Re-Developing Old Oil Fields in Tight Carbonate Reservoirs: Insights for Discrete Fracture Network Modelling

Abstract

Intense exploration activities led to discover several oil and gas fields across continental Europe and in the Mediterranean over the 1950’s, 60’s and 70’s. Many of these discoveries were then abandoned or only partially developed as a consequence of different reasons. For instance, some were considered non-economic at that time and also the discovery of larger fields led companies to gather investments for appraisal and development. During the recent oil downturn, the race to optimize revenue led a number of companies to focus on field redevelopment instead of exploration. This has also been stimulated by the enhancements in technology and recovery techniques during recent years, particularly true for tight carbonate reservoirs. Consequently, projects that were previously marginal or not economic have become viable. The modern standard tools (image logs, multi azimuth acoustic logs, high-resolution 3D seismic, multi-azimuth downhole seismic) to detect the presence and to characterize fracture networks in boreholes drilled in 70’s and 80’s were not available. Therefore, the characterization of fractured reservoirs was very complex and at the same time very limited leading sometimes to a poor understanding of the reservoir behaviors and the underestimation of the discovery potentials. Nowadays, the appraisal or redevelopment of old discoveries made in fractured tight reservoirs implies to deal with the lack of information regarding the fracture network characteristics and to handle with alternative approaches. In this study, a case of an old oil discovery, the Benevento discovery, made in fractured tight carbonates from the Southern Apennines of Italy is presented. The Benevento discovery, which dates back to the early 70’s. The reservoir units consist of Turonian-Lower Miocene calcarenites and limestone at a depth of around 3000m below mean sea level and bearing both oil and gas. The traps are mainly structural as faults and folds and the seal is represented by the Messinian evaporites and Lower Pliocene shales. The trapping style is represented by a roughly symmetric thrust-related fold belonging to the so-called buried Apulian thrust belt. The structure lies on the same trend of the largest and deepest oil fields in the region (Val d’Agri oil fields). This oil discovery has been re-evaluated and efforts for better understanding the fractured reservoir are needed before the new development drilling phase will take place. The dataset is composed of 3 wells with cores and vintage well-logs, 2D seismic lines, drilling and test data. Regional geological information from wells in the surrounding and from regional seismic is also available together with data from present-day seismicity and stress field. Within this context, an integrated approach was adopted with the aim to extract and squeeze the maximum amount of fracture information from the available data and to use a multi-scale and multi-scenario approach to produce realistic drivers for the discrete fracture network modelling. Restoration and forward modelling were also used to simulate 3D strain distributions used to drive the fracture modelling. The results of the fracture modelling were then used to populate a dual-porosity/dual-permeability reservoir model and to help planning the design of horizontal development wells.

AAPG Datapages/Search and Discovery Article #90341 ©2019 AAPG Geoscience Technology Workshop, Exploration and Development of Siliciclastic and Carbonate Reservoirs in the Eastern Mediterranean, Tel Aviv, Israel, February 26-27, 2019