Multilateral and extended-reach drilling is now being widely applied in the world's oil fields. Although dating back to the 1950s, multilateral technology has only in recent years reached mainstream status, driven by technical advances, economics and environmental considerations. The world's oil fields are increasingly brown fields. Multilateral and extended reach technology aims to get at more of the hydrocarbons remaining in these reservoirs and increase recovery rates. The following case histories demonstrate a variety of successes and mark the progress of what is now one of the industry's fundamental tools.

Unlocking the value of Alaska's heavy oil
Alaska's West Sak is a large, shallow heavy oil accumulation that overlies much of the ConocoPhillips-operated Kuparuk field on Alaska's North Slope. It contains from 7 billion bbl to 9 billion bbl of oil in place. Oil gravity ranges from 10° to 22° API at 75°F (23.8°C). However, because the reservoir is close to the base of the permafrost, in-situ viscosity for much of the oil is greater than 300 centipoise (cp). Initial oil production began in 1971 at approximately 3,000 b/d. By March 2005, production had increased to 16,000 b/d. Current development plans call for production rates of more than 40,000 b/d by 2007. ConocoPhillips and BP have cited multilateral/extended-reach technology as a significant contributor to unlocking the value of the West Sak reserves.
Multilateral well construction in West Sak began in 2000 with dual-lateral Level 4 systems. The success of the first three multilaterals confirmed the potential for significant savings in well construction costs. After a thorough review and analysis in early 2001 of the relative installed costs, construction risks and implications for long-term well operability and intervention of all available multilateral systems, ConocoPhillips changed from Level 4 to Level 3 construction using Baker Oil Tools' Level 3 Hook Hanger technology.
The Hook Hanger system provides mechanical support for junctions that join cased and cemented main bores with screened, openhole laterals in wells with commingled production. The system was the first to offer the option for re-entry into both the lateral and the main bore. Hook Hanger systems have simplified multilateral completion operations and have been used successfully in dual- and tri-lateral applications. Depending on the application, the system can be run with either of two anchoring systems.
Typical installation sequence:
• The lower lateral is drilled and completed through the casing shoe.
• A one-trip WindowMaster whipstock system with bottom trip anchor is run in position to drill the upper zone and set on the liner hanger.
• The casing window is cut and the upper lateral is drilled to total depth.
• The whipstock is retrieved.
• The Hook Hanger assembly is made up, run into the casing and landed in the casing exit window. The system's hook engages with the lower part of the casing exit window to hang the lateral well system off the main bore casing.
• The well is now ready for final completion and production.
Retrievable lateral and mainbore diverters provide future access. Both can be deployed and retrieved with either coiled tubing or jointed pipe.
Changing from Level 4 multilaterals to Level 3 Hook Hangers in West Sak resulted in six fewer trips per dual-lateral well. The system continues to be used for the tri-lateral configurations being drilled today and has been continually improved. For example, a 63¼4-in. bit rather than the smaller, earlier 6-in. bit is being used to drill the laterals out of 75¼8-in. casing. Wireline tools are no longer used to orient the lateral liner hangers prior to setting, and a completion system has been designed and installed with the multilaterals that is capable of hydraulically isolating between laterals and lateral intervention with coiled tubing and wireline tractors.
Drilling and completion optimization has also allowed progressively longer laterals to be successfully completed. The multilateral lateral section in 2001 was 4,600 ft (1402 m). By 2004, an 8,400-ft (2,561-m) lateral section had been successfully completed. During the same time frame, horizontal displacement increased from about 5,000 ft to 12,500 ft (1,524 m to 3,811 m).
West Sak development strategy has evolved from simple, low-oil-rate vertical wells to complex, high-oil-rate extended-reach multilaterals that have provided access to more reserves with fewer wells, thus cutting development costs and dramatically improving the value of West Sak projects.

North Sea well is converted to a multilateral
Located in the Norwegian sector of the North Sea, the Troll West field is approximately 50 miles (80 km) northwest of Bergen in 1,033 ft to 1115 ft (315 m to 340 m) of water. The field is presently producing approximately 200,000 b/d of oil, qualifying it as the second-largest field in the North Sea. This field provides about 11% of Norway's oil production.
The multilateral well concept on the Hydro Troll West field was introduced primarily to increase the total reservoir exposure from existing subsea template structures. To recover reserves from the thin oil-bearing sands before gas production commences, new horizontal producers are continually being drilled. A total of 109 wells drilled by March 2006 include 41 multilateral wells with 54 multilateral junctions. Wells drilled nearly 10 years ago are still producing, but reserves around the bore holes are not being produced.
