"Obviously the frac material is still so prominent that it is obsructing the flow volumes from the zone."
Thanks SAM-DAN for your analysis of the NR. I agree with your assessment that flow rates should increase as the frac fluid continues to be purged and that the NR is misleading given your DD findings. I am not very familiar with wellhead technology but I do have hands-on experience as a test laboratory engineer with the measurement of flowing gases, liquids and mixtures of the two. The flow of a fluid (a fluid can be a gas or liquid) through a conduit is governed primarily by the First Law of Thermodynamics (Conservation of Energy). If we neglect friction at the conduit wall there are three components of energy: 1. Internal (fluid composition, temperature and pressure), 2. Kinetic (velocity and density), and Potential (elevation change). If we further assume isothermal flow and omit for the moment the pressure changes that occur with elevation change (i.e. assume horizontal flow), simple flow equations can be deduced such as the Bernouilli Principle which governs the flight of winged aircraft.
Now consider the flow of pure natural gas (no contained liquids) at a wellhead choke. For given gas properties, I believe the flowrate achieved depends on the wellhead pressure and the diameter of the choke. Clearly, increasing the choke diameter should increase the flow rate (we can live with a pinhole leak in our vehicle's tire for a couple weeks but driving over a three-inch nail protruding from a board will likely stop us a short ways down the road). As the gas necessarily accelerates to a higher velocity at the choke, the gas pressure decreases right at the restriction in order to satisfy the First Law of Thermodynamics. In other words, kinetic energy is locally increasing at the expense of the internal energy component (pressure). Downstream of the choke, this process is reversed and much of the pressure is recovered again as the gas decelerates.
Now consider trickling in a dense liquid--the frac fluid--along with the gas flow. This introduces several energy losses which must be incurred at the expense of the inherent energy of the natural gas. First, a lot of energy gets chewed up by the process of accelerating the frac liquid through the choke. In addition, the energy of the gas phase is depleted by the acceleration of the frac liquid within the well bore itself and by the work done by the natural gas in lifting the frac liquid from the bottom of the well to the wellhead. IMO, all these energy losses translate into lower wellhead pressures and reduced natural gas flow rates.
I think a competent reservoir engineer should have been able to pull an estimate for this flow reduction off the top of his head, or, conversely, the flow increase which should occur once the frac liquids are thoroughly purged. This is why the NR was misleading. |
