One of the more puzzling phenomenon I have observed in gas field production happened during my tenure as an operator of wellhead compressors. One would intuitively assume that the faster the wellhead compressor ran, the more gas would be delivered through the sales meter. Normally, as the compressor speed was increased by manually screwing open the governor speed control valve, the compressor suction pressure fell. Of course, this also reduced the wellhead pressure and the gas flow would be expected to increase according to the formula:
The above equation is really of little use to the field troubleshooter because C,n and PR are unknown for partially depleted wells. But,the equation does positively indicate that gas flow will never decrease as the wellhead pressure is dropped. Much to my surprise, I began to observe that as I dropped the wellhead pressure by speeding-up the wellhead compressors that:
• 30% of the wells did not exhibit any observable increase in gas flow.
• An additional 10% of the wells actually lost production as the wellhead pressure dropped.
If the reader will consult equation 1, of the previous article, he will note that when wells have relatively high stabilized shut-in pressures, as compared to their flowing wellhead pressure, that the incremental gas flow obtained from a further reduction in the flowing wellhead pressure may be quite small. It transpires that there is another factor which tends to negate the effects of decreased wellhead pressure. This factor is water.