Using Halliburton's ReFlexRite system, an existing well has been converted to a multilateral well, a first for the field. Existing production well H-2 was required to be sidetracked to drill an additional lateral in an adjacent reservoir section not being drained. The original well was still producing 1,800 b/d of oil, and this production was to be retained after the addition of the new branch.
The ReFlexRite system, with a sealed flexible junction, is a combination of two major multilateral systems. The first system creates a high-quality geometrically defined milled window. The second system has a high-flow TAML (Technical Advancement of Multilaterals) Level 5 junction providing hydraulic pressure integrity at the junction through the completion.
The flexible junction provides connectivity to the main bore and lateral, hydraulic and mechanical isolation, and access to the branches. The flexible junction also offers an optimized flow area with two D-shaped legs providing mechanical stability and increased tensile/compressive strength. The ability to isolate the main bore or lateral above the junction is provided at a Y-block with polished bores for setting a bridge plug or for landing of a junction straddle seal sub. The system is also designed for low risk, short installation times and a robust, simple installation process from floaters.
Installation sequence on the H-2 well:
• Kill and de-complete the main bore.
• Set 103¼4-in. multilateral packer with a seal stinger designed to enter the cut off 7-in. blank pipe above the main bore screens and provide a tight sand seal.
• Mill a 15 ft (4.6 m) first-pass window in 1.5 hours. The drilling whipstock was run with shear bolted mills, and an 81¼2-in. full-gauge 5.0 m window was opened.
• The 81¼2-in. lateral section was drilled from the window exit at 6,145 ft and 17,312 ft (1,873 m to 5,277 m) measured depth. An openhole sidetrack was performed at 12,080 ft (3,682 m) and a second lateral drilled to 17,021 ft (5,188 m). Another openhole sidetrack was drilled at 13,024 ft (3,970 m) and a third lateral drilled to 19,006 ft (5,793 m). The total length of reservoir section added to the H-2 well is 16,857 ft (5,138 m).
• The drilling whipstock was pulled and replaced by a completion deflector. The lateral screens were run and entered the third and final openhole lateral since all sidetracks were performed low side. The flexible junction was installed with the screens and landed in the deflector to create a TAML Level 5 pressure- and sand-tight sealed junction.
Currently, the well is producing over expectation at a rate of 9,000 b/d of oil. This is 500% greater than the well was producing as a depleted, single lateral.

Strategic use of Rt data maximizes reservoir exposure
Producing heavy crude from South America with horizontal extended-reach wells requires geosteering to maximize reservoir exposure. Obtaining accurate true resistivity (Rt) measurements in real time for such projects is a necessity.
Logging in this environment is influenced by relatively fresh mud and, at times, highly laminated formations having a high contrast in resistivity between reservoir sands and interbedded clays and finer-grain sands. The typical recovery mechanism is solution gas drive using extended-reach horizontal wells. In these unconsolidated formations, the poor acoustic impedance contrast between the reservoir sand and shale severely limits the usefulness of seismic sections for target identification. The practice is to log every extended-reach well with logging-while-drilling (LWD) gamma ray and a resistivity tool. Use of the resistivity log is to maximize the lateral extent of the drainhole within the reservoir via real-time geosteering.
Operators are then faced with the option to instrument the bottomhole assembly with an measurement-while-drilling (MWD) tool and a 2 MHz propagation resistivity LWD tool, or to rely on a laterolog LWD-type tool. In these high-deviation wells in a laminated environment that exhibits high resistivity contrasts, propagation readings are greatly influenced by bed boundary effects, mud filtrate invasion, shoulder bed effects and resistivity anisotropy - even after application of sophisticated multidimensional inversion techniques. Entire log sections are saturated and can never be recovered.
As development in this heavy-oil belt progressed and thinner prospects were drilled, the operator needed more detailed and accurate formation evaluation. The early LWD propagation resistivity logs were re-analyzed in an attempt to glean more information than could be obtained from simple resistivity cutoffs. In so doing, an interesting discrepancy was noticed - the LWD propagation logs recorded while drilling always showed what seemed to be a deep conductive invasion profile having a diameter often greater than 20 in., whereas wireline laterologs in vertical wells showed little or no invasion. Such an invasion profile in formations such as these, where the oil viscosity is often greater than 2,000 cpa, is physically unlikely.
