Application of Pulse-Test Methods in Oman
Authors: Rijnders J.P. 
Published: December 2007

Single-well pulse testing using very short pulses is effective for measuring vertical permeability. A particular advantage is that, unlike the constant-rate method, it does not require fully built-up conditions.

Introduction

Major decisions concerning the future development of a reservoir must increasingly be made in the early stages. Numerical simulators are now used early to study the effect of various development schemes. The extent to which these model studies are useful as a method of comparing various possible development schemes depends largely on the reliability of the early available data. The lateral permeability development and the magnitude of the vertical permeability (for coning studies, for example) arc often critical parameters. Because subsequent production can disturb the reservoir to such an extent that representative permeability data can be obtained only initially, tests that can provide that early information are especially important. Recently, therefore, there has been a revival of interest in well tests, especially those that provide a direct measurement of the in-situ horizontal and vertical permeabilities. permeabilities. New test methods based on pulse disturbances (single or repeated) have been proposed to achieve this. The advantage of these methods over constant-rate methods is that, owing to the more complex response resulting from the pulses, they provide more selective information on the in-situ reservoir parameters. The pulse response can be characterized by only relative pulse response can be characterized by only relative pressures (see Fig. 1); therefore pulse tests can be pressures (see Fig. 1); therefore pulse tests can be performed even when a general back-ground pressure performed even when a general back-ground pressure trend is present. Although there are these theoretical advantages, they are counterbalanced by greater, experimental difficulties. The measured response usually is weak, so special high-sensitivity recording equipment must be used. Also, the weak response is more susceptible to small disturbances than the response of conventional tests. An additional problem is created by the fact that variable pressures must be recorded. To ensure that the pressures measured inside the observation wells truly reflect the pressure variations in the reservoir, these wells must be "hard" enough. This means that the time constant of the wellbore-perforation system must be smaller than the variations in pressure that are to be observed. A well can be made harder by reducing the total compressibility of the fluids inside the wellbore and additionally by reducing the resistance over the perforations (skin). This implies that it might be necessary to completely fill the well with a gas-free liquid or to use special packer arrangements. This is generally time-consuming and expensive. These interwell pulse methods generally are applied only to low-storage pulse methods generally are applied only to low-storage reservoirs with close spacing, since in such cases a reasonable response can be expected. In the Yibal oil field in Oman, an interwell pulse test was carried out over long distances.

JPT

P. 1025