A better understanding of the reservoir has resulted in the design of significantly more complex wellbores to optimize exploration and development. These advanced wellbore geometries have presented a major challenge to conventional openhole logging.

Complex well geometries

The shift from vertical wells to more deviated, S-shaped, horizontal, and multilateral wells has been achieved with a broad scope of innovative drilling technologies and techniques. Coiled tubing (CT), underbalanced drilling, and managed pressure drilling methods are increasingly common tools for improving drilling efficiency and limiting near wellbore formation damage.

However, growing wellbore complexity, tortuosity, hole instability, low-pressure zones, and a multitude of other issues have made conventional wireline conveyance problematic and often impossible. This comes at a time when a full set of logging data is often critical to optimizing the completion and long-term productivity. The need for formation data contrasts strongly with the risk of sticking or losing tools in the hole and the repercussions of limited or no openhole logs.

Poor hole conditions and convoluted trajectories result in excessive washouts that often create a zone that is impassable for tools under gravity or with conventional tool-push techniques. These tools have limited compressive strength and, being the first point of penetration, they tend to buckle in these zones, resulting in a series of incomplete operations, tool damage and fishing, and many times a failure to acquire data.

In particular, logging operations are commonly challenged on two key counts: well control and the ability to reach target depth. It becomes imperative to circulate and rotate to defy these challenges. Unfortunately, while wireline provides useful data, it also inhibits drillpipe rotation and puts constraints on circulation.

Technology advances

The solution relies on the marriage of a similarly diverse set of conveyance systems and a unique suite of logging tools. Weatherford is regularly logging complex and difficult wells in both real time and in memory using an array of Compact 2¼-in. openhole tools. The small-diameter tools acquire a full range of wireline-quality data to make basic and specialized petrophysical measurements including resistivity, porosity/lithology, formation testing, acoustic, and imaging. This year, the world’s first memory-capable 4.1-in. oil-based microimager is being introduced.

The flexibility to operate in real time or memory modes with the same set of tools provides a high degree of flexibility in planning and responding to specific wellbore challenges. It also eliminates logistical challenges posed by dedicated systems that operate in only one mode. Where wireline operations present well control issues, logging to memory provides a nonwireline alternative for acquiring data.

Options and applications

The ability to dependably log these wells has only come with the development and integration of two key components: a full suite of small-diameter tools and multiple conveyance options. The solution is twofold. It provides reliable acquisition of critical formation data in complex wellbores, and it reduces the risk of losing tools in the process. The success of this system assures future well designs are not constrained by the capabilities of the logging system.

Memory logging such as a well shuttle system protects tools inside the drillpipe until they are deployed into open hole. The technology logs in a single pass to save time in extended-reach wells, and it allows circulation and pipe rotation to deal with the hole conditions.

A memory-based drop-off method uses drillpipe with a special bottomhole assembly (BHA). The BHA is lowered to the target depth without the tools with full freedom. Once the drillpipe reaches desired depth, the tools are lowered inside the pipe on wireline and released into the BHA. The “drop off” conveyance technique enables the driller to have full control over the well, allowing mud circulation and conditioning while having logging tools deployed and providing the largest flow area.

Inherent to the utility of this versatile openhole logging system is the ability to log a full set of data. For example, in unconventional assets, the world’s slimmest cross-dipole sonic, microimagers, and spectral gamma ray tools are being used to identify sweet spots to guide perforation placement and fracture treatment.

In thinly laminated formations, microimaging logs provide the only alternative to coring for determining net pay thickness. In developing the Grayburg formation in the Permian basin, Apache Corp.’s collaboration with Weatherford has developed a fit-for-purpose logging program incorporating the Compact microimager. Initial oil production rates increased nearly 40% in the first 180 days when the imaging tool was included in the operator’s completions strategy.

Case histories

In western Russia, a drilling program converting vertical wells to highly deviated, complex-geometry horizontal wells required a formation tester log to evaluate pressure and permeability. The wireline through-pipe operations minimized cable sticking for reduced rig time. More than 40 out-of-pipe jobs were performed, including formation testing in memory, to achieve operational efficiency of 95% and a 99% sealing success rate.

The data were used to confirm formation saturation and fluid contacts as well as dynamic fluid model parameters. They also allowed more effective planning of the production well grid and saved drilling time when low pressure led to one planned horizontal candidate being reassigned as an injector well.

Wireline logging attempts in western Texas failed to reach total depth despite hole conditioning. To acquire the measurements, including rock mechanical properties, the 2¼-in. triple combo and cross-dipole sonic tools were run through the drillpipe, extending just below the well obstruction.

Highly deviated wells in a coalseam gas field in Australia challenged conventional wireline evaluation. Drilled from pad sites, the wellbore deviations reached from 60° to 85° in an 8½-in. hole. To log the wells, a nonwireline solution was selected that combined CT memory logging and shuttle conveyance. The shuttle used for openhole formation evaluation allowed constant circulation and pipe rotation to mitigate differential sticking in the tight hole. In addition to the traditional quad combo suite, an advanced microimager and a cross-dipole sonic were also run for full evaluation of the coalseams. CT memory logging was used in the cased hole to evaluate casing inner diameter and provide a cement bond.

To optimize hydraulic facture design in a complex volcanic reservoir development program in Argentina, a cross-dipole tool was deployed through the drillpipe into the open hole. The tool acquired data for rock mechanical properties, brittleness, and formation anisotropy to inform perforating and fracturing design. The same tool was later run in the cased hole to evaluate the fracturing results.

Global solutions

While this multiconveyance system brings reliability and predictability in the logging operations, it is also redefining the limits to acquiring logs. For example, in unconventional reservoirs it is being used in short-radius wells and long laterals common to multiwell pad drilling and stacked pay designs.

Recent tests have shown that certain tools can successfully navigate dogleg severities as high as 120°/30.5 m (100 ft) using pipe or CT conveyance. This system also allows logging a full suite of formation evaluation services in highly deviated or horizontal slim holes such as 37/8-in.

Each drilled well comes with its own challenges. Conveying tools to the desired depth and mitigating well control issues requires a versatile response using flexible tools and the most suitable conveyance method to reliably acquire critical data while minimizing risk.

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