A Procedure for Modelling Gas Gathering Systems

Abstract

Abstract It is not uncommon to hear gas producers complain that the success they had finding gas with the drill bit never seems to be fully translated to sales. Often they will use historical references such as, "sales volumes are expected to increase by only 70﹪, 50﹪, or even 20﹪ of the total gas volume added by new wells into the gathering system." In severe cases, new high pressure wells can cause older wells, especially those with liquid production problems, to shutdown. Operators quickly learn to avoid this situation by choking the new wells. In many situations, this is the best they can do, but it raises a big flag for anyone monitoring daily field logs. Inevitably, the question is asked, "Why are these wells choked?" Unfortunately, the operator can only say thatthis is what he has to do to avoid killing other wells, whereas, thosemonitoring the field's production cannot reconcile why all the new gas added to the system doesn't show up in the sales reports. This is a classic case of the confusion that often exists between office and field personnel. It is also a case that could be simulated using a gas gathering system model. A model could be used to demonstrate why new wells may need to be choked but more importantly, it can be used to test ideas for optimizing system production rates. FIGURE 1: Back pressure plot. Available in Full Paper. A gas gathering system model can be an important tool for testing the impact of various ideas proposed for optimizing a gathering system. The relationship, as shown, between deliverability (lower pressure to increase production) and the gathering system (increase pressure to increase capacity) is non-linear and opposing. This relationship can lead to the counter-intuitive situation where additional compression is installed to lower well back-pressures with the expectation of increased production, but the lower pipeline operating pressures reduce pipeline capacity and so the well back-pressures are only reduced minimally or worse, increased! Modelling offers the opportunity to test many ideas, virtually, before making expensive capital expenditures to lay pipeline or install compression that may not increase gas volumes appreciably. Consequently, proposals can be put to the test and can be quantified in terms of incremental gas rates and incremental recovery of reserves. When attempting to build a model to replicate an existing gathering system's behaviour, it is extremely important to first "tune" the model. Tuning requires that, at a minimum, a systematic approach be used to match pipeline pressure losses, each well's deliverability relationship and each compressor's capacity relationship to current operating conditions. The five most common mistakes made after building a model arenot tune the model,assume all the data is representative of true conditions,assume that all pressure losses are frictional pipeline losses,not confirm that well deliverability relationships match current operating conditions, anduse compressor design curves without confirming their validity with current operating conditions.