Applying Gas-Well Load-Up Technology (1991)

Summary This paper incorporates critical-rate and blowdown-limit technology intosystem-network-analysis (SNA) techniques to predict abandonment pressures fordepletiondrive reservoirs and demonstrates that SNA by itself tends tounderestimate the abandonment pressure. A number of practical operationalconsiderations pertaining to the use of this technology are also outlined. Introduction At the outset of this series, we said that the technology of low-pressuregas-well load-up would be presented to enhance the understanding of gas-wellload-up problems and to establish more consistent and accurate analyticalmethods for evaluating the economic abandonment of depletion-drive gasreservoirs. The first three parts of the series presented the technology fordetermining a presented the technology for determining a well's critical rate, for predicting wellbore hydraulics during load-up, and for calculating a newproducing limit known as the gaswell blowdown limit. This final part presentspractical methods for using this technology practical methods for using thistechnology to predict abandonment pressures, to evaluate productionalternatives, and to perform a reservoir depletion analysis. This paper alsosummarizes considerations for operating low-pressure gas wells and fields. Methods for Predicting Abandonment Pressures In the past, the most common method for predicting the abandonment pressureof a predicting the abandonment pressure of a depletiondrive gas reservoir wascomparison to a similar reservoir's past performance. Because of recentadvances in performance. Because of recent advances in computing technology andtwo-phase-flow calculation methods, an increasing number of engineers are usinga more rigorous SNA to predict reservoir abandonment pressures. This processinvolves integrating pressures. This process involves integrating the reservoirflow performance (inflow) and the wellbore/system flow performance (outflow)for various flow rates and static reservoir pressures. The results of theintegration can then be plotted and analyzed. Fig. 1 is an example of such aplot. The abandonment pressure for each well/ system configuration is typicallyinterpreted as the static reservoir pressure where the wellbore/systemperformance becomes unproducible-i.e., where the inflow and outflow performancecurves no longer intersect. Two-phase-flow correlations indicate that at thispoint the wellbore will load up and the well will die. Using the Liquid-Droplet-Model Critical Rate To Predict Abandonment Pressures If we compare this load-up point to the critical-rate calculation presentedin Part 1 of this series, we can construct a comparison plot like that in Fig.2. As this figure shows, as the static reservoir pressure declines, thetangency point diverges from the critical rate. Thus, the previous analysiswith the tangency point as the load-up point yielded a lower abandonmentpressure than is indicated by liquid-droplet-model criticalrate technology. Thedata presented in part 1 showed that the liquid-droplet-model critical rateaccurately predicts the load-up threshold for low pressures. We can concludethat the two-phase-flow correlation does not correctly interpret this samepoint of wellbore instability. Although examination of the causes of this difference is beyond the scope ofthis paper, it is worthy of a few comments. As a general rule, mosttwo-phase-flow correlations use an interpretation of vertical flow regime topredict where the transition from mist to slug predict where the transitionfrom mist to slug flow begins. This transition point is generally consideredthe point where the wellbore becomes unstable. The correlations, however, cannot determine the point where liquid droplets begin to be held up in thewellbore. This difference in load-up-point interpretations could explain thedivergence between the two methods. Using the Blowdown-Limit Model To Predict Abandonment Pressures A comparison of the abandonment pressures predicted with liquid-droplettechnology predicted with liquid-droplet technology combined with SNA tohistorical data for various reservoirs showed that the criticalrate abandonmentpressure is still not the correct ultimate abandonment pressure of mostdepletion-drive reservoirs. In fact, many reservoirs are depleted below thepressure predicted with this method. To pressure predicted with this method. Tounderstand the reason behind this, it is helpful to examine the technology ofthe blowdownlimit model and its application to the same SNA analysis. JPT P. 344