Well Testing

Pressure measurements recorded near the productive interval form the basis for transient well-test analysis, and are primarily used for determining reservoir-rock properties and producing-formation limits. To understand this important discipline, well-testing experts have developed definitions ranging from average reservoir pressure to DST and well-interference testing. All definitions have been reviewed by experts in this field, and many are augmented by high-quality graphics

Role of Well Testing

  • To evaluate well condition and reservoir characterization;
  • To obtain reservoir parameters For reservoir description:
  • To determine whether all the drilled length of oil well is also a producing mite;
  • To estimate skin factor or drilling- and completion-related damage to an oil well. Based upon the magnitude of the damage. a decision regarding well stimulation can he made

Types of Well Tests

The main objective of drilling a well is to test and evaluate the target formation. The usual method of investigating the reservoir is to conduct a well test and there are two methods which are available: 

• Drill Stem Test (DST). The scope is to define the quality of the formation fluid. This is a short term test which uses a combination of drillpipe/tubing and downhole tools to evaluate the reservoir. The formation fluid may not reach or only just reach the surface during the flowing period. 
• Production Test. The scope is to define both the quality and quantity of the formation fluid. The formation fluid, in this case, is flowed to surface. 


A drawdown test entails flowing the well and analysing the pressure response as the reservoir pressure is reduced below its original pressure. This is termed drawdown. It is not usual to conduct solely a drawdown test on an exploration well as it is impossible to maintain a constant production rate throughout the test period as the well must first clean-up. During a test where reservoir fluids do not flow to surface, analysis is still possible. This was the original definition of a drill stem test or DST. However, it is not normal nowadays to plan a test on this basis.

A multi-rate drawdown test may be run when flowrates are unstable or there are mechanical difficulties with the surface equipment. This is usually more applicable to gas wells but can be
analysed using the Odeh-Jones plot for liquids or the Thomas-Essi plot for gas. It is normal to conduct a build-up test after a drawdown test. The drawdown data should also be analysed using type curves, in conjunction with the build up test.

A build-up test requires the reservoir to be flowed to cause a drawdown then the well is closed in to allow the pressure to increase back to, or near to, the original pressure which is termed the pressure build-up or PBU. This is the normal type of test conducted on an oil well and can be analysed using the classic Horner Plot or superposition. From these the permeability-height product, kh, and the near wellbore skin can be analysed. On low production rate gas wells, where there is a flow rate dependant skin, a simple form of test to evaluate the rate dependant skin coefficient, D, is to conduct a second flow and PBU at a different rate to the first flow and PBU. This is the simplest form of deliverability test described below.

A deliverability test is conducted to determine the well’s Inflow Performance Relation, IPR, and in the case of gas wells the Absolute Open Flow Potential, AOFP, and the rate dependant skin coefficient, D. The AOFP is the theoretical fluid rate at which the well would produce if the reservoir sand face was reduced to atmospheric pressure. This calculated rate is only of importance in certain countries where government bodies set the maximum rate at which the well may be produced as a proportion of this flow rate.