Determining Bottomhole Pressures in Pumping Wells

The use of the acoustic well sounding (AWS) technique to determine bottomhole pressure (BHP) requires an estimate of the gas-void fraction (f.) in the liquid column of a pumping well annulus. Three correlations relating the annular superficial gas velocity to fg are available for saturated oil columns. These correlations were developed by Godbey and Dimon, Podio et al., and Gilbert as reported by Gipson and Swaim. Use of these correlations for determining the BHP, either flowing or shut in, involves a stepwise numerical integration often performed by a computer. This work addresses three aspects of estimating the BHP from AWS data:estimation of the superficial gas velocity,development of analytical solutions for a single-step BHP calculation, andcomparison and interpretation of the predicted BHP's by use of the three correlations for the field examples. A mathematical model, based on the principle of mass balance of the annular. gas phase, is used to determine the superficial gas velocity. This model rigorously accounts for the time-dependent pressure, volume, and the gas deviation factor in the liquid-free annulus. Analytical solutions are obtained for both the Godbey-Dimon and Podio et al. correlations to calculate the BHP in a single step. These analytical solutions provide a significant improvement over the numerical stepwise integration technique, because a hand-held calculator can be used for the BHP calculations. The field examples studied indicate that both the pumping liquid column height and the superficial gas velocity pumping liquid column height and the superficial gas velocity play a key role in estimating the gas void fraction-an play a key role in estimating the gas void fraction-an essential element in calculating the BHP. We observe that only the early-time shut-in pressures are affected by the presence of gas bubbles in the liquid column. Because the presence of gas bubbles in the liquid column. Because the bottomhole flowing pressure (BHFP) is dependent on the correlation used to predict the fg, both skin and productivity index calculations are affected. Estimation of the productivity index calculations are affected. Estimation of the permeability/thickness product and the static reservoir permeability/thickness product and the static reservoir pressure, however, are independent of the fg correlation pressure, however, are independent of the fg correlation used. Introduction The majority of the oil wells in North America are on some form of artificial lift system. Brown gives a comprehensive review of these artificial lift systems. Typically, the oil is lifted up the tubing string while the gas is vented through the annulus to avoid gas-locking the pump. Sucker-rod (beam) pumps are the most popular and pump. Sucker-rod (beam) pumps are the most popular and widely used lift system in the industry. The process of gas venting through the annular liquid column (oil and/or water) has a profound effect on the liquid density. Because knowledge of the gas-lightened liquid column is the key to a meaningful BHP (flowing and/or shut in) estimation, a better understanding of the physical process is essential, so we explored the relevant physical process is essential, so we explored the relevant works available in the literature to provide an overview of the state of the art for estimating BHP's in sucker-rod pumping wells. pumping wells. A knowledge of the BHFP (pf) is an essential element in predicting a well's productivity index (J) and its inflow performance relationship (IPR). This information is instrumental in proper artificial lift design. A pressure buildup test conducted on a pumping well can provide an array of valuable information-such as permeability/thickness product, skin, and static reservoir pressure. The last piece of information is necessary for a meaningful J estimation. We will examine the methods available that permit estimation of pwf and subsequent shut-in pressures, pws, for a buildup analysis. Because of the mechanical constraints, a subsurface pressure recorder normally cannot be run down the pressure recorder normally cannot be run down the tubing string to monitor the in-situ pressure in a sucker-rod pump. After the pump and rods have been pulled, pump. After the pump and rods have been pulled, however a recorder can be run downhole to record pressures in the conventional mariner. This method has several problems. First, a rig is needed to pull the pump and rods and problems. First, a rig is needed to pull the pump and rods and rerun them following the test. The cost of the test may be prohibitive, especially for marginal wells. Second, the early-time data, including the p is lost because of the very nature of the operation. Permanent downhole recorders are used sometimes to monitor pressures in key wells of a field in certain cases. Because of their permanent nature, the recorders have a very limited application. Nind described several other alternatives-such as depression of the annular liquid column and two methods involving the use of a dynamometer. These methods are time-consuming and have other limitations. They are capable of estimating only the pf, and, consequently, have no application in pressure buildup testing. The use of AWS with an echometer has been a very popular method for estimating both the flowing and popular method for estimating both the flowing and shut-in pressures in pumping wells. Thomas et al. and McCoy describe the principle of the method. SPEJ P. 823