Formation testing, or FT, has many applications including formation fluid characterization, pressure measurements and reservoir characterization.
Pressure data can be analyzed as it varies with depth. Multiple pressure measurements are taken in the well to determine fluid density gradients
and potential fluid contacts. With the extraction of the formation fluid, the pressure can also be analyzed as it varies with time, essentially as a
mini well test. Depending on the extent of the tests, reservoir parameters such as permeability, skin and well
deliverability can be characterized.
Azurite offers an integrated environment with the ability to process raw FT data, switching seamlessly between the vs time and vs depth views of
that data. The test types and qualities are identified automatically, fluid gradients/contacts can be determined and a comprehensive PTA analysis performed.
- Fluid Gradient Definition
Data loading in KAPPA-Workstation
Azurite can load an unlimited number of stations (vs time) and static passes (vs depth) in LAS, DLIS, LIS, ASCII format, or from clipboard or manually.
An FT tool is run with pressure gauges to record the reservoir response during the test. The reference pressure may be the gauge belonging to the tool module, used during the operation (probe or packer) with higher resolution (quartz vs strain gauges).
Azurite uses the pressure gauge information to compute key pressures (mud pressure, stop and drawdown pressure), the test quality, and identify the test type.
Test type identification
A station may contain different test types. Depending on the sobjectives of the operation, a variety of tests are carried out to read the response of the reservoir (pretest, miniDST, sampling, etc.).
Azurite follows a series of criteria to identify automatically the test type and validity per station. This criteria depends on the reference pressure and the computed rates behavior.
Test quality is used to identify the confidence of the tests recorded at a station. This range may vary between excellent to low quality tests.
Azurite calculates the quality from the pressure standard deviation, the build-up pressure stability, the build-up temperature stability, and the drawdown mobility.
Pf - Mdd identification
An objective of FT is to determine Formation Pressure (Pf). The drawdown mobility (Mdd) is calculated by making the assumption of steady state spherical flow. This mobility depends on the rates, pressure difference and the tool geometry.
By default, Azurite defines Pf as the last pressure of the test, read on a line obtained by regression over the last 60 seconds of the build-up. As with Pf, mobility is calculated for the Pretest and the Mini-DST tests.
In practice, during an FT operation, the cumulative volume is measure vs time. From this data, it is possible to compute rates.
Azurite calculates rates from volumes. The Compute Q calculation is launched on the station volumes (total volume, VTT) in order to finally get total rate (QTT). The calculation of rates can be performed by simple point-to-point derivative (three point derivative) or either using the first order derivative (Two point derivative).
Azurite offers, a comprehensive built-in analytical catalog combining well, reservoir and boundary models.
Additional capabilities include changing wellbore storage, vertical and horizontal (dual packer only) anisotropy.
Azurite includes constant as well as changing wellbore storage models to fit early time behavior. Changing storage models include Hegeman, Fair and Spivey.
To match transient data and depending on the tool geometry, the following well models are available in Azurite:
- Single Probe Model (circular and oval probes)
- N-probe model (dual probe and SaturnTM)
- Vertical dual packer (limited entry)
- Horizontal dual packer
- Slanted dual packer
To match transient data and effects due to the reservoir, the following reservoir models are available in Azurite:
- Pseudo steady state
- Slab model
- Sphere model
A range of boundaries can be modeled analytically in Azurite including:
- Single fault
- Parallel faults
- Intersecting fault discrete angle
Data vs depth plot
In general practice it is common to use open hole log data to identify the 'best spots' where a pressure point or sampling can be taken.
Azurite displays the static passes (e.g. GR, Resistivity, Density, Neutron porosity, etc.) and pressures/mobility vs depth that may help in the identification of different zones/formations.
Open hole log data
The open hole logs are very useful for the zone definition, especially the gamma ray tool (GM) where different rock types can be identified.
Open hole logs can be loaded into Azurite for visualization purposes. They may be integrated into a single user track (e.g. Density-Neutron porosity) to assist with interpretation.
The user can define different zones if different pressure trends may identify zones when open hole logs are not available.
Azurite can identify different zones if different pressure trends are noticed to diverge from the expected pressure change due to the density of the reservoir fluids.
Pressure vs depth
Running a formation test tool allows the construction of pressure profiles (vs TVD). This data is used to identify fluid gradients and the potential contacts in the reservoir.
After the automatic identification of tests, Azurite selects the higher quality tests per station. The pressure is plotted vs depth and the gradient defined and hence the fluid in situ identified.
When several pressure points along the wellbore exist, a gradient analysis may be performed. The main objective is to quantify pressure variation with TVD which is used to estimate in situ fluid density.
In cases where different fluids coexist in the reservoir, pressure points may identify the contacts due to the density difference between these fluids.
In Azurite, the intersection between two fluid gradient lines is identified as a contact displaying its depth and pressure value.