Estimating IPR Curves in Intermittent Gas Lift Wells From Standard Production Tests

Abstract

Abstract Currently in Lake Maracaibo there are nearly 2000 wells on Intermittent Gas Lift (IGL) with an average production of 150 BPD, so a method to estimate the IPR curves in such wells is necessary and important. Considering that in IGL the bottom hole flowing pressure is an instantaneous value continuously changing, then the correct and precise way to determine the IPR curve is through a production test along with a downhole pressure survey. However, many times this method is not economically convenient in highly depleted wells, as it does in a typical intermittent gas lift well. Since there is no publication reported on this subject, the purpose of this work is to get the equations that relate Vogel's model with field data for IGL wells and a numerical method to solve these equations in order to obtain an IPR estimation. Through these equations more realistic values can be obtained, and therefore a better match can be reached between simulator's outputs and field data. These equations will help the field engineer to properly analyze and design these types of wells. Introduction IPR curves are easy to obtain in continuous flow, because with only two different pairs of values - bottom hole flowing pressure (measured or estimated) and its corresponding rate - Vogel's equation is directly defined. However, in Intermittent Gas Lift (IGL) wells, the bottom hole flowing pressure is an instantaneous value continuously changing, and therefore the inflow rate is also changing, so a special analysis is needed. This fact is reflected in the wellhead pressure charts where the transient essence of the intermittent gas lift production method is highlighted. Two alternating and periodic processes can be easily appreciated from these charts.Slug formation and casing pressurizationSlug rising and casing depressurization. This work mainly concerns the slug formation, and takes into consideration the fall back estimation from the slug raising process. This paper will show that fall back is not only important in design, simulation and operation of IGL wells but also it is a very important parameter in IPR estimations. The main idea in this work is to obtain a function for the formation of the liquid slug capable of predicting the pressure buildup that takes place during this stage, taking into consideration the liquid fallback and the fact that the slug formation is a periodic process. By this approach different problems can arise: from the easiest, which is to calculate Vogel's maximum rate (Qmax) knowing reservoir pressure (Pr) and considering one test rate, up to the most difficult case i.e. to estimate N parameters that define any arbitrary inflow model from M known test rates (M>N). In this paper Darcy s and Vogel s models will be solved simultaneously to show how the new approach works, besides it provides some reference for comparison and analysis.