Drilling Technology

Openhole logging without a wireline in high-angle wells

The usual ways of getting formation-evaluation data from high-angle and horizontal wells are wireline PCL and LWD; but under such conditions, reliability has been a problem

Peter Elkington, Reeves Oilfield Services Ltd., Aberdeen

Pipe-conveyed logging (PCL) failure rates as high as 1 well in 4 are most often attributed to failure of the wireline and wet-connect system, and although LWD is a more elegant solution, measurement quality still needs improvement. PCL has the added drawback of being slow, and LWD remains too expensive for many onshore locations. This article discusses a new, more reliable way of performing PCL that is especially well-suited in rugose, high-angle wells. Brief case studies are also presented.

Shuttle Conveyed Logging

The Reeves Well Shuttle seeks to mitigate inherent weaknesses in PCL and LWD. In the new system, measurements are of a type and quality associated with wireline tools, but the sonde is conveyed inside drill pipe. The tools are "garaged" at surface, and then run-in at tripping speed past any bad hole conditions that might impede conventional PCL tools.

A "dart," pumped from surface, unlatches the tools, causing them to move into openhole, suspending them from the end of the drill pipe by a collar in the bottom-hole assembly. Data are acquired in memory mode while tripping out. It has a reliability advantage because the tools are not exposed to vibration, potentially catastrophic loads when tripping in and elevated temperatures for extended periods; and the wireline – the main cause of PCL failure – has been eliminated. If needed, pipe can be rotated and reciprocated to help get to bottom, and if a well-control situation occurs during logging, there is no wireline to hinder recovery, allowing a safer operation in that respect.

The new method can be more cost-effective than LWD when real-time logging is not needed to guide the drilling process. In some cases, the logging-only run can be eliminated by combining the shuttle with a hole-cleaning trip, which makes the economics look attractive even where rig costs are high.

Shuttle Components

Formation-evaluation measurements are made with 2-1/4-in. OD compact-format tools. The company introduced them in 1998 as replacements for previous generation openhole tools. Since then, they have logged about 7,000 wells, mainly in wireline mode. Key to their success in conventional operations has been their ability to deliver the measurement quality of conventionally sized tools in all hole sizes commonly found at TD. Their compact size is exploited in the new application, where they operate without a wireline.

Power comes from battery modules. Low power consumption means a single module powers a triple combo (array induction, Pe-density, neutron porosity and gamma ray) for 19.3 hr – more than enough for most operations. Data is written to non-volatile memory, and the process of generating depth logs from time-sampled data is the same as that used in LWD. The surface system comprises a PC and patch box – no wireline / skid unit is used.

The logging tools are retained inside the last section of drill pipe by a latching sub. Inside is the running tool, which locks the logging tools in place until activated by a dart, pumped from surface (see figure). The dart releases the running tool and logging string, which then deploy out the end of the drill pipe. Pumping seals on the dart are newly designed and behave effectively and predictably throughout the length of the drillstring, even where the drill pipe ID changes.

Fig. 1. Logging and running tools are housed within landing and latching subs. The dart is pumped from surface and releases tools.

A circulating and landing sub gives a positive pressure indication at surface that the logging tools have been correctly deployed. It also provides an open path for drilling fluid – preventing the pipe from being pulled wet. An optional rasp joint above the latching sub provides additional hole cleaning. A float valve sub, run above the latching sub, contains two non-return valves to prevent well pressure from forcing the logging tools up inside the drill pipe in the event of a kick.

Case History

Shuttle operations began in December 2001, initially onshore, moving offshore in April 2002. The early land operations were in horizontal wells, but build sections and deviated wells that could not be accessed with gravity-fed wireline were also logged.

The shuttle was run in Forest Oil Corp. Redoubt-4 well offshore Alaska, drilled as a deviated delineation well in its 100%-owned Redoubt Shoal field. The well encountered a coal-bearing sequence known to produce severe ledges that have been linked to the failure of wireline PCL and LWD elsewhere in the area. It reached 20,203-ft MD, making it the deepest deviated well in the Cook Inlet area. Although the deviation never exceeded 60°, equipment was lost in the well before the shuttle was called for. In a successful operation that lasted 19.5 hours, the shuttle recorded important formation density, porosity, array induction resistivity, hole size and gamma ray data.

The caliper from the density tool showed ledges and extensive washouts to more than 15 in. over an interval with bit sizes of 8.5 and 6.75 in. Gravity-fed wireline tools would not have slid past the ledges, and it was determined that wireline PCL could have taken 72 hours with a low probability of success.

Before the operation, the total internal pipe volume was determined (this is the volume of mud that needs to be pumped to transport the dart to the latching sub). The mud-pumping rate needed to achieve a reasonable deployment speed was calculated, together with a total pumping time (a cautious 55 min. in this case). An important consideration was the type of pumping seal used by the dart. It was known from flow-loop tests that simple cup seals risk severe damage travelling past variations in ID encountered in most drill-pipe strings. As a result, a novel seal design with more predictable behavior and better wear characteristics was developed and used in Redoubt-4. During the logging operation, variations in the mud pressure (monitored during deployment) were used to confirm the dart’s progress in the well, which in turn confirmed the efficacy of the seals.

After 19.5 hours, the log data were downloaded and replayed. Analysis showed 229 ft of net oil pay in the Hemlock formation below 18,872 ft, and extended the lowest known oil by about 50 ft without encountering the oil-water interface. Forest Oil estimates the recoverable oil in Redoubt Shoal field will be at least 100 million bbl. The well also encountered 589 ft of net gas pay in multiple shallow sands. This well, combined with successful drilling in other regions, means that Forest’s predicted production for the year may be achieved in the first quarter of 2002.

In another operation, the KTU-32H well, located in Marathon’s Kenai gas field, encountering similar coal-bearing formations. Conventional wireline-logging techniques failed to reach the objective horizon, so, the shuttle was deployed to TD at 9,450 ft and returned data to surface after 20 hr. During this operation, gas was safely circulated out of the hole with the shuttle still in place, something that would not have been possible in conventional wireline operations. The shuttle data satisfied Marathon’s formation-evaluation requirements at a substantially lower cost than LWD.