Tony Leavitt, Schlumberger
Drilling development wells to target reservoirs beneath the Louann Salt in the Gulf of Mexico has been problematic in the past. A new approach, which integrates several technologies and techniques to achieve success, is to drill directionally through the salt using concentric reamer with concentric polycrystalline diamond compact (PDC) bit to minimize the effect of the rubble zone and to use rotary steerable systems.
Deepwater GoM development drilling is on the rise. Innovations in seismic acquisition and processing allow operators to image exploration targets beneath the salt that blankets vast areas offshore Texas and Louisiana. Many subsalt discoveries have been confirmed, and several fields have been delineated. They now are ready for development.
Salt concerns
A significant number of the discoveries lie beneath water more than a mile deep. Ultra deepwater floating rigs can have spread rates greater than $500,000/day. Attention has focused on developing techniques to cost-effectively drill and land deepwater wells that target reservoirs beneath the salt. The closer the reservoirs lie to the base of the salt, the more difficult and time consuming they are to drill. Salt is particularly tough to drill, and in the GoM its thickness can range from 2,000 ft to 12,000 ft (610 m to 3,659 m).
Most operators start exploration programs with vertical wells, but subsequent discoveries lead to directional wells for reservoir development. Maximizing field development with minimal investment sometimes requires drilling from a single template, and reservoir connectivity can only be achieved via directionally drilled wells, similar to a fixed platform field development concept. Drilling minimal wells from a central location introduces further challenges in getting the required departure from the seabed template to the drilling targets. Further departure means kicking off the well at shallower depths, sometimes within salt bodies. These formations are known to cause directional control problems.
No easy targets
Although many challenges face operators trying to develop deepwater subsalt plays, one of the most vexing has been drilling through the overlying salt bodies. Some of the very characteristics that make salt bodies excellent trapping mechanisms for hydrocarbon reservoirs also present the greatest drilling problems. When drilling salt, conventional wisdom has held that the less salt drilled the better. The general approach has been to drill through the salt vertically, then kick off the well to its final target. Unfortunately, this technique can cause problems. Firstly, there usually is a rubble zone immediately beneath the base of the salt, an artifact of the salt migration. This rubble zone is hard to negotiate, especially if it is necessary to kick the well off sharply in it. But when the target reservoir requires a long step-out or if it lies close up against the base of the salt, high dogleg severity in the rubble zone may be unavoidable.
New angle on salt drilling
If a well could be kicked off and drilled directionally through the salt, it would be possible to build a gentle ramp in the desired direction to avoid high-angle maneuvering after drilling out of the base of the salt. The idea has several potential benefits. Operators willing to consider directional drilling in the salt could optimize their surface (seabed) location based on the least risk of shallow gas hazards. They could optimize the salt exit location to minimize the effect of the rubble zone on borehole stability. They could more easily sidetrack to evaluate the extent of pay zones, as well as increase step-out distance to displaced target zones. And they could avoid inclusions, or areas of high geopressure where salt creep is particularly problematic.
Building and holding a gentle slope in the salt offers many advantages compared to a vertical trajectory with its deeper kick-off resulting in more radical doglegs below the base of the salt
Under the overburden pressures caused by the overlying sediments and augmented by subsurface temperatures and low permeability, salt formations can exhibit pseudo plastic flow. This phenomenon is known as “salt creep” and can close the borehole around the drill string. For salt formations, the in-situ stress generally is accepted as being equal in all directions and equal to the overburden weight. Wellbore closure rate due to the salt increases with temperature and increasing differential pressure between the formation stress and mud weight hydrostatic pressure. This means that salt creep will be faster in higher temperatures where the hydrostatic differential pressure is significant. Salt creep calculations indicate that wellbore radius is directly proportional to closure rate; i.e., a 12-in. (30.5-cm) hole has a closure rate twice that of a 6-in. (15.3-cm) hole. Accordingly, one of the principal objectives of salt drilling is to get through the salt quickly and case it off before creep shrinks the borehole size to less than the casing diameter. One way to do this might be to drill using an oversize bit, but this is inefficient and time consuming, and in deep water, time is money.
Creep solution
Another solution to the creep problem could be to use a hole-opener device like a bicenter bit. These give better penetration rates (ROP) through the salt than oversize concentric drill bits, but often bicenter bits can cause destructive vibration that damages the cutters or the instrumented bottomhole assembly (BHA). Experience shows that the use of a concentric hole-opener or adjustable reamer following a concentric polycrystalline diamond compact (PDC) bit provides the best ROP with the least vibration when drilling salt sections. Concentric reamers are most useful to reduce torsional, otherwise called “stick-slip,” vibration that is the most troublesome type encountered when drilling salt. Concentric reamers are very reliable. They are run collapsed through the surface casing or conductor pipe, and then opened when they reach the open hole. Opening is facilitated by a ball-drop or, in some cases, simply by mud pressure. When the pressure is released, they close automatically. The reamers contain mud ports so circulation can be used to clean and lubricate their cutters and well as assist in circulating out the cuttings. Best results are obtained when reamers are run with a stabilizer just below the tool and another stabilizer about 30 ft (9 m) above it.
