PRESSURE TRANSIENT ANALYSIS (PTA)
PTA has been the primary reason for the development of the tools we use today. It was initially called Well test Interpretation. Originally this type of analysis was performed on data acquired during operations refereed to a well test. A typical well test set-up is shown in that picture, temporary equipment is installed downhole and at surface, the well is put on production under a predefined program and the diagnostic is performed, generally on a shut-in period after a stable production phase during which the producing rate was measured.
To perform a Pressure Transient Analysis the rates from the tested Wells and, where applicable, nearby wells are required. In addition the pressure response, preferably from downhole measurement, and generally acquired during pressure build-ups, are recorded. However it is always recommended to acquired the pressure response during the full production history of the test. Additional information needed includes the fluid physical properties; pressure, volume and temperatura (PVT) and possibly logs and geology.
The first PTA methods were introduced in the 1950’s with specialized plots (semilog, MDH, Horner) initially focused on a specific flow regime called Infinite Acting Radial Flow (IARF),where both well productivity
and the main reservoir properties could be determined. Specialized plots for other flow regimes (liner, bi-liner, pseudo-steady state, etc) were also developed.
In the 1970’s log type-curve matching techniques were developed to complement straight line techniques. The principle was to plot the pressure response on a log-log scale tracing paper and slide this plot over pre-printed until one was selected and matched. The choice of the type-curve and the relative position of the data on this type-curve, called the match point, were then used to calculate physical results. These methods were of poor resolution until the Bourdet Derivative was introduced.
In 1983, the bourdet derivative, the slope of the semilog plot displayed on the loglog plot considerably increased the diagnostic capability, resolution and reliability of a new generation of type-curves. However, the mid 1980 saw the development of PC (Personal Computer) based dedicated software, with the possibility of directly generating models integrating
superposition effects. These packages are based on modern pressure transient analysis and the use of sophisticated and user-friendly computer programs running on state-of-the-art PCs. Advanced mathematical models are used to match the measured pressure response to any disturbance, taking into account the complete pressure and flow rate history thus generating the exact model corresponding to the actual test history.
Models are diagnosed through pattern recognition of the different flow regimes present in a response and using the Bourdet derivative, which defines these flow regimes easily. The engineer can decide which should be the most appropriate model to apply.
Downside in the methodology of Pressure Transient analysis basics
The methodology has a downside in that solution found are not always unique so the engineer is challenged to search for the most consistent answer by considering all data available to him from all sources, not only the well test. Gone are the days of most straight-line analysis. MDH, Horner and other specialized analysis plots have become redundant as it is the model and the match with the real data that governs the validity of these analysis. In addition, nonlinear regression to improve results, and the development of powerful PCs, has brought the methodology to the point it is today:
The development of new analytical models in the 1980/1990’s and processor hungry numerical models in the 1990/2000’s converged with the availability of increasing volumes of reliable data and high speed desktop computers.
The application of this methodology spread rapidly beyond well tests as other field operations could produce candidate data for such processing. So the name drifted from Well Test Interpretation to the more generic term Pressure Transient Analysis, although the name Well Test (WT community, WT Forum, WT monograph) remained.
Pressure Trasient Analysis was the correct terminology because the basic process was the interpretation of the pressure signal after correction taking into account the production history (superposition time, convolution, deconvolution, etc).
Pressure Transient Analysis was about making a diagnostic, and then using this to take decisions, including remedial action on the well and/or using the resulting model to simulate future well behavior.