Sunday, December 2, 2018

Logging and Perforating with Coiled Tubing

Like CT drilling , coiled tubbing logging has come of age only in the 1990s. One of its key selling points revolves around the stiffness of the tubing, enabling penetration into horizontal and high-angle sections. Additionally, wireline inside coiled tubing offers the potential to pump fluids downhole and log at the same time.

Successful application of CT logging requires the reliable interface of the coiled tubing and logging units. Wireline log acquisition systems are driven by depth. To supply real-time depth data for CT logging, an encoder relays a depth signal from the injector head into the logging unit through a dedicated interface. This depth information is also used by the monitoring system that records coiled tubing and pumping parameters. 

A newly designed coiled tubing head is now available to attach the logging tools to the CT. The modular head secures the cable in place, allows fluid to be circulated through a dual flapper valve during logging, and provides for electrical connection and mechanical release.

Fundamentally, a CT logging operation is not much different from its wireline counterpart. However, the tubing is stiffer than wireline so it tends not to stretch as much, and the injector head provides a stable speed. Coiled tubing may deploy most logging tools, as long as they are slim enough to fit inside the wellbore. The scope of slimhole logging -whether the wells were drilled using CT drilling or conventional techniques - has been limited by the availability of slimhole hardware. Now that scope is broadening.

Originally, slimhole logging tools were developed to gather petrophysical information in deep, usually hot, wells that required extra strings of casing, thereby reducing the final well diameter. Alternatively, they were needed to log through drillpipe under difficult hole conditions. These rigorous environments ensured that such tools were of necessity simple, reliable and rugged.

Today, CT drilling and slimhole wells are being used, or contemplated, for a broader  range of well types that have more sophisticated needs. To meet these needs, many standard and new high-technology imaging tools have been reengineered to perate in more restricted boreholes. For example, the DLL Dual Laterolog Resistivity tool and the combinable Litho-Density tool have been repackaged with diameters of 2 3/4 inch and 3 1/2 inch, respectively.

In addition, new instruments have been designed , such as the SRFT Slimhole Repeat Formation Tester tool for sampling the formation, the sourceless RST Reservoir Saturation Tool , and the Pivot Gun for slimhole perforation. Combinability of tools and coiled tubing logging capability are standard features.

To date, coiled tubing is most often used for production logging, sometimes combined with CT-conveyed perforation. As usual, the production logging tool string measures a range of parameters, including spinner revolution, fluid density, pressure and temperature; a gamma ray tool and a casing-collar locator are also included.

Production logging of high-angle or horizontal wells presents a tremendeous challenge. For example, there may be stationary fluid  stationary fluid, back-or cross flow - some zones may be accepting fluid produced by other zones. Only a fraction of the fluids "seen" by the tools may actually be moving. To overcome these difficulties, the production logging program must be sufficiently flexible to respond to changes in well behavior.A typical CT production logging job involves the following steps: 
  • Rig up and pressure test equipment.
  • Run in hole stopping to check CT weight.
  • Correlate depth with a reference log using casing-collar correlation and gamma ray logs -vital because the CT tends to form a helix in the well.
  • Log the well while shut in.
  • Log in both directions with typically four passes at say, 40,60,80 and 100 ft/min.
  • Observe well anomalies, making some stationary log measurements to look for backflow.
  • Pull out of hole.
 Another production-related CT logging service employs pulsed neutron logs and borax solution. The borax is pumped into the CT-production tubing annulus at a pressure above that of of the reservoir but below the fracturing pressure. Because borax is more effective than reservoir fluid at slowing neutrons, pulsed neutron logs can trace where it has gone and hence confirm the location of a suspected channel and indicate high-permebility zones. With additional openhole log data, initial reservoir saturation information may also be derived. 

After the production profile of a well and potential hydrocarbon saturated zones have been identified, reperforation using CT-conveyed guns may be necessary.



Matrix Treatment

The most traditional of all coiled tubing services is the delivery of fluids downhole. No account of the practical uses of coiled tubing would be complete without describing at least one pumping application - a role that has become more important with the proliferation of horizontal wells.
  As in other areas, increasingly sophisticated pumping services are available. For example, a relatively new matrix treatment tackles an old problem, diversion. Unless a stimulation fluid is successfully diverted into the areas that most need it, the fluid will channel into the high-porosity, high-permeability formation that least requires improvement. Horizontal wells generally have a much longer reservoir section than their vertical counterparts, so the problem of diversion is proportionally more difficult. To compound this, few horizontal wells are completed in a way that allows reven rudimentary zonal isolation.
  Traditionally, diverting materials - like calcium carbonate or rock salt - are introduced to temporarily plug the zones of the formation taking most fluid, redirecting flow to more needy parts of the wellbore. But the plugging must be reversible- by dissolution in acid or reservoir fluids- and leave the formation undamaged. Not an easy criterion to meet. 

A successful alternative employs stable foam that is generated in the "thief zones"   as a diverter. Alternating stages of acid and the foam - made from water containing surfactant and nitrogen are pumped. The diverter enters the formation that is taking fluid. Some 10 minutes or so are allowed for the foam to build up and, when pumping restarts with a new acid stage , a pressure increase is seen at surface as the foam ensures the acid enters some other part of the formation. Pressure gradually decreases until it is time to pump the next foam stage. Once production starts, the foam breaks down and flows out of the well leaving undamaged, acidized formation.

Coiled tubing is an ideal way of targeting the delivery of the treatment fluids to the formation, particulary in horizontal wells. Furthermore, because the volume inside CT is relatively small, a flexible treatment program may be employed, based on pressure responses observed during pumping.













No comments:

Post a Comment