Predicting Gas Well Load-Up Using Nodal System Analysis

Abstract

SPE Member Abstract Predicting the onset of gas well load-up is essential to optimize the performance and maximize reserve recovery from mature pressure-depletion gas reservoirs. The widely used approach of nodal system analysis to evaluate well performance has previously been considered unreliable in predicting the producing rate and reservoir pressure at which load-up occurs. This paper will demonstrate that compositional wellbore fluid modeling is required for applications of nodal analysis on low-pressure gas wells to accurately match performance and reliably predict the occurrence of load-up. Introduction Gas well load-up is frequently the controlling factor in the abandonment of mature pressure-depletion reservoirs. Load-up occurs in gas wells at low producing pressures when the flow rate velocity becomes insufficient to carry and continuously remove the produced fluids from the wellbore. Operators strive to delay load-up and increase reserve recovery by installing compression and optimizing wellbore hydraulics. The accurate prediction of the producing conditions at which gas well load-up will occur is essential to assess operational modifications and determine reserves. Nodal analysis is often used to evaluate the performance of a well by analyzing the pressure-rate relationship at various points or nodes throughout the well's producing system. In the past, however, nodal system analysis has been considered unreliable for low-pressure gas wells since applications have routinely underestimated the reservoir pressure and rate at which load-up occurs. Alternate methods are available to predict the rate at which load-up will occur, but they do not provide the corresponding pressure that is needed for reserve determination and flow optimization. The minimum flow rate required to keep a gas well unloaded, known as the critical rate, can be calculated from the liquid-droplet flow model developed by Turner et al. A recent study Of low-pressure gas depletion by Coleman et al. confirmed the validity of this calculated critical rate with field tests on numerous low-pressure (less than 500 psig) gas wells forced into load-up (see Figure 1A). In an attempt to more closely determine the associated reservoir pressure at load-up, Coleman et al. proposed applying the calculated critical rate to well performance predictions obtained from nodal analysis (see Figure 1B). Although this approach has significantly improved estimates of abandonment pressure in depletion-drive gas reservoirs, field performance continues to show that even these adjusted pressure predictions are too low in comparison with actual pressures observed at load-up. Based on extensive computer wellbore modeling, we have found that past applications of nodal analysis have not accounted for the water that is contained in the -produced gas and/or the phase change of this water that occurs in the wellbore. The presence of water can have a significant impact on well flow performance. This fact is substantiated by the work of Ferrini et al. on horizontal pipeline flow, which concluded that "water radically modifies the pressure-loss mechanism" and must be included when predicting flow performance. P. 983^