Measurement While Drilling for Geologists, What You Need to Know about Your data!
Although Measurement While drilling systems have been commercially available since 1978. It is only in the past decade that usage of this technology has become standard on land drilling operations as the number of directional wells increased and the subsequent proliferation of horizontal drilling.
Typically geologists are presented with gamma ray data from an MWD system that is provided by the same service company that provides the directional drilling measurements. Unfortunately there can be wide disparity in quality control amongst these service providers. Issues can arise not only from faulty equipment but also from improper calibration and data handling. In the early days of MWD and logging while drilling (LWD) much effort was placed on accurate depth control and log quality control. Unfortunately as this technology has become more and more viewed as a commodity, attention to this detail has gone by the wayside in many areas. Common issues that are observed by geologists are lack of consistency in measurements between tools in the same wellbore and a lack of consistency from well to well.
Since the gamma measurement is logged to depth and is used for correlation from well to well or to try to get a tie-in with seismic in addition to the gamma data it is imperative for the geologist to understand how depth data is acquired and what the limitations are on this data also.
Understanding the directional measurements, their limitations and how these can effect a geological interpretation is critical for a geologist to understand. A simple quality control method has been devised that is applicable to data from any MWD service provider.
There are several methods currently in use today for measuring depth. Most often this data is now provided through a subcontractor to the rig contractor. Most MWD companies will utilize this depth measurement to tie into their gamma data. It is important to understand that the depth measurement is always tied into the driller’s physical measurement of each individual piece of drill pipe which is made by a tape measure. Although this method is usually very accurate on a piece by piece basis many times the drill pipe is not picked up in the proper order which will lead to inaccuracies. Additionally it is imperative that the driller calibrate depth every “Kelly down” or every 90’ in the case of a top drive.
AAPG Search and Discovery Article #90154©2012 AAPG Eastern Section Meeting, Cleveland, Ohio, 22-26 September 2012