Although we have been talking about wellhead pressure (both shut-in and flowing tube), the wellhead pressure is just an indirect indication of the really important parameter-that is, the bottom hole pressure. It is the pressure inside the casing at the level of the perforations that determines gas flow. By lowering a pressure sensing instrument suspended on a wire-line to the proper depth, bottom hole pressures can be directly measured. But this is an expensive and time consuming procedure, and beyond the scope of the options available to the field troubleshooter.
So we do not usually know the actual bottom hole pressure. If we knew the density of the column of fluids (i.e. the mixture of gas, condensate and brine) inside the tubing, we could calculate the bottom pressure as follows:
It is the difference between the bottom hole pressure (Pp) and the pressure in the surrounding sand formation that determines the rate of gas flow from a well. From equation (2) we can see that the bottom hole pressure will increase as the density (SG) in the tubing rises. This increase in Pp reduces the gas production from the well according to the formula:
The main point that the troubleshooter must absorb from the preceeding paragraphs is that any increase in the average fluid density in the tubing will surpress gas flow. An increase in this density is always due to the accumulation of condensate and/or brine in the tubing. Unfortunately, there is no way to measure this accumulation. Hence, the troubleshooter cannot really make direct use of equation (3). However, with a little experience, it is possible to determine the approximate effect of liquid loading on many wells.