Formation pressure and magnetic resonance logging-while-drilling technologies reduce deepwater drilling costs and enable novel formation-evaluation programs.

Shell Offshore Inc. successfully drilled and completed a commercially productive well in the Ram Powell Unit in Viosca Knoll field, Gulf of Mexico as part of a redevelopment program that began in 2003. The target was relocated after finding a water zone in the initially drilled section. The company plugged back and sidetracked to an updip location. In its decision-making for drilling and the overall completion, the company used formation-pressure-while-drilling (FPWD) measurements and saved more than US $1 million by eliminating the need for two difficult wireline-pressure measurement runs on drill pipe.
FPWD measurements were successfully used on a subsequent well, eliminating the need for a third pipe-conveyed wireline run. Magnetic-resonance-while-drilling data provided important information regarding fluid viscosity, rock texture (net sand calibration), permeability and grain-size estimations used in the design of the well completions.
Ram Powell redevelopment program
The Ram Powell Unit covers eight blocks in the Viosca Knoll field, eastern Gulf of Mexico, located about 125 miles (200 km) east-southeast of New Orleans and about 80 miles (128 km) south of Mobile, Ala. Situated in 2,000 ft to 4,000 ft (610 m to 1,220 m) water depths, its production began in September 1997 from a tension-leg platform (H&P 205), making Ram Powell one of the most mature oil fields in the deepwater Gulf of Mexico.
Ram Powell development has undergone the typical lifecycle of a deepwater oil field. Following initial development drilling, subsequent production surveillance data and a 4-D seismic survey conducted in 2001-2002 were used to identify near-field exploration potential and undrained infill opportunities. In January 2003, Shell initiated Phase 2 redevelopment activities at Ram Powell and noted the new wells to have two main differences from initial development drilling. First, the well targets have less recovery potential and/or more subsurface uncertainty. Second, the target locations are more remote, making the wells more risky and difficult to drill directionally because they must pass through zones destabilized by depletion. This has also added risk and cost to formation evaluation.
The drilling of redevelopment wells typically requires more data gathering than development wells because of the lack of well control and reduced tolerance for uncertainty. Formation pressures and sidewall samples for permeability and grain size are often deemed crucial in decision-making. These data are traditionally acquired on wireline. Advances in measurement while drilling (MWD) and logging while drilling (LWD) have provided an answer to these data-gathering challenges. In fact, new LWD measurement technologies have enabled novel formation evaluation programs to be created which benefit redevelopment project economics.
The recent Ram Powell wells were drilled with bottomhole assemblies (BHAs) that included a rotary steerable system (RSS), a suite of 6.75-in. formation evaluation and imaging-while-drilling tools and an FPWD tool - all run below a hole opener that enlarged the well bore after measurements were made. The LWD tool suite included a gamma ray/resistivity tool, a density/neutron tool and a nuclear magnetic resonance (NMR) tool. The FPWD tool provided direct pore-pressure and mobility data for reservoir pressure management and mud weight optimization.
Data from all tools were pulsed to surface in real time using the MWD tool, which provided high-speed telemetry and downhole power to the LWD tools.
With the FPWD tool located between a pilot bit and a reamer, the stabilizer size dictated when pressures would need to be acquired. An 8½-in. stabilizer was used on the Ram Powell A-10 well, which allowed pressures to be measured in hole sizes up to 10½-in. Thus, the pressure measurements had to be taken after drilling but prior to passing through with the 105/8-in. hole opener. On the Ram Powell A-19 well, a 9¼-in. stabilizer was used so that pressures could be measured after total depth (TD) while pulling out of hole. The latter option was found to be much more time-effective.
Formation pressure data acquisition
The main purpose of pressure data acquisition was to aid completion design and verify dynamic reservoir models. The advantages of obtaining formation pressure MWD, as opposed to using wireline, are to reduce rig usage and borehole exposure times, both crucial from economic and operational standpoints in deepwater operations. Performing pressure measurements on LWD can potentially save several days of rig time, including the time needed for a cleanout trip and a wireline or pipe-conveyed run. Perhaps even more importantly, using a drilling BHA to acquire pressures significantly reduces the overall risk profile of the well by minimizing borehole exposure time. Since the FPWD tool was part of the drilling BHA, pressure data acquisition was now an integral part of the drilling sequence.
