History Match and Reserve Evaluation of Hydraulically Fractured Wells: Low Permeability Field Case History

Abstract

Abstract Due to the production decline of mature fields, the possibility to drain low permeability formations became recently of great interest in the Congo basin. Some of these reservoirs can produce at economic rates only if massive hydraulic fracturing jobs are performed. In the Congo off-shore environment, the capital expenditure associated to such kind of interventions can be critical. It follows the need to accurately identify the candidates, and then properly design and model such hydraulic fractures in order to have sound production profiles for economic evaluations. Accurately modeling hydraulic fractures and mid-long term production profiles of slanted fractured wells can be extremely challenging. Analytical methods can be misleading in estimating production profiles of hydraulically fractured wells due to multiphase flow and the reservoir heterogeneity. The paper describes the successful history match process for highly deviated wells with pre and post frac production history in Kitina 3A oil field. The static and dynamic 3D model is calibrated with dynamic data with a step by step approach, starting with the data gathered in the prefrac production history. Different criteria to establish cuf off are tested in order to properly describe the reservoir net pay. The post frac data give insight into fracture properties. A huge effort has been made in order to properly modeling both hydraulic fractures geometry and conductivity in a consistent way with the data obtained with the treatment interpretation. The hydraulic fractures are described in a full field 3D model using refined grids around the wells and tuning the grid dimensions in order to have both production accuracy and acceptable run time. History match process gives indications that fracture conductivity may differs significantly compared with the value of the frac job interpretation. In addition post frac history match is improved greatly by the knowledge of formation permeability through for example well test interpretation. On the base of the experience gathered on the simulation of such wells, some general guidelines are drawn for a wider application. Introduction Layer 3A is the upper oil bearing pool of Kitina reservoir (offshore Congo), characterized by permeability of 1–5 mD and an average porositiy of 15%. The layer can be divided in 2 main zones: 3A1 and 3A2. These zones belong to Sequence 3, that in Congo offshore is interpreted as a drowning sequence. The lithology of 3A zone can be considered an alternation of silt and sandstone beds with limestone (wakestone and packstone). The oil production started in 2005 with the well KTM107 after a work over and a change of production interval. During 2005 and 2006 other 2 wells were opened in layer 3A, KTM111 and KTMW6ST2 respectively. Start up oil rate was interesting, but the production decline was strong. Due to the increasing oil price scenario in 2007, the three wells were hydraulically fractured in order to raise oil production [Ref. 1, 2, 3]. The stimulation operations were characterized by a high expenditure compared with the most common on shore treatments.