Smooth, fast steering
Steering in the salt can offer additional problems. Experience with conventional steerable BHAs that used mud motors to drive the bit and adjustable bent subs to steer was less than optimal. Not only were the systems difficult to control so they would steer in a straight line, but in sliding mode, they were very inefficient and yielded reduced penetration rates. The solution has been to drill the salt using rotary steerable systems. The system offers several features that facilitate salt drilling. Firstly, no drag shoes are required to establish a steering reference. Everything rotates, so friction is minimized. This feature aids in producing high quality straight boreholes as well as delivering maximum power to the bit. Another valuable aid is the annular pressure-while-drilling (APWD) sensor. It can act as a “creep detector” by sensing increasing differential mud pressure as the borehole narrows. Finally, the system has a direction and inclination (D&I) hold program that works like an auto-pilot on an airplane. Once the directional driller establishes the direction and inclination for the tangential section through the salt body, the D&I hold is engaged and the tool steers itself straight along the pre-determined course. These features allow drillers to select more aggressive bits that help boost ROP while drilling a high quality borehole and staying below the threshold of destructive vibration.
Tools running in salt
Various logging-while-drilling (LWD) tools are located in the BHA below the reamer to help identify drilling hazards in the salt section. These can vary according to the situation, but may include an array resistivity compensated tool as well as a sonic-while-drilling tool or a seismic-while-drilling tool. These latter devices correlate with existing seismic data and help feed geomechanics models. Recently, some operators have included a formation pressure-while-drilling service to get a quick measurement of pore pressure immediately below the salt body. High-speed telemetry provides the added benefit of four-axis real-time vibration measurement. Recently, drillers have been guided by real-time vibration indications to adjust drilling parameters like weight-on-bit, rotation speed, and mud weight to maintain maximum safe penetration rates.
As previously mentioned, a key objective is to drill and case the salt section before salt creep sets in. To help achieve this, it is recommended to run a near-bit gamma ray (LWD) sub about 7 ft (2 m) behind the bit. This enables the driller to know immediately when the bit drills out of the salt. Drilling unnecessary rat hole into the unstable rubble zone is to be avoided, and the real-time gamma ray log provides early warning that the rubble zone has been reached.
A properly constructed salt hole is characterized by low kick-off point and landing ramp inclination build rates of about 1.5° to 2.0° dog-leg severity. Experience shows that the best overall results are obtained when synthetic oil-based mud is used in the salt section. This mud has been shown to reduce borehole tortuosity and washouts, as well as to minimize borehole instability problems commonly associated with salt drilling.
Case study No. 1
A couple of recently-completed deepwater GoM wells illustrate the effectiveness of using advanced rotary steerable systems to directionally drill salt sections. The well in case study No. 1 was drilled vertically and cased to 10,500 ft (3,201 m) with 16-in. (40.6-cm) diameter casing. Top of salt was estimated at about 12,000 ft (3,659 m) and base of salt was estimated from seismic at 20,532 ft (6,260 m) TVD. The drilling plan called for the well to be kicked off from vertical in the salt at 15,521 ft (4,732 m) with inclination build rate of 1.5°/100 ft to a total inclination of 30°, then to hold deviation and inclination until the base of salt was reached at about 21,911 ft (6,680 m) measured depth. A 14 3/4-in. (37.5-cm) PDC bit was run with a 14 ¾-in. x 16 1/2-in. (37.5-cm x 40.6-cm) AnderReamer staged above the LWD tools. The P-50 drill plan allowed 51 days to reach the 13 5/8-in. (34.6-cm) casing point just beneath the base of salt. In fact, using a rotary steerable system and real-time LWD data to maximize drilling efficiency, the entire interval including the 9,881-ft (3,012-m) salt section was drilled in 17 days on a single bit run. The intermediate string of 13 5/8-in. (34.6-cm) casing was run successfully without incident.
Case study No. 1 intermediate string of 13 5/8-in. (34.6-cm.) casing was run successfully without incident.
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Case study No. 2
The No. 2 well was drilled and logged locating the top of salt around 12,400 ft (3,780 m), and base of salt was estimated at 19,400 ft (5,914 m) measured depth. The 16-in. (40.6-cm) surface casing was set in the salt at 13,000 ft (3,963 m). Seismic analysis had determined that a safe salt exit could be made at a particular location and that spot was picked as the drilling target objective. The plan was to drill out of casing using a 14 3/4-in. (37.5-cm) PDC bit followed by a 14 3/4-in. by 16 1/2-in. (37.5-cm x 40.6-cm) concentric reamer as a hole-opener. The kick-off point was about 14,400 ft (4,268 m) and the plan was to build to about 15°of inclination, which would point the bit directly at the targeted safe exit zone. The inclination build was completed at 15, 980 ft (4,872 m) measured depth. At that point inclination was 14.98° and borehole azimuth was 54.94°. The automatic D&I hold was activated and the tangential section was drilled, hitting the safe exit zone target at 19,365 ft (5,904 m) measured depth with inclination at 14.99° and azimuth still locked on 54.94°. Immediately upon exiting the salt, the wellbore was steered to an azimuth of 24.9° and inclination was built to 20.27°. The targeted casing point for the 13 5/8-in. (34.6-cm) liner was reached at 20,605 ft (6,282 m). The entire interval was drilled from shoe-to-shoe in a single run.
It can be said that deepwater oil and gas are simply conventional reserves in an unconventional setting. Through the proper application of drilling technology and field-proven techniques, exploitation of subsalt reserves can be effective and rewarding. Drilling risk traditionally associated with subsalt plays can be managed, opening greater opportunities to access and produce valuable reserves.
