Compilation of Completion and Workover Information
The compilation of completion and workover information onto maps and logs is critical to understanding effects of previous reservoir-production strategies and what portion of the reservoir the data are associated with. Posting the initial completion date on a base map is the first step in compilation and illustrates both the initial drilling pattern and the relative spatial timing of production. Next, logs should be annotated with initial completion perforations and any subsequent reperforations and workovers, as well as any known production data, such as production rates or cumulative fluid volumes. This information illustrates which genetic units were perforated and produced, along with the timing and production results. Next, completion intervals for each well are posted on the gross sand maps generated when determining the reservoir architecture. This map shows the spatial distribution of completions within each genetic unit and is a first look at possible missed pay zones.

Production Time-Series Graphing and Mapping
A time-series analysis of fluid flow consists of graphing and mapping of oil, gas, water cut, fluid levels, and pressure-depletion variation over the producing life of a reservoir. Graphing of all produced fluids and pressure is done on both the reservoir and well levels. Reservoir-level graphs illustrate the production of all fluids and should also include the number of wells on line. The wells should be annotated with the timing of any production strategy changes, such as the implementation of secondary or tertiary recovery. Graphs are developed to capture the shortest interval in which data are taken and then are annotated with any workovers that may affect the well performance, such as pumps, well stimulation, tubing changes, and recompletions.

Next, fluid-flow trends within the reservoir are established from a set of production-performance maps that illustrate initial potential, cumulative and current production, gas:oil ratio (GOR), fluid levels, water cut, and pressure depletion on a per-well basis. Analysis of these maps captures the historical changes in fluid production throughout the reservoir and highlights important trends in fluid flow. Areas of best production (sweet spots), as well as areas showing impedance to fluid flow, are readily identifiable. Water-cut and pressure maps record the pattern of water migration as reservoir pressures decrease and can highlight preferential pathways of fluid migration. These patterns will most likely indicate fairways of high transmissivity and, thus, reservoir communication. Anomalies in these maps can indicate barriers to fluid flow, which may also indicate reservoir compartmentalization.

Four-dimensional seismic is a newly emerging technology in production time-series analysis. It involves the recording of 3-D seismic at two different times in the production life of the reservoir and comparing the differences. Any difference in seismic attributes observed occurs from a change in fluid saturation and thus indicates fluid flow. For example, the encroachment of an aquifer or the expansion of a gas cap could be interpreted from the seismic attributes.

Comparison of the initial hydrocarbon-fluid characteristics and the time-series analysis is the primary aid in determining the initial drive mechanism. Monitoring GOR can indicate whether gas-cap expansion or solution-gas drive, or both, are the functioning drive mechanism. Rapidly increasing GOR near the crest of the structure can indicate gas-cap expansion in a reservoir containing initial oil characteristics at or above the bubble-point pressure. A fairly uniform increase in GOR around the field can indicate a solution-gas-drive mechanism and uniform pressure depletion. A time series of the water:oil ratio that displays increases up structure over time can indicate a water-drive mechanism.

Another key is whether the oil in the reservoir is undersaturated or saturated with solution gas. An undersaturated oil reservoir can produce substantial volumes of oil with a significant pressure drop before gas will come out of solution in the reservoir. This situation will be seen in a steadily producing GOR at a value near the initial-solution gas:oil ratio. In contrast, saturated oil begins to produce at elevated GOR's soon after production with only a minor pressure drop.