Interpretation of Water Injection/Fall Off Test (Response to M.Levitan,SPE 77532)

Abstract

Abstract Water injection /fall off tests are normally associated with water flood project. Recently, interested in this type of well tests has developed in the area of reservoir appraisal. In the vast majority of situations associated with exploration activities, there is no infrastructure and equipment in place to collect and export the hydrocarbon produced during well test. The common practice used in the industry is to burn the produced fluid. The demands to reduce emission during well tests put enormous pressure to avoid these tests together. This brings large uncertainties to the reservior appraisal and increases the investment risk if a decision is made to sanction a project and to develop the field. Replacing a production/build up test sequence by an injection/fall off test sequence solves the problem of emission. Levitan [1] stated that three main problems due to using water injection / fall off test might happen which are listed below: The first problem is that the character of the system changes and Instead of single-phase flow we face now with two-phase water-oil flow by their own relative permeabilities. The second problem is injection of cold water includes temperature changes in the formation and brings additional complication to pressure behaviour through temperature effects on the oil and water viscosities and the third one is injection of water may result in the formation fracturing and in coupling of rock mechanics and fluid flow problems. It is therefore, important for successful test interpretation to avoid fracturing and to inject water at below the formation fracturing pressure. This paper is divided into two parts. In the first part we are going to compare Numerical and Levitan's Analytical model for different injection and fall off periods using numerical part of Saphir well test software. In the second part as far as we have validated Saphir Numerical model compared with W-O Levitan Analytical model, now this model is used to generate pressure responses in order to investigate the influence of reservoir parameters. Methods of interpretation will be used and some specific advantages of the numerical model will be shown. Introduction General description of Saphir: Saphir is a well test interpretation package based on the methodology of the pressure derivative. The basic logic is to guide the user through the complete interpretation process using this methodology, while providing easy access to complementary side facilities. Numerical Option: Here, some applications for numerical model in Saphir are listed below:Similar to reservoir simulator but specific features for transient behaviour are defined.Grid system (unstructured Voronoi, PEBI): Possibility to create zones delimited by faults / Composite boundaries.Used to build type curves (linear, monophasic) produced by superposition. Numerical Multiphase option: Saphir is very much similar to reservoir simulator by using complete multi component reservoir simulation, which is used with real production history. Moreover, using one, two or three phase flow with related relative permeability curves and PVT also implementing gas or water injection are the most advantages of Saphire well test software. Part-1 Comparison between Numerical and Levitan's Analytical model: Fig (1) shows the flow sequence, which includes four injection periods (1,2,4,5) and two-pressure fall off (3,6). The bulk of injection in this sequence takes place during injection periods 4 and 5 between the two-pressure fall off periods 3 and 6.