Veslefrikk is a mature oil field in the Norwegian sector of the North Sea. It has a layered structure with several pressure-independent layers and three main fluid systems: the Statfjord Formation, Intra Dunlin Group and Brent Group. Each fluid system has distinct formation water compositions but similar oil compositions. Seawater injection has been used for pressure support and many wells have commingled production. The principal scale control challenges are scaling potential and placement of scale inhibitor into zones with different pressure. Although production logging is performed regularly to aid scale squeeze designs and production allocation, this is costly, involves well intervention risks and it cannot always be undertaken when required.
In this study an innovative approach to obtain ‘production logging type’ information from wells producing from two zones has been evaluated. It involves undertaking multi-rate well tests where produced water samples are collected at each rate. Depending on the conditions for each test, either qualitative or quantitative integrated analysis of the well test results and produced water compositions can be performed. Information obtained from the tests for each zone includes: Pressure, productivity index, water cut, produced water composition and seawater fraction.
The methodologies adopted for the multi-rate tests and the analysis of their results are described, and the results for four multi-rate tests are presented. The multi-rate tests were undertaken under different conditions and the influence of these conditions on the type and quality of results obtained are discussed, and assumptions and associated uncertainties are identified.
The case studies demonstrate that information useful to scale, well and reservoir management can be obtained from multirate tests. In addition, produced water compositions can be obtained from these tests without the need for downhole sampling. However, in each case it is important to assess uncertainties associated with the results.