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Openhole logging is a well-established means by which E&P companies better understand reservoirs and determine where and how to complete their wells.
Traditionally, the conveyance technique of choice has been wireline, which delivers formation evaluation logging tools to total depth (TD) such that accurate completion decisions can be made for the optimal long-term viability of the well.
In laterals, conventional wireline logging becomes more difficult, more prone to failure, and more risky. This is largely a result of two limitations: first, because conventional wireline logging involves pushing the tool downhole, any ledge or other obstruction encountered runs the risk of hindering forward motion or damaging the tool.
The second limitation is the cable itself, which does not provide freedom to the driller. This is particularly relevant in shale wells, which are inherently sticky by nature. Getting the tool past a sticky section of the well bore requires rotating the pipe, which is not possible using conventional wireline logging. Hence this conveyance technique presents additional risks in the form of incomplete datasets, increased capital costs, and safety concerns for the rig crew, who are simultaneously tripping pipe and running wire downhole.
The industry's common approach has been to simply avoid logging in laterals altogether. In long-lateral shale oil and gas wells, rather than convey logging tools to identify those zones that are most conducive to hydraulic fracturing, operators may decide to fracture the entire section.
This often proves to be a costly decision because the act of blindly fracturing an entire interval can result in a high number of perforation clusters contributing to a small percentage of total production. Typically, the largest cost of completing an unconventional well is the fracturing stage, with each stage costing up to US $250,000. As many shale gas wells require up to 20 stages, this failure rate can translate to an operator overspending on a single well by several million dollars.
Weatherford has a suite of smaller diameter, shorter length memory-based logging tools called Compact tools. All functions of the tool can be deployed to TD via conveyance methods well suited for laterals, including monopole, cross-dipole sonic and formation testing, basic resistivity, and nuclear services. These solutions ensure that wireline data – with the same set of tools, sampling rates, and output – can be obtained reliably without using wireline.
Pump-down drop-off is a through-drillpipe conveyance system used to deploy a memory logging tool string that is run on wireline through the drillstring after the driller has reached TD. The tool string is dropped off into the bottomhole assembly (BHA). The wireline is then retrieved and the drillstring is pulled out of hole (POOH), while the attached logging tools record formation data to memory. This conveyance system incorporates a drop-float valve in the BHA to allow tools to pass through it for drop off or pick up, which also helps maintain well control at
all times during the operation. In addition, a reamer or open-bore drill bit can be run when applying the pump-down drop-off technique.
Another conveyance method, the well shuttle messenger, transports logging tools inside the drillpipe such that they are isolated and protected from the borehole environment. No wireline is required for conveyance, and therefore the shuttle can be run into the well at tripping speed and rotated up to 60 rpm to move past sticky sections of the well bore or other obstacles with circulation as and when required. The tools are deployed at TD, and logs are recorded into memory. Again, drillers have the freedom to circulate at any time and back-ream, within operational limits, if required. When integrated with a full drillstring-conditioning trip, the well shuttle helps eliminate a logging-only run.
Success in complex trajectory
The well shuttle system has been used to deliver and deploy logging tools for laterals around the world. An operator in China used this conveyance system to log the kickoff and horizontal section of a complex-trajectory well in a sandstone formation after previous techniques failed. The operator had tried a tool-push conveyance logging system into the openhole onshore well, but after six runs over the course of 22 days, the system failed to reach TD. The failure resulted in a cost of $300,000 in rig time and irreversibly damaged sonic and induction tools.
The company provided a logging string consisting of a gamma ray tool, array induction tool, sonic sonde tool, and borehole navigation tool, all deployed with the well shuttle. The shuttle protected the logging string during run in and successfully conveyed it to 5,730 m (18,799 ft) TD. The tools were deployed into the well bore and POOH to successfully log the entire well bore. The complete logging data profile was acquired in 45.5 hours, just 10% of the time the operator had spent on attempting to log the well previously with the conventional logging tools.
Logging shale wells
The successful deployment of logging tools via the well shuttle and pump-down/drop-off conveyance techniques has given operators the confidence to begin logging shale wells. The two factors most critical for shale resource development – optimal well placement and accurate fracture stimulation in the zones of interest – particularly benefit from the log data obtained from cross-dipole sonic measurements and microresistivity imaging.
The industry's first 2.5-in cross-dipole sonic tool provides acoustic data for a variety of formation evaluation applications. It incorporates three high-powered transmitters – one monopole and two wideband, low-frequency dipole transmitters perpendicular to each other. A receiver section consisting of an array of eight receiver stations – each one consisting of gain-matched piezoelectric geophones aligned with the dipole transmitters – record up to 96 high-fidelity wideband waveforms. These sections are separated by an isolator section that prevents direct flexural wave transmission to the receivers through the tool body. Acoustic data from the cross dipole tool is used in the evaluation of rock mechanical properties, which informs the design of hydraulic fracturing programs.
Additional reservoir insight, including identifying the presence and orientation of faults and fractures, evaluating secondary porosity, and identifying sand-shale facies, is possible by analyzing the microresistivity formation images provided by the compact micro imager (CMI). The CMI has eight arms located in two different planes. The upper caliper arms are cross-linked to help centralize the tool and provide two diameter measurements, while lower caliper arms are independently articulated to maintain the good borehole contact necessary for high image quality. The CMI can be deployed through drillpipe into open hole or into extremely tight-tolerance boreholes.
For shale plays, the deployment of cross-dipole sonic and CMI tools has broader implications if the datasets are incorporated with information from other evaluation tools, including mud logging, core sample analysis, and wellsite geochemical analysis. Taken together, these datasets provide a comprehensive understanding of the reservoir such that it can be developed with minimal uncertainty and risk and help to identify sweet spots, which is a much awaited solution for the industry.