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Abstract This paper presents a well test performed on a high permeability North Sea reservoir producing from a water zone. The test and the following interpretation were designed to be of a conventional North Sea format. However the pressure behaviour from the formation combined with the limited quality of the bottom hole pressures resulted in a complex analysis sequence. The interpretations had to be performed using pressure derivative methods and semilog analysis in combination with pressure history simulations to provide consistent results. Introduction The use of the pressure derivative method has become a standard for analyzing well tests. Clark and van Golf-Racht have clearly demonstrated the efficiency of the method on a high permeability North Sea reservoir. They have also demonstrated some of the limiting factors in the analysis of well tests in high permeability reservoirs. The method is dependent on data of a good quality in order to provide a good resolution to the observed pressure responses. The introduction of the pressure derivative method made this problem very clear. Several methods have been suggested to smooth the pressure derivative data without distorting the reservoir responses. The mostly used smoothing technique has been the method introduced by Clarck and van Golf-Racht. Malavazos and McDonough also presented a paper on how to interpret high permeability reservoirs showing compartmentalised sand bodies using loglog/pressure derivative plots. The paper addresses how various reservoir effects influence the bottom hole pressures during a flow period of a test. This method requires data of a good quality which is not always the case when dealing with high permeability reservoirs as discussed by Cinco-Ley et al. Several factors, such as inertial and frictional effects, may give unusual pressure behaviour on bottom hole pressure in high permeability reservoirs. These effects are difficult to eliminate and consequently the derivative analysis could be of limited value. In this paper, we present an example where the pressure derivative method is of limited use to describe the reservoir geometries that are influencing the well test data. Pressure oscillations due to water hampering effects and rig heave made it difficult to obtain a proper match using the pressure derivative method. Further, the reservoir limits detected during the test were not identified within a single constant rate flow period. Using a combination of pressure derivative techniques, semilog analysis and analytical simulations, reliable analysis could be performed providing consistent results. P. 79^ |
