RESERVOIR MODELING
WITH CONFIDENCE

This paper presents the integration of two surveillance technologies, inflow & inter-well tracers in an Abu Dhabi Offshore field, for the first time to monitor a complex multi-layered reservoir with early stage water injection. Inflow tracers are chemicals with unique identities, installed into the lower completions that penetrate different layers in the reservoir. Inter-well tracers are also unique chemicals injected into injection wells. Tracer technologies allow non-electric wireless monitoring of the reservoir for many years while reducing cost and operational efficiency. In both technologies, data capture requires to fluid samples to be collected and analysed to detect the unique tracers in the production wells. Inflow tracers provides both qualitative and quantitative estimates of zonal flow as well identifying the source of water breakthrough. Inter-well tracers introduced in the injection wells can identify the primary flow paths and rate of movement of injected fluid in the reservoir. This information can be used to evaluate water channelling and improve water flood conformance plans to maximize sweep efficiency.

The planning and design stage included a selection of pumpable inter-well tracers, volume calculation and breakthrough time simulations to determine the tracer mass required to meet the monitoring objectives. On the other hand, inflow tracers were designed to account for high temperature and highly acidic stimulation fluids and placed along the producing intervals in the lower completion. Quantification models will be utilized to quantify the zonal influx while accounting for expected cross flow. The execution plan is defined by the optimum production and injection rates, equipment and lab test requirements, and optimized field sampling schedule for transient and steady state flow campaigns.

The co-implementation of both tracer technologies in the early stage of waterflood development offers unique ability to monitor the reservoir, eliminating the need for expensive, complicated offshore logistical arrangements for time-consuming wireline interventions to manage the reservoir more efficiently and maximize recovery.

The success of any improved oil recovery (IOR) project is largely dependent on how much oil is remaining to be mobilized within the targeted area of the partially depleted or mature reservoir. Partitioning tracers are generally used to measure residual oil saturation (Sor) or remaining oil saturation (ROS) in the near wellbore region via a single well chemical tracer test (SWCTT) or in an inter-well region via a partitioning inter-well tracer test (PITT). There is a limited repertoire of nonradioactive and environmentally friendly inter-well partitioning tracers for measuring ROS. A new class of environmentally friendly partitioning tracers was field tested, in a giant carbonate reservoir undergoing peripheral waterflood, for measuring ROS in inter-well regions in a depleted area.

The new partitioning tracers were qualified via laboratory experiments and are deemed to be very stable at reservoir conditions (213°F and a salinity range of 60-200 kppm). The field pilot was conducted concurrently with a set of non-partitioning inter-well chemical tracer test (IWCTT) to determine reservoir connectivity, water breakthrough times, and injector-to-producer pair communication in an area selected for an IOR/EOR field pilot. An elaborate sampling and analysis program was carried out over a period of 30 months.

This paper reviews the complete design and implementation of the test, operational issues, and the analyses and interpretation of the results. The breakthrough times of the passive and partitioning tracers are reported, and inter-well connectivity between the paired and cross-paired injectors and producers are analyzed. The ROS measured by a majority of the novel tracers is comparable to the saturations obtained via SWCTT, core and log derived saturations.

The combination of conventional IWCTT and the novel partitioning tracers via PITT has been very useful in analyzing well interconnectivity, understanding the reservoir dynamics and quantifying remaining oil saturation distribution in the reservoir. This has led to better reservoir description and an improved dynamic simulation model.

To obtain improved oil recovery (IOR), it is crucial to have a best-possible description of the reservoir and the reservoir dynamics. In addition to production data, information can be obtained from 4D seismic and from tracer monitoring. Interwell tracer testing (IWTT) has been established as a proven and efficient technology to obtain information on well-to-well communication, heterogeneities, and fluid dynamics. During such tests, chemical or radioactive tracers are used to label water or gas from specific wells. The tracers then are used to trace the fluids as they move through the reservoir together with the injection phase.

At first tracer breakthrough, IWTT yields immediate and unambiguous information on injector/producer communication. Nevertheless, IWTT is still underused in the petroleum industry, and data may not be used to their full capacity—most tracer data are used in a qualitative manner (Du and Guan 2005). To improve this situation, we combine tracer-data evaluation, 4D seismic, and available production data in an integrated process. The integration is demonstrated using data from the Snorre field in the North Sea. In addition to production data, extensive tracer data (dating back to 1993) and results from three seismic surveys acquired in 1983, 1997, and 2001 were considered.

Briefly this study shows that

• Seismic and tracer data applied in combination can reduce the uncertainties in interpretations of the drainage patterns.
• Waterfronts interpreted independently by tracer response and seismic dimming compare well.
• Seismic brightening interpreted as gas accumulation is supported by the gas-tracer responses.