Shell's petrophysicist used LWD density data to identify target sand locations and select FPWD measurement depths. The tool was then activated and a correlation pass was made to tie in. Upon arriving at a pressure station, the FPWD tool was deployed and automatically began a drawdown-wait-retract sequence (Figure 1). It was not necessary to orient to the low side of the hole because the FPWD tool deploys a backup arm opposite the probe to press it against the formation and obtain a strong seal. This design is highly advantageous, as tool orientation can take several hours at each measurement station. After the tool had been positioned, it was stationary for about 6 minutes while taking a measurement. Circulation continued throughout the entire process, and formation pressure values were pulsed to surface in real time. This enabled the petrophysicist to ensure that the tool was reading a valid formation pressure. After the pressure was measured, the probe was retracted and the tool moved to the next station. Throughout the pressure acquisition sequence, communication was maintained between the FPWD engineer and the drilling crew to ensure that the drillpipe remained stationary during a pressure measurement and the pump rate remained unchanged while tripping to avoid an accidental deployment.
Once pressure data was obtained on the A-10 well, FPWD measurements showed that the low resistivity zone at the bottom of the target sand was swept oil, indicating a higher than expected oil-water contact. This data led Shell to relocate the reservoir off-take point and sidetrack the well updip. The LWD measurements showed producible sands in the sidetrack, so casing was run to bottom. A well schematic of A-10 and A-10 ST is shown in Figure 2.
A total of 25 FPWD measurements were made - 12 while drilling the well's initial section and 13 while drilling its sidetrack. The FPWD tool fully demonstrated its value since formation pressures were considered a critical piece of information for making decisions regarding the sidetrack and overall completion. Had LWD pressures not been acquired, difficult wireline runs on drill pipe would have been necessary for both the original hole and the sidetrack.
The FPWD tool was used again on the subsequent well. Pressures were successfully acquired in both massive and low net-to-gross laminated sands. An example of tested formations and acquisition density are shown in Figure 3. Shell plans to continue using FPWD measurements to save the time and cost of wireline pressure-measurement runs.
NMR data acquisition
The NMR-while-drilling tool was also run on the wells, the largest benefit of which was reduced uncertainty of the net sand volume in the laminated sand-shale reservoir.
Ram Powell reservoirs provide a classical example of thin-bedded channel-levee turbidites, and their petrophysical well evaluation methodology involves performing Thomas-Stieber analysis calibrated with NMR. For these recently drilled Ram Powell wells, a method utilizing the separation of NMR and neutron porosities was used to calibrate net sand counts and determine shale volumes. The NMR logs were also used to provide an independent estimate of water saturation (Sw) to calibrate the Thomas-Stieber Sw calculation. After irreducible water saturation was established, the NMR estimate of permeability was calculated using the Coates formula with parameters fitted to field performance data. In addition, log estimates of porosity and permeability were used to estimate average sand grain size, which was in turn used to design completions.
NMR fluid property data were acquired using station logging, a new diffusion-based measurement in NMR LWD. This technique distinguishes different formation fluids and their respective saturations. It also indicates diffusivity and thus viscosity of hydrocarbons in a reservoir. The asset team found it was logical to combine the fluid typing capabilities of NMR measurements with FPWD mobility data. There was a natural synergy between NMR station logging and formation pressure testing; the FPWD tool was stationary during pressure measurements for periods of 5 to 10 minutes, which was enough time for the NMR tool to acquire five to 10 scans of a station log sequence. Each station log represented a number of echo decays with two different wait times (TW) and up to five different inter-echo spacings (TE).
Fluid viscosity, which was estimated from the station log data, was of value in converting FPWD drawdown mobilities to formation permeability. This alone provided a useful application of station log data since uncertainty in viscosity has always hampered the use of mobilities for calibrating permeability. In Figure 3 (second log track), NMR station positions are shown as red circles, and permeability estimates are shown as purple squares. Given the laminated nature of the sand,
it was considered good agreement between these estimates and log permeability estimates from Coates (red line) and in-house grain size-porosity correlation (blue line).
As illustrated at Ram Powell, recent advances in LWD technology have introduced specialized measurements that were previously available only by running wireline logs. It is now feasible to perform complex evaluations using LWD data alone. The resulting step-change down in the risk profile of deepwater development wells has reduced borehole trouble time and well costs, thereby improving overall redevelopment well economics.

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