When natural gas from high pressure and temperature reservoir is produced, due to cooling of gas in wellbore tubing and in gas gathering pipelines, the amount of saturation water dropout increases. Furthermore, as gas reservoir pressure declines due to gas depletion, amount of connate water production also increases. If aquifer exits, it also adds to the amount of water production. The problem of lifting and carrying increasing amount of either saturation or mobile connate and aquifer water while gas production rate is declining, poses an interesting problem of maximizing both gas deliverability and gag recovery considering wellbore and surface gathering network. For example, large tubing diameter will result in initial higher gas deliverability due to lower frictional drop when gas rate is high and able to lift and carry water, but it will also result in inability to lift and carry water when gas rate declines in future, and thereby, limiting gas recovery.
Factors affecting gas deliverability, such as, tubing diameter, delivery pressure, compression ratio, water dehy /separator/line heater installation, pipeline diameter, pipeline contour, amount of free water etc. are studied with view to offer certain guidelines on gas field development aspects when saturation and/or mobile water production problem is expected. Specifically, this paper deals with:
enumeration of the most important factors affecting gas deliverability and its recovery,
aid in planning and conceptualizing gas pool development, and
calculation procedure for estimating amount of saturation water dropout.
Production, collection and distribution of natural gas requires interconnected pipeline network because of large gas volume. Mathematical simulation which simultaneously solves equations describing fluid flow in reservoir,r tubing, pipelines, and associated surface equipment, such as, dehy, compressor, separator, chokes, etc., and other contractual constraints, is called gas network analysis. The objective of the simulation is to find the maximum gas deliverability. The mathematical simulationand solution techniques are fairly well known, and therefore, will not be described in this paper commercially available software(l,2) to do the gas deliverability analysis are used in this paper. The dry gas simulator(l) uses Cullendar and Smith (3) and Bureau of Mines (4) Monograph 10 method for calculating pressure drop in tubing and gas flow in pipelines. As most user's assign certain efficiency factor to the pipeline pressure drop calculations, the model uses the turbulent region friction factor with relative roughness of 0.0005 (partially rough pipe). This is done to simplify the concept.
The gas water deliverability simulator(2) uses two phase pressure drop calculation procedures outlined in references(5,6,7,8,9). Gas density is calculated using the Standing(10) chart for the compressibility factor. The amount of water vapor in gas is calculated using the data of McKetta and Wehe (11).
The calculated gas density and amount of saturation water is shown in Figure 1 at the pressure and temperature ranges encountered while simulating the gas deliverability network. Note that the gas density (inversely, gas volume and tubing/pipeline gas flow velocity) could be decreasing in the multiples of 100's as it travels from the reservoir to a given delivery point.