By opting to use a laterolog type tool, the Schlumberger geoVISION downhole MWD/LWD imaging tool, one operator witnessed the benefits of running such a tool whose measurements remain less influenced by environmental effects than the propagation tool. LWD laterolog type tools, while also affected by invasion, behave in a predictable manner and are more easily corrected to yield an accurate value of Rt.
The LWD laterolog type tool has an integral, cylindrical electrode that delivers a high-resolution, lateral resistivity referred to as RING resistivity. In addition, three azimuthally-focused button electrodes, spaced longitudinally along the axis, provide multiple depths of investigation that permit qualification of invasion profiles. The tool also makes a bit resistivity measurement that allows real-time coring-point and casing-point selection in both conductive and resistive mud environments.
The occurrence of RING resistivity with the use of the LWD laterolog type tool prompted an entire study to be conducted on measuring these sandstone reservoirs that range from thick amalgamated fluvial deposits to more heterolithic, tidally influenced fluvial and distributary channel deposits. The effect of these discreet thin conductive layers on laterolog and propagation measurements as the well gets close to horizontal was the subject of the study. By modeling individual environmental effects observed with each tool response, the conductive profile invasion phenomenon was investigated in detail.
The actual data obtained using the laterolog LWD type tool demonstrated the same high-resolution, accurate measurements for real-time interpretation as those derived in the comparison study. It was confirmed that the conductive invasion profile seen on propagation logs from this region is mostly produced by the thin near-horizontal clay layers seen on the borehole images. While this did not imply that there was no filtrate invasion at all, analysis of the laterolog LWD type resistivities showed much less invasion than the often massive invasion that had been previously suggested with the use of propagation logs.
The tool's resistivity measurements while drilling provided the real-time geosteering guidance required to maximize the length of well bore within the reservoir sands. The measurements behaved predictably and were corrected to yield an accurate Rt value in the high-resistivity-contrast, laminated environment that had proven conventional propagation resistivity measurements difficult to correct and interpret
.
Simplify casing exits at extreme depths
In Equatorial Guinea, an extended-reach well of more than 19,000 ft (5,792 m) had missed its target. To avoid a total loss, the operator wanted to exit the well with a lateral at 19,297 ft (5,883 m). Attempts to use standard casing-exit technologies on a similar well in the same area had failed to deliver a simple, cost-effective sidetracking solution that is routine at shallower depths.
The operator knew to expect (1) sinusoidal pipe buckling from the length of the drillstring; (2) limitations on applied weight and torque; (3) difficulty controlling and positioning downhole tools; and (4) the difficulty of pushing the tool all the way into the hole. Even if a standard whipstock could be run to depth, would it be possible to apply enough additional force to break the shear attachment bolt and free the milling assembly? The operator also wanted to make certain of the exit location by touching down on the cement top at the bottom of the well and then backing off an exact distance. If the tool did make it downhole, would the impact of tagging the cement break the shear attachment and result in the wrong location and direction for departure of the new well bore?
The operator selected Weatherford's StarBurst hollow whipstock technology, which incorporates a number of innovations on standard whipstock technology to construct a downhole junction in three trips.
Installation sequence:
• First trip: A running tool deploys a whipstock using a shouldered design that puts pressure on the whipstock without putting strain on the shear attachment bolt. The whipstock is anchored by slips that prevent axial and radial movement in either direction. The running tool is pulled up, which breaks the shear attachment bolt, thus avoiding buckling problems. The whipstock has two other unique features: it is hollow, and the first part of the ramp is a brass lug.
• Second trip: A starter mill is deployed which has three full-diameter blades so that the initial casing breakout will be full gauge. This mill also features a "nose," about 11¼2 ft (.46 m) long, that rides along the brass lug and guides the mill. The mill chews up the soft brass as it goes along, while the nose directs nearly all of the milling force towards the casing, allowing less pressure to be used. The nose also causes the mill to start the casing breakout about a meter above the top of the whipstock, ultimately resulting in an exit hole that is 15% to 20% longer than a conventional exit. This helps ensure a smooth transition for subsequent bottomhole assemblies. The starter mill is removed when the nose stops against the casing at the end of the brass lug.
• Third trip: A multilateral-system (MLS) mill is run-in. To avoid damage to the hollow whipstock, this mill is designed to cut straight ahead only. The MLS finishes the smooth transition into the new lateral.
The job went forward without a hitch, allowing the tool to create the casing exit with minimal weight and with a longer exit window. The entire process took about a week to complete. While this technology was used here only for a casing exit, the hollow core of the whipstock can be perforated so that fluids can also be produced from below the whipstock.

Comments