lost due to friction loss, assuming there are no elevation changes. Therefore, along the flow in the pipeline, pressure keeps dropping and actual gas velocity keeps increasing.  
4. Erosion velocities for pipelines are reported for pure single phase flow and can be very high. However, solids in gas will quickly erode the pipeline even at normal pipeline operating conditions, which would be much less than the single phase erosion velocity.  Solids in gas can compromise the surface facility and well bore integrity very quickly and it is imperative to address and stop the solids production at the earliest before obtaining regular production from the well. This may be by reducing the drawdown - by reducing choke size, or by other downhole intervention methods.         

Hope this clarifies your queries.

Puneet Kishore

Response to various queries are as under:

1. Purpose of Choke is to control the drawdown on the reservoir which is achieved by the choke limiting the quantity of well fluid that can be produced from the well. It is not used to control Flowing well head pressure.
2. Choke has critical flow when Downstream pressure <=0.55 times upstream pressure, for natural gas wells. When choke is having critical flow, for the given upstream pressure and size of choke, a constant and maximum mass flow rate is achieved. Therefore even in the event of pipeline failure and downstream pressure going down to atmospheric, the production mass flow rate from the well remains same and does not increase. Conversely, as downstream pressure increases to >0.55 upstream pressure, the choke is no longer critical and flow reduces and starts mimicing a pipeline flow.
3. Gas being a compressible fluid, a constant mass flow rate from a choke does not imply constant gas velocity in the downstream pipeline(s), as actual gas velocity of gas in the pipeline depends upon the prevailing line pressure. For a constant diameter pipeline, higher line pressures give lower actual flow velocities and lower pressure give higher actual flow velocities. Line pressure reduces along the pipeline as there is pressure lost due to friction loss, assuming there are no elevation changes. Therefore, along the flow in the pipeline, pressure keeps dropping and actual gas velocity keeps increasing.  
4. Erosion velocities for pipelines are reported for pure single phase flow and can be very high. However, solids in gas will quickly erode the pipeline even at normal pipeline operating conditions, which would be much less than the single phase erosion velocity.  Solids in gas can compromise the surface facility and well bore integrity very quickly and it is imperative to address and stop the solids production at the earliest before obtaining regular production from the well. This may be by reducing the drawdown - by reducing choke size, or by other downhole intervention methods.         

Hope this clarifies your queries.

Puneet Kishore

Decreasing choke size at the wellhead reduces the mass flow or drawdown from the reservoir, as a result the bottom hole flowing pressure increases, the friction loss in the tubing also reduces due to lesser flow rate and higher pressure, thus you get higher flowing tubing head pressure.
Genesis of pressure is from the reservoir, not from surface. 
You can control either  pressure or flow at a time, not both simultaneously.  So by reducing choke size, we are trying to control flow i.e., mass output from the well, so pressure will change as per hydraulics..

It is basically the fluid hydraulics in the reservoir and well that determines the flowing tubing head pressure for any given choke size where flow is critical.
Hope this clarifies.
Apologies for not responding earlier.

Regards.
Puneet Kishore 

Response to various queries are as under:

1. Purpose of Choke is to control the drawdown on the reservoir which is achieved by the choke limiting the quantity of well fluid that can be produced from the well. It is not used to control Flowing well head pressure.
2. Choke has critical flow when Downstream pressure <=0.55 times upstream pressure, for natural gas wells. When choke is having critical flow, for the given upstream pressure and size of choke, a constant and maximum mass flow rate is achieved. Therefore even in the event of pipeline failure and downstream pressure going down to atmospheric, the production mass flow rate from the well remains same and does not increase. Conversely, as downstream pressure increases to >0.55 upstream pressure, the choke is no longer critical and flow reduces and starts mimicing a pipeline flow.
3. Gas being a compressible fluid, a constant mass flow rate from a choke does not imply constant gas velocity in the downstream pipeline(s), as actual gas velocity of gas in the pipeline depends upon the prevailing line pressure. For a constant diameter pipeline, higher line pressures give lower actual flow velocities and lower pressure give higher actual flow velocities. Line pressure reduces along the pipeline as there is pressure lost due to friction loss, assuming there are no elevation changes. Therefore, along the flow in the pipeline, pressure keeps dropping and actual gas velocity keeps increasing.  
4. Erosion velocities for pipelines are reported for pure single phase flow and can be very high. However, solids in gas will quickly erode the pipeline even at normal pipeline operating conditions, which would be much less than the single phase erosion velocity.  Solids in gas can compromise the surface facility and well bore integrity very quickly and it is imperative to address and stop the solids production at the earliest before obtaining regular production from the well. This may be by reducing the drawdown - by reducing choke size, or by other downhole intervention methods.         

Hope this clarifies your queries.

Puneet Kishore