This paper describes special aspects and problems encountered when analysing transient pressure tests in low permeability formations. A constant rate approach has been widely used in oil and gas industry for analysing well tests. However, in low permeability formations special problems arise in applying the constant rate analysis method.

First, using the constant rate approach the pretest borehole history must be approximated. As it acts as a long term constant pressure injection or production event while drilling through the test interval it results in the creation of a non-uniform pressure distribution in the vicinity of the borehole. Numerous field examples show that after the packer system is set a distinct pressure build-up or fall-off occurs. The non-uniform pretest pressure distribution cannot be neglected in the analysis of the subsequent test events e.g. DST sequence. Significant errors may occur in estimation of the reservoir parameters, in particular the initial pressure.

Second, the use of the constant rate approach in analysing the DST sequence in low permeability formations leads to errors unless the rate sequence, and thus superposition function, is constructed properly. A new method of rate discretisation for the initial discontinuous events (the beginning of the 1st. and 2nd. DST flow period) is presented and the errors resulting from neglecting the true rate sequence are discussed. Field examples present the application of both concepts.

The main objective of this paper is to show that by simple measures it is possible to take account of borehole history and apply rate discretisation to improve the accuracy of interpretation when using standard constant rate analysis software.

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