Gas Deliverability Forecasting - Why Bother?
Authors: Trick M. 
Published: March 2010

Because...
The goal of gas reservoir management is to optimize gas recoveries and profits. This goal can be achieved by early recognition of opportunities, and by continually updating operating plans as economic scenarios change. These basic principles apply both to producing gas fields and to new or potential acquisitions. Early recognition of opportunities requires the careful engineering assessment of the production capability and ultimate recovery under different depletion strategies. This means that gas production forecasting is a necessary step in achieving the goal of good gas reservoir management; it is one of the most important responsibilities of the engineer in charge of a gas reservoir.

Why Do You Need a Computer Model?
To model a gas field properly, such that all potential bottlenecks are taken into account, the following factors must be included in the modelling process:Reservoir performanceSandface inflow performanceWellbore pressure lossesGathering system pressure lossesField facility performance
It is simply not practical to attempt to model all of these factors by hand. Traditional estimation procedures based on rules-of-thumb, generalized nomographs, and other simplified procedures can introduce large errors, particularly when multiphase flow conditions occur. Important interactions between different parts of the overall production system are ignored, again at further cost to accuracy.
On the other hand, the integration of all these important factors in a single computer- based model permits one to evaluate many different development strategies as easily and quickly as commonly used approximate methods. A good computerized gas deliverability forecasting model can help you do all of the following:Predict the most cost effective way to improve your competitive position in terms of your portion of total gas production when sharing a reservoir with other parties (i.e., competitive drainage analysis).Determine if it is economically viable to try to meet your current contractual requirements.Estimate how much gas is available for spot market sales.Evaluate whether drilling wells or adding compression will produce the most reserves, and thus, which option yields the most profit for the money invested.Determine the optimal placement of compression (Le., at the plant, or at various locations in the field) for maximizing production while minimizing capital cost.Evaluate whether changes to the gathering system (e.g., pipe looping) will increase production and/or decrease compression requirements.Determine if production from existing wells can be increased by wellbore modifications.
Furthermore, a good forecasting model will take into account the effects of time and cumulative production to determine the economic optimum field development over the entire field life. The addition of compression must be evaluated over the life of the field. A compressor sized according to current conditions is almost invariably not the most economical choice.

Tune Your Model to Match Actual Performance
Everyone has heard the old adage about computer programs-"garbage in, garbage out." For the record, software developers seldom provide you with a modelling program that will tell you that you got "garbage out."