Those experienced in the natural gas industry know that tests conducted early in the life of a well, in a tight formation, tend to yield optimistic deliverability forecasts. It is not uncommon for producers to report to government agencies, or to purchasers, the results of such deliverability tests, and then to discover, after the well has been placed on production, that rates and pressures have declined continuously to as low as 10-20% of the initial test quantities.
By understanding what controls this transient flow behavior, sometimes called "flush" production, and by collecting appropriate pressure data during the deliverability tests, we can make use of pressure transient analysis to accurately predict the "stabilized" gas rates that the well will eventually achieve.
It is to the producers' advantage to make practical use of current deliverability testing regulations, and to apply the latest state of the art technology to Pressure Transient Analysis. From the pressure buildup data, and the informed use of ADVANCED well test analysis software, one can obtain completion and reservoir characteristics to accurately predict the well's long-term deliverability behavior.
Unlike conventional reservoirs, production from tight formations must be forecast utilizing the "transient flow" equations rather than the "stabilized flow" equations. It is no longer necessary, nor is it acceptable, to use simplistic reservoir models.
Realistic forecasts must take into account the reservoir characterization and model description derived from flow and buildup test analysis. The transient flow forecast in tight formations may control the economics of production, and must therefore be modelled rigorously.
The majority of gas well testing regulations in North America require that the deliverability tests consist of several flow rates and durations with at least one of these being a STABILIZED point. The intent of the multi-point test is to define the reciprocal slope, n, of the back pressure plot and evaluate the extent of turbulence. The intent of the STABILIZED point is to locate the position of this "back-pressure" plot.
The above procedures work well in high and medium permeability reservoirs. In low permeability, and particularly in tight gas formations, these tests are not only unnecessary but can lead to misleading results. The reason that they are unnecessary, is that, because of the relatively low flow rates, the turbulence term can be neglected. The reciprocal slope, n, value is 1.0. The reason that they can be misleading, is that it is impractical to flow the well until STABILIZATION is achieved, and so, in general, the operator will flow this well for an EXTENDED (but NOT STABILIZED) period usually 24-72 hours, and anchor the deliverability line (back pressure plot) through this 72-hour test point.