Proper characterization of a producing well has significant impact on asset management. Production schedules from different wells and further infill drilling in a field depend on the effectiveness and the pattern of the production drive mechanism. Whether the reservoir displays partial aquifer influx or is waterflooded, the flow profile of produced/injected water as well as the reservoir pressure, permeability, fluid saturations, and formation compaction are useful to properly evaluate the overall sweep efficiency. This information is more crucial in a field with multi-layered reservoirs having different permeability, pressures, and structure. Data acquisition should be scheduled and compared to baseline data such as a baseline pulsed neutron capture, production, or compaction logs. There should also be plans to monitor specific wells over the life of the project to observe and optimize the reservoir performance, characterize the reservoir response, and in the case of waterflooded reservoirs monitor the performance of the waterflood. Multi-rate multi-zone production logging and pressure transient testing are two excellent methods to achieve these goals. Introduction Production in Mars field in the Gulf of Mexico began in 1996 and peaked in June of 2000 at 208,000 B/D of oil and 217 MMscf/D of gas. The deepwater Mars field is a prime candidate for secondary recovery since the predicted primary recovery has limited aquifer support. Monitoring the waterflood with cased hole logs is critical to improving the performance and increasing the overall volume recovery of the Mars field. The ability to acquire carbon oxygen data for monitoring sea water injection and sigma data for reservoir fluid changes, combined with multi-rate production logging and testing is essential in reducing unnecessary risks of well intervention. Many of the Mars wells produce from multiple reservoir layers so multi-rate production logging and well testing surveys are valuable methods to estimate the layer flowrates, pressures, permeabilities, skin factor of individual layers, and distance to and type of boundaries. A surveillance logging program has been conducted in the Mississippi Canyon Block 807 area since 2004 with initial baseline surveys to monitor and to optimize the production performance of the field and to inspect the waterflood progress. This approach allows reservoir characterization without requiring zonal isolation. It also helps to evaluate where the waterflood has influenced pressure maintenance. The results of the multi-rate multi-zone (MRMZ) production logging and testing combined with saturation trends confirm and provide valuable data for the reservoir modeling. Another benefit of the step-rate production testing is the evaluation of the layer contribution to the total production. This data together with estimated individual reservoir layer pressures helps to determine oil reserves and assist in waterflood management. The data acquisition for these wells is being conducted in realtime to optimize the stabilization times for each test period. Thus, the required data is being acquired without recording unnecessary information or conversely, not obtaining sufficient data. The combined pressure transient testing and the MRMZ production logging of several of the wells tested in the Mars field will be described in detail to illustrate the monitoring technique and to show the time-lapse changes occurring in the Mars field. The flow pattern of the produced water, the reservoir sweep, the reservoir pressure profile, permeability, skin damage, and fluid saturations will be provided and discussed in detail to highlight the results of the time-lapse monitoring of the reservoir. Both the waterflooded and the non-waterflooded case studies will be discussed. While pulsed neutron capture and formation compaction logs are mentioned as surveillance tools, they are not discussed in detail.

Waterfloods serve many purposes and their performance can have major economic impact on the drilling, production and management of hydrocarbon reservoirs. The primary purpose of the Mars (Mississippi Canyon 807) waterflood in the deepwater of the GOM is to increase recovery efficiency in three main reservoirs. In addition, the waterflood helps maintain reservoir pressure in selected sands, which minimizes compaction and subsequent well failure. Surveillance of the waterflood through carbon/oxygen logging, of the formation through compaction logging, and of the individual reservoir layers through multi-rate production logging is critical to the success of the Mars field. The program has consisted of obtaining time-lapse logging and reservoir pressures combined with reservoir modeling. Data has been obtained in the injection, producing, and monitor intervals of several Mars wells and proven to be vital to the evaluation of waterflood efficiency and the prediction of the waterflood front. A monitoring logging program has been conducted in the Mars area since 1996, with initial baseline surveys, to the present day monitoring surveillance program. The logging program has had two purposes. The primary purpose of the program has been to monitor the "sea water" waterflood saturation fronts with carbon/oxygen logging and integrate this data into the reservoir models. A second purpose has been to monitor strain as a result of reservoir compaction and to monitor the effects of the waterflood on the rate of strain. This data is used to help determine wellbore integrity and ultimately to predict wellbore failure. It also provides calibration data for the compaction model used in reservoir simulation. Selected Mars well examples are described in detail to highlight the results of time-lapse monitoring of the waterflooded reservoirs. A comparison of log data with simulation modeling predictions demonstrates the benefit of the data acquisition and evaluation methods. Discussion is focused on best practices learned during the 12 year program, how log responses have helped verify modeling parameters, and on justification of future activities. Introduction The Mars Field (Figure 1) consists of six OCS leases in the Mississippi Canyon Area - Blocks 762, 763, 806, 807, 850 and 851 - located in the Gulf of Mexico about 130 miles southeast of New Orleans. The leases were acquired in 1985 and 1988 and the first discovery well was drilled on Mississippi Canyon Block 763 in 1989. Shell and their partner BP announced plans in 1993 to develop Mars utilizing a 24-slot tension leg platform (TLP) to be installed on Block 807. The TLP was installed in May 1996 in a water depth of 2,940 feet. Production began July 8, 1996 and peaked in June 2000 at 208,000 BOPD and 217 mmcfd. Mars consists of a series of Miocene to Pliocene age turbidite sands deposited within a minibasin bounded by the deeply rooted Venus salt body to the southeast and the more tabular Antares salt body to the north and west. The northeastsouthwest trending basin becomes narrow and more confined within the deeper Miocene interval. The geologic age of the Mars formations above 14,000 feet are Pliocene and the deeper reservoirs are Miocene. Exploratory and appraisal drilling encountered 14 major and 10 minor moderately geopressured pay bearing sands between 10,000 feet and 19,000 feet sub sea.