K.B. Nolen and S.G. Gibbs
Greenshot, LLC
America needs energy. Good techniques (Vogel’s Method and Constant PI) exist for estimating production from wells. These are phrased in terms of pressure. Pressure is not easy to measure especially in deep gassy wells. Fluid level is a logical remedy for the problem and can be measured at virtually any depth. But this creates the need for a method for deriving pressure (PIP) from fluid level. Wally Gilbert derived the first method in 1955. It was not used widely because casing gas (mcf/d) had to be vented while measuring it. This was a pollution no-no, then and now. In 1983 Echometer (J. McCoy and A. Podio) developed a method based on pressure buildups (psi/min). The Echometer method became the industry standard.
Recently a new device appeared which can supply Gilbert with casing gas in a non-polluting fashion. Thus Gilbert takes its place alongside Echometer as methods for deriving PIP from fluid level. Data for comparing accuracy of the methods was obtained by shooting fluid levels on ESPs equipped with downhole sensors to measure PIP (only 7 wells-a small sample). The results indicated a vast over-prediction of casing gas with the Echometer method. Echometer themselves recognized that casing gas below the fluid level should not be included. Echometer’s reckoning of casing gas improved. When deriving PIP, both Echometer and Gilbert proved inadequate in computing PIP (always low) in high pressure gassy wells. It is apparent that an accurate PIP can not be derived from a single pressure buildup when gas “heading” is occurring. A glaring fact surfaced. After decades of use, the industry still does not have a good method for deriving PIP from fluid level in all types of wells, especially in gassy wells with high fluid levels. More work is planned to increase the data set. Also heading in the casing is thought to be affecting the results. The ‘permanently installed’ casing gas rate device will be used to collect buildup data over long periods of time to offset the heading effects. New correlations (S curves) are needed to handle, high and low pressure wells.