Abstract

Commercial exploitation of the large unconventional gas reserves of the lower portion of the "Resources Triangle" such as the tight Cretaceous Sands of the Elmworth-Wapiti deep basing and the S.E. Alberta Milk River-Medicine Hat low grade formations has proven that prediction of gas well performance utilizing commonly accepted methods and short term well test data can lead to serious errors due to the unsteady-state effects. The need for alternative approaches capable of extending the range of applicability beyond the limitations incurred by the former method, reinforced by seven years of fully documented anomalous performance of the multilayered, tight Debolt carbonate of the Dunvegan field, prompted the development of the unsteady-state deliverability model presented in this paper. This program utilizes the Laminar Inertial Turbulent approach suitably modified to accommodate the effect of time on the deliverability factor "A" in accordance with recent theories described in the literature and confirmed by the author's empirical observations.

The model features a complete suite of routines designed to effectively endow it with diagnostic powers comparable to those afforded by grid-based simulators, While providing for easier handling and fasterprocessing of the data base.

When future appreciation of reserves is anticipated as a result of development or infill drilling, the initial gas in placeterm of the material balance equation can optionally be revised at each time step to include a proper estimate of the inherent volume accruals. The periodical update ofthe pool's average back pressure parameter "A" subsequent to attainment of pseudo-steady-state regime is also advocated as the proper means to account for dynamic readjustment of flow patterns resulting from development and production practices (i. e. infill and/or development drilling, shut-ins etc.)

An example run using Alberta & Southern's tank-type deliverability model as a basis for comparative analysis illustrates the ability of the proposed program to readily pinpoint optimistic estimates of productive capacity arising from the application of steady-state assumptions and short term AOF tests results to the solution of an essentially unsteady-state problem. This can prove instrumental in an operator's assessment of its ability to meet contract obligations and to devise remedial measures aimed at insuring adequate supply.

Reliability of results is enhanced by a built-in calibration technique based on inputted "control points" (bottom hole or wellhead pressure measurements that generates appropriate corrections to initial controlling parameters so as to insure general compliance with the actual combined performance of the reservoir and gathering system.

COMPARATIVE ANALYSIS

Test runs designed to compare conventional tank type projections with the unsteady state deliverability forecasts proposed by the subject model revealed that:

  1. Short falls as highs as 20–30% of the flow rates predicted by conventional methods may be expected in low permeability heterogeneous reservoirs commencing in the early years of production. This is largely due to the failure of a tank type model to recognize inevitable future deteriorations of productivity determined by prevailing unsteady-state flow regimes and inherent time-dependency of deliverability parameters.

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