Well Test Analysis
Well test analysis is a critical tool to understand well and reservoir performance. Unlike any other oil and gas surveillance activity, a pressure transient test provides information about well and reservoir performance (permeability-thickness KH and skin S), hydraulic connectivity over large volume and average reservoir pressure. It gives the ability to understand reservoir flow behaviour in a simple and powerful way.
More and more information about the wells and reservoir are extracted from the rate and pressure data by using modern well test analysis tools and techniques. Minimum connected volumes, compartmentalization, field-derived relative permeability curves, prediction of water breakthrough and diagnosis of production problems are some few examples of such information. Oil and gas field recovery strategies can even be assessed and improved by using well test analysis.
In Exploration and Appraisal, well tests can be carefully designed, optimized and interpreted to bring the best quality data, achieve the subsurface objectives and reduce reservoir uncertainties. In production, opportunistic free shut-ins could be evaluated to monitor the wells and reservoir.
In the Well Test Library section, the theory behind well test analysis is explained for a producing well in a new reservoir and the flow behaviour is described for different types of wells. This theory is then extended to any shut-in period. A minimum number of equations are given to avoid complicating the issues in well test analysis.
Well Test Analysis Tools
To analyze the PBU test, the analysis toolkit includes 4 main tools:
• Data Plot, also called Production History Plot. This shows the bottom-hole pressure and rate versus time.
• Superposition Plot, showing the shut-in pressure versus the superposition function,
• Log-Log Plot, showing the pressure signal Δp and derivative curves versus shut-in duration Δt using a log-log scale,
• Deconvolution, showing the deconvolved Δp and derivative plots versus the test duration Δt using a log-log scale.
Well Test Analysis Techniques
Once fully familiar with the tools, well test analysis can be performed to recover the well and reservoir information from the data.
1. The Data-Plot will first help you with a qualitative interpretation, giving some indication on skin, permeability-thickness and pressure support over time.
2. The evolution of the derivative shape over time for the different PBU tests should be investigated. This will bring confidence in the derivative data and detect any reservoir and non-reservoir effects. With favourable reservoir conditions, deconvolution should be investigated.
3. The conventional and deconvolved derivatives are the driving tool for the analysis and should be used to detect the flow regimes and define some possible well and reservoir responses.
Deconvolution will help to investigate the presence of boundaries at late times. This could be supported by a decreasing pressure behaviour during the whole test sequence in the production history plot and by a downward shift of the different pressure build-up data in the superposition plot.
4. Once a possible combination of flow regimes is obtained over time, a well and reservoir interpretation model, also called “type-curve model” can be selected and flow regimes can be spot with a series of straight lines fitting the derivative data. This is the straight line analysis.
Well and reservoir parameters for some flow regimes can be subsequently obtained directly from the derivative, without matching the data with a complete type-curve model. For instance, the permeability thickness product can be calculated directly from the radial flow stabilization line and the wellbore storage can be obtained from the intersect of the radial flow stabilization and unit slope straight lines.
From the stabilization line, the permeability-thickness product, the skin and the radius of investigation are directly calculated.
5. Well and reservoir parameters are then refined by matching the pressure derivative for that interpretation model with the derivative of the field data. As for the ΔP curve, the match will probably be obtained by adjusting the skin and wellbore storage values.
6. The results should be checked by looking at the other tools and all the plots should be matched. For gas wells, the turbulence factor and mechanical skin could be tweaked while keeping the total skin constant so as to improve the match on the flowing periods.
7. Repeat steps 4 to 6. Since well test analysis is a non-unique process, more than one solution should be given.