“Mendoza, this meter is broken”, I complained. “Every time we increase the compressors speed to pull-down the wellhead pressure, the recorded gas flow drops. I just raised the rpm from 375 to 425, and the wellhead pressure fell from 220 PSIG to 15 PSIG. But the metered flow decreased from 180 MSCFD to 150 MSCFD. That’s impossible; the meter must be broken”.
“Yes Sir”, responded Mendoza, “you mentioned the same thing last week about that old well down near the river. But when we checked it out, the flow meter was okay”.
“So you think it’s water again”, I ventured.
Mendoza settled himself comfortably on the trucks tailgate and explained. “Yes Sir, it looks like we’re just sucking water into the well. The harder we suck with the compressor, the more water we bring up”.
“I can see that, but why don’t we increase our gas make too”.
“You know more about these things than I do Sir. But what I’ve been told is that the gas in the reservoir is floating on top of a pool of brine. Once the gas pressure in the reservoir is pretty much reduced, the brine starts working it’s way towards the casing perforations. The lower the pressure at the perforations, the more easily the brine flows up into the gas formation and into the well. That’s called “coning”, because the water is supposed to be flowing up at an angle to the perforations. Once the water enters the production tubing, gas flow from the wellhead always drops off’. “Yes Mendoza, the water rising to the surface interfers with gas production. This happens because of the following:
1. The average density of the fluid inside the subsurface tubing increases.
2. The downhole pressure increases relative to the surface pressure.
3. The pressure difference between the reservoir and the casing is diminished and hence the flow of gas from the sand formation through the casing perforations slows”.
“After all”, I continued, “the rate of natural gas production is really not a direct function of the wellhead pressure. Rather, the controlling variables are really the reservoir pressure and the downhole pressure:
“Mr. Lieberman”, interrupted Mendoza, “aren’t you getting off the subject again. What I want to know is what do we do now”. “Suppose we get a sample of water from the high pressure separator”, I answered. “We could get it analyzed for salt. If the salt content of the water is a lot lower than that of the brine produced when the well was first put on line, we can assume that water is leaking around the outside of the casing. A “squeeze-job” (i.e. forcing more cement around the casing) is supposed to correct this problem. But if, as you say, we are promoting water flow (i.e. coneing) from a water zone below the gas bearing sand formation, we had better just slow the wellhead compressor back down to 375 rpm. After all, the flowing water is probably promoting the formation of channels that will make future coneing of brine into the well even worse”.
