Predicting future deliverability of gas to one or more points (gas plants) from one or more pools connected to the same gathering system requires a rather sophisticated computer program. Such a program should be able to handle both water influx and volumetric decline, vertical and deviated wells, changing well deliverability with decline, regulatory constraints, variations in gas composition in different parts of the system, and differences in elevation at various points in the piping. FORGAS is capable of all of the above, and in addition has many other useful features.
Long term gas production and deliver ability forecasts are necessary for the development of reservoir depletion plans and the design of gathering systems and other facilities. The ability of a gas prediction model to differentiate properly between different depletion strategy alternatives play a key role in assessment of the economic viability of these alternatives. This requires that the prediction model be able to accurately simulate with depletion:
The performance behavior of the reservoir pressure,
The gas inflow into the wellbore,
Gas flow up the production string (tubing),
Gas flow in the surface gathering system, and
The field facility.
Furthermore, it is also necessary that the gas forecast model be sensitive to different depletion strategy alternatives and contracts. Most of the diverse models presented to industry for gas field forecasting can be grouped into four main categories:
Models which stress the reservoir performance behavior. (Agrawal and Beveridge 1972, Mattes et al 1973, Van Beek and Troost 1979, Collier et al 1981).
Models which stress the gas gathering system and cope with looping and/or other complex pipeline configurations. (Stoner 1969, Dempsey et al 1971, Flanigan 1972, Agbi 1981).
Models which are directional economic optimum depletion strategy tools. (Winter and Edgington 1964, Van Goldfract et al 1966, Wentink et al 1971, Bradshaw and Kingsbury 1971, Huppler 1974, Padget and Tuer 1980).
Single or multipool and/or single or multiwell gas forecasting models. (Joyner and Lovingtoss 1970, Bennet and Forgerson 1974, Spoldi 1981, Dietz et al 1981, Baldwin 1980, Sullivan 1982, Trice and Powell 1977).
Because of the specific orientation of these models and the desired computational characteristics (easy, fast and/or cheap), some limiting assumptions and simplifications are inherently applied to the scope of each model. The model described in this paper should be viewed as belonging to a new or fifth category, based on the above outline, which has as its characteristics:
Simultaneous production of independent multi "gas plant-gas gathering system" with each system producing out of the same and/or independent reservoir(s),
many of the unique features of the aforementioned categories in a multiwell multipool and multicontract configuration.
For each of the companies involved a proper assessment has to be made of the alternative of producing all reserves under an agreement or joint arrangement, or unitized conditions by a single field/plant operator, compared with the viability of producing the reserves competitively.