Decision making with uncertainty while developing multiple gas/condensate reservoirs: Well count and pipeline optimization

Well-developed methodology exists for handling uncertainty for a single reservoir. However, development of multiple fields presents a significant challenge when uncertainty in a large number of variables, such as gas in place and liquid yield, occur in each reservoir. Some of the challenges stem from our need to forecast the system behavior involving a coupled reservoir/wellbore/surface (CRWS) network for the entire spectrum of variables so that facilities can be designed for the range of fluid composition and throughput. Of course, assessing well count and sequencing well drills are some of the important objectives. This paper describes probabilistic production forecasting with a compositional CRWS network model for nine reservoirs involved in delivering gas supply to a liquefied natural gas (LNG) plant in Nigeria. Our main objective was to use an economic indicator to select the optimal design of two main pipelines, each transporting 200 and 300 MMscf/D from the two production platforms, located 15 and 5 km, respectively, from the processing platform. Rate and cumulative profile s showed that sustained deliverability of gas could be realized for approximately 11 years before the decline occurred in high-permeability reservoirs. In other words, uncertainty in gas in place did not surface during the plateau period, only during the decline period lasting another 5 years after the first 11. In contrast, the liquid rates exhibited a large uncertainty band throughout, a direct manifestation of the condensate yield issue. The uncertainty band among each of the 12 components aided facilities design. Differences in net present value (NPV) and discounted profitability index (DPI) were used as discriminators for discerning optimal pipe size from the standpoint of project economics.