Summary Low salinity water injection is an emerging EOR technology, applicable to mixed-to-oil-wet sandstone reservoirs. Flooding with low salinity water causes desorption of petroleum heavy ends from the clays present on the pore wall, resulting in a more waterwet rock surface, a lower remaining oil saturation and higher oil recovery. A secondary flood application is discussed in the Omar field in Syria showing a change of wettability from oil wet to a water-wet system. This change in wettability is supported by the observation of dual steps in watercut development. In between the two steps the watercut was constant. This behaviour is a known indicator of changing wettability. Moreover, direct connate water banking measurements confirm the change. The field observations are supported by spontaneous imbibition experiments in core material and a single well Log-Inject-Log test in an analogue field. From the field observations, the change in wettability is estimated to be nearly complete, leading to an associated incremental recovery of 10–15% of the Stock Tank Oil Initialy In Place (STOIIP). The significance of this work is that this is one of the very few documented proofs of concept on a reservoir scale. Work is ongoing to prove this concept in a tertiary flood as well.

With an approximate STOIIP of 760 MMbbls, the Omar field is the largest field in Al Furat Petroleum Company's portfolio. The field -located in the Euphrates Graben 45km SE of DeirEzZor -was discovered in 1987 and holds a maximum undersaturated oil column of more than 500m with two original oil-water contacts of 3750 and 3778 meters subsea. The oil production almost exclusively originates from two sandstone formations: the Cretaceous sheet-like shallow marine Lower Rutbah (RUL) and the Triassic coastal fluvial plane Mulussa F (MUF) formation. The Omar Field is formed by an elongated, high relief tilted horst block, which is internally compartmentalised.Originally, the field produced naturally at a peak net oil rate of some 80kbpd but production declined rapidly because of the lack of any pressure support. Following the implementation of water injection from 1991 onwards a plateau production of around 60-70kbopd was achieved for some five years (1994)(1995)(1996)(1997) declining to the current net oil production of 20 kbopd.Despite the structural complications, the injector-producer connectivity in the laterally extensive RUL sands could be established rather confidently and recoveries in excess of 55% should eventually be achievable. Predicting water-flood efficiency in the Mulussa F 3D sand channel labyrinth turned out more complicated. As a matter of fact, it was demonstrated that the resolution achievable by static reservoir modelling was not sufficient to predict the water-flood efficiency meaningfully. As a consequence a statistical infill campaign was launched with a focus to infill the existing major gaps between the MUF wells and secondly to establish a line drive waterflood pattern while investigating the merits of a dense five spot.The results of this infill drilling campaign (executed in 2005-2006) and a new 600-fold high-resolution seismic survey gave a multi-disciplinary team the challenge to improve in identifying more attractive targets while reducing the downside drilling results observed during the infill campaign. A combination of the new structural data with a regional geological well correlation fully and iteratively integrated with dynamic well information and production data, indicates that the recovery in the MUF formation could well be optimized through a more deterministic instead of the previously adopted statistical infill drilling approach.

Complexity and heterogeneity inherent to naturally fractured reservoirs pose important technical, economic, and managerial challenges to operating companies. In order to meet these challenges, operators ought to apply innovative technologies and tailored exploitation strategies. This paper summarizes the reservoir surveillance and characterization approach, as well as the operational strategies used in the development of the Cretaceous fractured carbonate Cogollo Group in La Concepción Field, western Venezuela, with emphasis on the successful application of selective technologies and methodologies. Discovered in 1948, the Cretaceous reservoir had produced over 90 MMSTB of 36°-API oil from 29 wells up to January 2005. Since 1998, Petrobras EnergÍa Venezuela's operations have resulted in a significant increase of the total reservoir oil throughput from 2500 to a maximum of circa 17000 BOPD, with an associated cumulative production of 23 MMSTBO. A key factor in the successful redevelopment of the mature Cretaceous reservoir was the timely recognition of the major features driving the reservoir productivity and performance, in addition to the opportune identification of the main operational risks and the suitable technology to mitigate them. Then, as we drilled and intensively acquired data, we refined the reservoir description, improved operational indicators and costs, and implemented new technology and risk management strategies to face the increasing challenge of the remaining reservoir targets. The integration of the new structural model (en echelon transpressional stepover zones), derived from the 3D seismic data, modern well-logs and dynamic (production logs and pressure tests) data acquired from re-entries and new wells, and rock description and analysis from 950' of core, allowed us to define a clear relationship between fault-related fracture systems and production. In brief, the Cretaceous reservoir is classified as a type-II fault-related fractured reservoir containing a micro-fissured pseudomatrix. Currently, based on our model, the main development strategy is to drill wells that cross "seismically visible" faults with the aim of intersecting their associated fracture networks. Discontinuity seismic attributes are used to delineate the fault zones and optimize well locations. Reprocessed seismic volumes are now used to diminish the uncertainty in the spatial geometry of the targeted faults. In addition, the structural interpretation is being tested and enhanced with the results from physical experiments (sandbox analog modeling). Presence of discrete highly productive fractured zones, in conjunction with low reservoir pressures due to depletion, led to the adoption of under-balanced drilling (UBD) with Nitrogen injection as the drilling technique in the reservoir section to avoid severe circulation losses. During the UBD phase, a calibrated drilling dataset is used along with mudlogging data to check for geological events and monitor well productivity in real time. Furthermore, the complexity and uncertainty associated to the targets impose the use of strict directional control to conform to the designed complex well trajectories. The presented field development strategy has proved to be successful since it has enabled to achieve a sustained economic oil production in the naturally fractured Cretaceous reservoir of the mature La Concepción field.