Exploring, drilling and completing hydrocarbon and other wells are generally complicated, time consuming and ultimately very expensive endeavors. In recognition of these expenses added emphasis has been placed on well logging, profiling and monitoring of well conditions. Over the years, the detecting and monitoring of well conditions has become a more sophisticated and critical part of managing well operations.
Initial gathering of information relative to well and surrounding formation conditions may be obtained by running a logging tool in the well. Typically, a logging cable may be used to deliver the tool into the well by means of a winch at the surface of the oilfield. A device positioned near the winch at the oilfield surface records the amount of cable lowered into the borehole and thereby indicates the depth of the tool in the well. With the tool positioned downhole, the cable is then pulled uphole as the logging application proceeds. In this manner a log revealing an overall profile of the well may be established, with measurements being recorded continuously as a function of depth in the well.
For subsequent logging passes, perhaps containing different sensors, recorded measurements may be aligned with those of the above noted reference log previously acquired. That is, typically, the first log acquired in a well is considered the “reference”, and all subsequent runs are adjusted in depth to match this reference. This process, referred to as “depth correlation” ensures that corresponding measurements from the same section of the formation that is penetrated by the well are seen to be coincident when the logs are compared. The various measurements from the disparate sensors may then be combined to produce a more complete interpretation of the nature of the formations traversed by the well.
On occasion, some logging tools may be run which, by their nature, are to be positioned accurately at a specified depth, and remain at that depth for an extended period of time while measurements or other operations are performed. Such operations may include the measurement of fluid properties in the formation, the taking of fluid or rock samples from the formation for later analysis at the surface, or even the perforation of the metallic casing commonly used to isolate the formation from the wellbore once the wellbore is completed. Regardless of the particular application, knowledge as to the actual depth of the tool may be of substantial importance.
Unfortunately, it is frequently observed that, as the winch is stopped as the tool is brought to the required station depth, the tool continues to move for some time. This effect is sometime referred to as “creep”. As a result, the depth of the tool as determined with reference to the stopped winch at the surface fails to reflects the actual or true position of the tool downhole during the creep period. This, in turn, may lead to serious operational problems due to the lack of precise knowledge as to the location of the tool. For example, difficulty may arise in correlating data acquired with the tool in a stationary position with data recorded during the reference log with a moving tool. Similar difficulty may arise in correlating fluid or rock samples from the stationary tool with the dynamically acquired reference logging data. This may in turn result in the ultimate delivery of the tool to the wrong station or target depth within the well for the application to be performed.