Evaluation of the initial reservoir pore pressure, the current average prevailing reservoir pressure, and the deliverability of a well commonly results in the need for one or more pressure or rate-transient tests of a well's performance to be conducted in order to properly characterize the properties of the reservoir and the well completion efficiency. Conventional pressure build up or multi-rate drawdown tests have commonly been used for this purpose. These types of transient tests can last for days or even months in low-permeability unconventional reservoirs, resulting in prohibitive production deferments and operational expenditure. The reservoir pressure and the well deliverability are key indices that are used to properly characterize the reservoir properties and the expected ultimate recovery of a well.
This paper presents the development and application of an elegant production performance analysis technique that can be used to accurately determine the initial and/or average prevailing reservoir pressure of both conventional and unconventional reservoirs (for any permeability level), for wells of any type, time level, inner and outer boundary condition and flow regime, using only transient well production data (flow rates and bottom hole flowing pressures as a function of time). With the improvements in accuracy that can now be achieved in determining the average reservoir pressure, reliable and accurate transient Productivity Index values can be obtained for producing wells.
In addition to the improvement in determining the initial/average reservoir pressure, the newly developed production analysis procedure also provides an excellent means of Quality Control for the validity and consistency in the reported well flow rates as a function of the system drawdown. Examples are presented in this paper in which a variety of different flow rate measurement or estimation techniques have been utilized. These include a state-of-the-art advanced and extremely accurate surface multiphase flow rate metering system, flow rates computed from downhole flow control valve measurements, as well as an example in which well flow rates have been estimated using measurements obtained for monitoring the performance of an electrical submersible pump.
The acquisition of extremely accurate, high-frequency production performance data greatly enhances the evaluation of the reservoir pressure obtained with the analysis technique presented in this paper. Temporal measurements of the sandface flowing pressures and flow rates, obtained for a time scale on the order of minutes or hours is often adequate for accurately determining the initial or current average reservoir pressure. However, the use of high-frequency production data obtained with permanent downhole gauges and surface (or downhole) continuous flow rate metering systems offer even greater precision in evaluating the reservoir pressure with this analysis.