Slimhole triple-combo LWD overcomes severe doglegs

For a Middle East operator, adding a flexible, slimhole triple-combo logging-while-drilling (LWD) system to its directional drilling toolbox has revitalized a mature field and created additional recoverable reserves. The Weatherford LWD 43?4-in. system has made it possible to gather directional, gamma ray, density, neutron, and resistivity data in high-dogleg (up to 68 degrees/100 ft or 30.5 m), ultra-short-radius wells without making multiple
logging runs after landing the well.

Short-radius drilling
With the development of advanced tools and technologies, short-radius drilling is poised to redefine the nature of horizontal wells. Among the inherent advantages of drilling a short-radius well bore is the reduced footage drilled in the curved section and the corresponding additional horizontal section drilled in the pay zone for a given displacement.

Short-radius drilling offers benefits for operators seeking to economically maximize production from aging fields. (Images courtesy of Weatherford International)

Short-radius drilling provides a simple, cost-effective solution for driving maximum production benefits from aging fields. The technique makes it possible to re-enter and sidetrack existing vertical wells, targeting the limited hydrocarbon column horizontally. In addition to being less expensive and impactful than drilling a new well to the kickoff point, it also reduces risk. Openhole exposure is reduced. Unstable zones can be bypassed. The curve can be drilled fully in the target zone, and drilling can take place from a light workover rig. An additional advantage is that borehole trajectory through the production zone is near the original well bore, where much is known about the reservoir from previous logs and production history. This knowledge is particularly beneficial in addressing problems associated with “sick” wells and rejuvenating those wells with minimal interventions.

Short-radius drilling is not without limitations. High dogleg severity (DLS) can make it difficult or even impossible for tools such as LWD and rotary steerable tools to pass through buildup curves. High stresses on the drillstring pose the potential of stuck pipe, twist-off, and resulting fishing and hole-cleaning operations. High friction force between the bottomhole assembly (BHA) and the hole wall, particularly inside the curve section, increases the possibility of slide mode drilling difficulties.

Application in the Middle East
An operator has been using short-radius drilling in a challenging carbonate reservoir in the Middle East to enhance recovery from a mature field by re-entering “dead” vertical wells originally drilled in the 1950s and drilling horizontal sidetracks. The decision to use the short-radius system was based on the ability of the shorter curve section to reduce cost and drilling time and minimize drilling through difficult formations such as the anhydrite above the carbonate reservoir.

Vertical and lateral variation of formation dip and lithology in the field can create uncertainties for landing wells in the sweet spot. However, with limited total vertical depth to build to horizontal, the required buildup rate was too severe for existing LWD tools, and acquiring real-time triple-combo data through the severe doglegged sections was not possible. As a result, geosteering relied solely on gamma ray correlation and possibly resistivity or making multiple logging runs after landing the well. Additionally, an extra BHA run was required to soft-land the well to accommodate the LWD tools.

The development of the Weatherford 43?4-in. LWD system enabled the operator to acquire triple-combo data while drilling doglegs of up to 68 degrees/100 ft. The LWD data was used to control the landing and more effectively geosteer the horizontal section, reducing drilling cost and potentially improving production. Having complete log data while drilling the curve helped the petrophysicist and geologist redefine their interpretations concurrently and land in the targeted column. Landing in the sweet spot improves the potential for achieving targeted production rates.

Meeting the challenge
Meeting the challenge of directional control in short-radius well bores is dependent upon meeting three criteria:
1. Designing a BHA that can drill the short radius curve section at a predictable build rate;
2. Maintaining direction in the lateral section; and
3. Providing a measurement-while-drilling (MWD)/LWD tool with measurements closer to the bit that can reliably negotiate severe curvatures in the build section and geosteer in the horizontal section.

For the short-radius program in the Middle East, the most important consideration for the LWD system from the drilling engineering perspective was its ability to pass through and survive the proposed short-radius buildup rates. The planned BHA for the build section consisted of a 6.125-in. tricone bit, a 43?4-in. positive displacement motor with 2.9-degree bend housing and 3.06-degree offset kick pad, two 43?4-in. short flex chrome manganese non-magnetic drill collars, a 43?4-in. MWD transmitter sub, 43?4-in. screen sub, drillpipe in the curve section, heavy-weight drillpipe (HWDP) in the vertical section, and drillpipe to surface. To avoid damage to the motor and MWD assemblies, it was necessary to restrict drilling and tripping to sliding mode only while in the curve.

This log is an example of real-time data acquired in a short-radius well in the Middle East using the Weatherford 43?4-in. triple-combo LWD system.

The BHA for the soft landing and lateral consisted of a 6.125-in. PDC tricone bit, a 43?4-in. positive displacement motor with 1.15-degree bend housing, a 57?8-in. motor with top stab, a 43?4-in. short flex chrome manganese non-magnetic drill collar, a 43?4-in. LWD transmitter sub, a 43?4-in. LWD transmitter sub, a 43?4-in. screen sub, drillpipe in the curve section, HWDP in the vertical section, and drillpipe to surface. Drilling alternated between sliding and rotary mode.

The LWD tool was designed with full functionality including gamma ray, resistivity, density, neutron, and azimuthal density image. It also was designed to operate in hole sizes as small as 6.125 in. and building angle at a rate of approximately 68 degrees/100 ft.

Three-well program
The ultra-slimhole LWD system was applied in a three-well onshore re-entry program. All three wells were old oil
producers in a carbonate reservoir, using gypsum mud as the drilling fluid. Original casing size was 7 in., and windows were cut from permanent casing. Hole size for each well was 61?4 in. DLS ranged from 59.59 to 61.90 degrees/100 ft.

One of the wells was drilled to revive a “dead” well. The other two were completed as horizontal open holes to reduce water production, maximize oil production rates from the target reservoir, and add reserves.

In each well, the objectives were to plug back the original open hole; cut a window in 7-in. liner or casing, sidetrack the well and drill a short-radius horizontal section across the target reservoir, and complete the well with a 7-in. packer (and, in one case, an electrical submersible pump assembly on the tubing). All wells met their objectives. One well required two runs to complete, and in the case of the previously abandoned well, production was recovered.

As horizontal drilling and re-entry technology evolve, new drilling systems provide more predictable angle-building, more precise steering, and faster drilling rates that enable operators to produce profitably from more challenging targets. Providing valuable formation evaluation data with LWD tools allows for real-time data evaluation and decision-making. In addition, the ability to measure accurate LWD formation data in wells featuring a variety of high doglegs represents an opportunity for safe and cost-efficient wellbore construction.