MULTIPHASE TECHNOLOGY: Minimizing water cut by geosteering horizontally through unconsolidated zones

Since 1995, new technology has helped reduce well evaluation costs by two-thirds.

Recently, work performed at YPF Maxus' Widuri field served to further improve the company's track record from both technical and economic standpoints. A team consisting of specialists from YPF Maxus and Schlumberger introduced a combination of azimuthal LWD tool and geosteering technologies in an attempt to optimize borehole placement and borehole stabilit, and increase penetration rates and cuttings transport. The combined technologies were used in drilling the Widuri B-22 well.

The well provided a series of challenges never before encountered when using the technology combination. The well positioning had to be extremely precise in order to tap the thin, discontinuous hydrocarbon strata while avoiding water-bearing zones. The team also had to overcome a series of technical hurdles when designing a reliable bottomhole assembly (BHA). The BHA had to provide sufficient real-time data to guide the geosteering.

The first step was to drill a pilot well in order to optimize landing point for the horizontal well. Next, the well was side-tracked to land the 9 5/8-in. casing as close as possible to the pilot well within the target 33-Series reservoir.

Last, a 500-ft long, 8 1/2-in. horizontal hole was drilled, maintaining the wellbore within the top 10 ft true vertical depth (TVD) of the reservoir. These tasks were achieved by developing a BHA that combined a geosteering tool (GST) with a resistivity-at-bit (RAB) LWD tool, which provided azimuthal resistivity measurement while rotating the BHA. This combination allowed measurements to be recorded virtually at the bit for the GST and approximately 23 ft away from the bit for the RAB tool.

The GST is an instrumented downhole mud motor, with focused gamma ray and resistivity sensors mounted below the power section, which places them closer to the bit than conventional LWD tools.

The tool also provides information on inclination and drill string rotation rate. This tool is routinely run to drill and evaluate 8 1/2-in. Maxus' horizontal wells. Although the tool provides focused information at the bit, the BHA must be stopped to take measurements azimuthally. Furthermore, the GST has a relatively shallow depth of investigation compared to other LWD services.

The RAB tool is a laterolog-style resistivity tool that allows the wellsite team to see into the formation from all angles - up, down, right, and left - as the tool is rotated. It also provides a full borehole image of shallow, medium, and deep resistivity readings and sees resistivity events deeper into the reservoir than the GST, but much further behind the bit. The combination of these two systems allowed the team to see changes in reservoir characteristics before bit penetration while continuously rotating the BHA. One of the technical challenges the team faced was getting the tools to communicate to the surface in real-time mode while being run in combination.

The process

Resistivity data from the pilot hole were used to perform integrated forward modeling that provided information to help guide the geosteering along the horizontal section. Log data from the pilot hole was squared to create the lithological model for the forward-modeled cross section. By improving the team's understanding of the target formation, these cross-sections (resistivity and gamma ray) were used to steer the well horizontally along the 33-Series reservoir, thereby facilitating well positioning. In addition, the forward-modeled log was used in conjunction with the actual logs to more accurately determine the wellbore position and precisely guide the horizontal section along the target formation.

The increased resistivity and decreasing gamma ray at 5,670-ft MD (measured depth) was used as the marker to stop drilling and set the 9 5/8-in. casing. The 8 1/2 in. horizontal section was drilled using a 6 3/4-in. tool combination, as well as the GST and a measurement-while-drilling (MWD) telemetry system.

When coming out of the shoe, the plan was to build to 88 degrees south and follow a structural dip that appeared from seismic data to be about 3 degrees to the south. The string was steered high side to 5,685-ft MD, where the GST inclination indicated 87.9 degrees. The forward-modeled formation layer also showed a structural dip with a shallower than expected slope. In order to follow the formation dip from this point onward, the drill string was rotated while controlling the rate of penetration (ROP) at 80-120 ft/hr.

When the resistivity measurement dropped to 40 ohm-m at 5,799 ft, an azimuthal resistivity survey was performed using the GST to determine the borehole location relative to the top of the sand layer. The survey revealed that the well was approaching a silty section from above, which was confirmed by the model prediction and corroborated by azimuthal data.

As a result, the project team monitored closely the data generated by the tool combination as the drill string was rotating. An 80-120 ft/hr ROP was maintained while the string rotated down to 5,950 ft.

At about the middle of the sandstone, the bit resistivity measurement increased to 100 ohm-m, indicating the bottom of the silty section. At this point, the team decided to build the well in order to place it in the upper reservoir section, rotating the string and maintaining ROP at about 200 ft/hr. While the wellbore was still in good-quality pay, the well was terminated at 6,050 ft MD.

The solution

Successfully drilling the Widuri B-22 well proved the multiphase geosteering and LWD tool combination as the optimum technical solution for horizontal drilling applications in this field. The BHA provided good directional control of the drillstring. Excellent ROPs also were achieved with 95% rotary drilling.

The ability to rotate the motor and bent sub in 95% of the section improved cuttings removal and reduced the risk of the packing-off problems often associated with poor hole cleaning in high-angle wells like this. Improved hole stability also reduced the risk of potential problems encountered while running screens and pipe to the bottom of the well during the completion phase.

While drilling the horizontal section, the increased use of rotary drilling and associated azimuthal-plus-inclination measurements presented an enormous advantage. The readings confirmed the wellbore's location relative to the top of the reservoir and substantially reduced the risks associated with this kind of operation.

The continuous inclination data from the MWD in the landing section were used to select the optimum location for high capacity electrical submersible pumps. Without tangent sections, the pump shaft can suffer damage, resulting in short pump run life and expensive maintenance. Furthermore, the directional drillers use this data to eliminate sinusoidally shaped wells.

In addition, the data transmission rate of 6 bits/sec enabled real-time control of the job, and ensured both high ROPs and good-quality log data. Real-time data from drilling operations like the Widuri B-22 well has helped control the shape of well trajectories - an important factor when long, straight borehole sections improve gravel pack efficiency.

Cost benefits

The tools combination allowed YPF-Maxus to optimize horizontal well placement in a thin, complex unconsolidated sandstone reservoir. Furthermore, this combination of services allowed 95% rotary drilling through the horizontal section resulting in reduced drilling time, improved hole cleaning, reduced sinuosity, and optimum gravel pack conditions.

The direct cost benefits from increased ROP resulted in a cost savings of four rig hours or about $15,000. The indirect cost benefits, resulting from optimum well placement and more efficient gravel packing, were difficult to quantify. However, the well was one of the best 33-Series reservoir producers before pressure depletion caused the pump to fail. The well is currently waiting on a pressure maintenance program to pressure up this portion of the reservoir.

In addition to technical and operational achievements, RAB/LWD and geosteering technologies have served to cut YPF Maxus' average well evaluation costs to $100,000 per well from $300,000 during a five-year period. They have enabled the company to eliminate almost all wireline logging services and drill more wells per year. Additionally, the LWD strategy has helped the company to shorten average well evaluation times from an average of 66 hours in 1995 to an average of six hours in 2000.

Acknowledgment

YPF Maxus Southeast Sumatra Ltd and PT Schlumberger Indonesia granted permission to publish this article.