Pressure transient analysis (PTA) is traditionally used to characterize well and reservoir parameters from well tests based on shut-in periods. PTA was widely used for reservoir management and decision making before reservoir simulation became the main tool. Presently more and more reservoirs are surveyed by permanent (downhole) gauges. These gauges provide vast amount of pressure transient and rate data which may be interpreted using improved PTA approaches to gain more knowledge on reservoir dynamics. This has opened new prospects for PTA applications in field studies.
Permanent pressure and rate measurements cover both well flowing and shut-in periods occurring during normal operations. These measurements allow for analysis of time-lapse pressure transients and comparative interpretation of flowing and shut-in periods. The analysis of time-lapse data provides time-dependent description of well-reservoir parameters. The comparative interpretation gives understanding of flowing reservoir properties which are often different from those estimated from well shut-in periods as in the classical PTA. These approaches provided basis for an improved methodology of interpreting permanent pressure measurements, where the scope of the standard PTA application may be extended to integrate new data sources.
Application of the developed methodology has been demonstrated with data from fields on the Norwegian Continental Shelf. For many wells the comparative interpretation revealed significant difference in well-reservoir parameters estimated from flowing and shut-in periods. It was also found that the flow regime near a well may vary. For example, the dominance of a hydraulic fracture observed during a shut-in period may be significantly reduced during the flowing period with simultaneous changes in reservoir conductivity. Dynamic behavior of natural and induced fractures due to pressure (stress) changes, wellbore cross-flows and variable contribution of reservoir layers are considered to be possible reasons for these effects. Analysis of time-lapse pressure transients provided description of long-term changes in well-reservoir parameters, e.g. reservoir conductivity, hydraulic fracture properties and boundary effects.
The key advantage of applying this methodology is characterization of the more representative flowing, rather than shut-in, well-reservoir parameters as well as their permanent monitoring during reservoir lifetime. The monitoring may reveal ongoing changes in the reservoir characteristics, production impairment risks etc. The methodology uses data readily available for many fields and provides an updated description of well behavior and hydraulic reservoir properties. The reward may be improvement in everyday field operations and well performance optimization, reduced uncertainty of input to reservoir models and better decision making through enhanced model predictability.
