Well testing is one of the most important diagnosis tools in the life of a well. A properly conducted well test involves a temporary completion of a well to safely acquire information about reservoir pressure and drainage boundaries, well characterization, and representative formation fluid samples. From these data operators can gain a good understanding of the well and make an informed decision on how to proceed with the asset.

Proper characterization essential

In deepwater environments well testing is a very expensive process coupled with heightened HSE concerns. Properly characterizing a well has a significant impact on asset management. Although wells are routinely stimulated to remove formation damage, the subsequent testing to determine the skin after stimulation is seldom carried out because of high cost and safety factors.

Production schedules from different wells and further infill drilling in a field depend on the effectiveness and 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 data to properly evaluate overall sweep efficiency. This information becomes more crucial in a field with multilayered 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 also should be plans to monitor specific wells during the life of a project to observe and optimize the reservoir performance, characterize the reservoir response and, in the case of waterflooded reservoirs, evaluate the performance of the waterflood. Multirate multizone (MRMZ) production logging with pressure-transient testing is an excellent method to achieve these goals.

Assessing multilayered reservoirs

The Mars Field has limited aquifer support, so a waterflood was designed for three of the pay zone sands. The objective of this waterflood was to prevent reservoir pressure to go below bubblepoint pressure, inhibit formation compaction and increase the overall volume recovery. A surveillance logging program has been employed in the Mars Field with initial baseline surveys to help monitor and optimize production performance as well as to track waterflood progress.

This downhole surveillance approach allows reservoir characterization without requiring zonal isolation. It also helps evaluate where waterflooding has influenced pressure maintenance. The ability to acquire sigma data for monitoring seawater injection and carbon-oxygen data for reservoir fluid changes, combined with multirate production logging and testing, has become an essential technology in reducing the unnecessary risks of well intervention.

The application of production logging in the Mars Field has facilitated optimal reservoir management by identifying the source of excessive water production. This has helped increase production by isolating water-producing zones and lowering operating costs of handling excessive water production. In a recent job in the deepwater Gulf of Mexico (GoM) a fourfold increase in well productivity was observed using an acid-stimulation treatment after MRMZ logging and testing identified in real time that the high skin around the wellbore was the cause of the production decline.

Rethinking stimulation strategy

A deepwater GoM operator routinely injected xylene into its wells to remove formation damage associated with asphaltene but did not have a method of monitoring the effectiveness of the stimulation treatment apart from flow rates observed at the wellhead, which can be misleading.

The operator’s challenges were twofold. Since it had been observing different salinity water on the separator relative to the formation water salinity, it was of the opinion that there may be an aquifer farther below the producing sand contributing to the observed water on the surface via either a casing leak or from coning through the bottom perforations.

As with other wells in the GoM, asphaltene deposition poses a limit on the allowable drawdown. The well had been routinely injected with xylene throughout its history for asphaltene mitigation. The flowing bottomhole pressure was considered to be near or below the asphaltene onset pressure. Characterizing the effectiveness of the treatment and whether the treatment needed to be changed was a key challenge for the operator. If the treatment was ineffective, it had to be identified in real time so a remedial action could be taken.

Halliburton recommended running MRMZ production logging across the targeted pay intervals. For the MRMZ process, the well flows at three different flow rates followed by a shut-in period (Figure 1). During these different rates, traditional production logging measurements were carried out at each choke setting followed by a stationary measurement at a predetermined depth for 10 to 15 minutes. Following the last production logging run and the stationary pressure measurement between the two pay zones, the well was shut in for a pressure buildup. The pressure data were monitored until radial flow was fully established.

The traditional production logging passes would assist in determining the entry point(s) of water, gas and oil along with hinting of its source, while the pressure buildup would help evaluate pressure, permeability and the skin for each layer of the well after the initial stimulation treatment.

Of particular interest was the sequential increase in skin damage with increasing flow rate (Figure 2). Though the production had doubled after the initial stimulation treatment with xylene, the high skin observed in real time from the MRMZ pressure-transient analysis prompted the operator to change the formation damage removal program and stimulate with hydrochloric acid rather than only xylene. This resulted in an immediate fourfold improvement of productivity index with a potential of further doubling.

The highlight of the job was the real-time pressuretransient monitoring to lower the testing time and the identification of high skin with the MRMZ test, allowing the operator to rethink the stimulation strategy with a relatively low-cost procedure. This change to the operator’s program saved $2 million.

Low-risk method for acquiring essential data

Evaluating a well’s producing characteristics, whether influenced by a waterflood or by natural-aquifer influx, is essential to field management. The use of MRMZ testing has demonstrated a relatively low-risk, low-cost method for acquiring valuable data. The data may be used to recalibrate reservoir models; propose workovers to isolate water-producing zones; and stimulate high-skin, high-potential reservoirs.

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