AWS Automated Buildup Test

Bottom-hole Pressure Build–up Surveys

The fluid in the reservoir rock is under pressure and is forced out into the wellbore. How quickly it leaves the rock depends on both fluid and reservoir characteristics.

The fluid produced can easily be sampled at surface and sent to a lab for analysis. Determining the reservoir characteristics requires another approach – the well must be shut-in and closely monitored. As soon as a well is shut-in, its bottom-hole pressure begins to rise, or build-up. A build-up survey monitors the pressure response of the well to determine the reservoir characteristics, that is, how quickly and in what manner the pressure in the wellbore equalizes with the reservoir itself. The build-up test requires producing the well until a stabilized production rate and pressure is achieved, then shutting in the well for a period of time from 2 days to 4 weeks.

The pressure data can be recorded using:

• An acoustic well sounder with dead weight gauge at surface
• Bottom-hole recorders

For pumping wells, it is advantageous to record the pressure using acoustics as the pump and rods do not have to be pulled out of the hole.

The pressure data, change in shut-in pressure verses time, is plotted on a grid to evaluate the reservoir parameters.

Analysis of the SHAPE and SPEED of the build-up curve reveals specific reservoir properties:

• Capability of reservoir to produce by determining the permeability of the reservoir rock
• Damage to wellbore referred to as skin damage, is a region of reduced permeability
• Positive skin is bad, negative skin is good
• The static reservoir pressure
• Size of the reservoir pool from the extrapolated pressure

This information is used by several regulatory bodies and different groups for various purposes:

• Evaluate production practices
• Determine remaining reserves
• Assess recover performance in secondary recovery schemes
• Resolve equity questions

Reservoir

• Recoverable reserves still in place
• Pool performance verses predicted performance
• Secondary/tertiary recovery candidates

Production

• Workover / recompletion candidates
• Artificial lift problems

A point of summary for conducting a sonic build-up survey on a pumping well:

• Ensure accurate production rate recorded prior to shut-in and that rates are stable
• Record producing fluid level with acoustic well sounder and if…high fluid levels, conduct a Fluid Depression test to determine producing bottom-hole pressure or if…very low fluid levels, shut-in well.
• Time interval between fluid level shots should be based on well’s reservoir characteristics and pressure history
• Generally high productivity wells requires more frequent shots than low producers due to rapidly changing pressures and fluid shots
• As the test proceeds, time intervals between shots can be extended. Early time data requires higher shot frequency
• Record casing pressure with a dead weight gauge or digital pressure transducer

Click here to view ACOUSTIC PRESSURE SURVEY BUILD UP TEST