The Subsurface Telemetry Problem-A Practical Solution

Abstract

Abstract A practical solution to the inherent "blindness" of the rotary drilling system has been developed and field tested successfully. It consists of a novel telemetry system which makes it possible to obtain at the surface, concurrent with the drilling of a well, continuous logs on one or more physical parameters of the freshly penetrated formation at the bit. The method will also be made available for various warning and control systems pertaining to the condition or operation of the drilling equipment at the bottom of the hole. A measurement at the bottom of the hole is translated into a pulse code which periodically triggers a hydraulic mud valve in the drill collar. Partial closings of this valve for a short period of time create pressure pulses in the in-going mud stream which are readily detected at the surface as small increases in pressure. After decoding this pressure-pulse pattern arriving at the surface, the resulting readings are automatically plotted on a recording paper strip which moves as the drill progresses downward. Examples of continuous logs are shown, and a general description of the equipment used in the telemetry system is given. Introduction The Need for Continuous Logging in Rotary Drilling of Exploratory WellsThe basic purpose of drilling exploratory wells is to find and evaluate new oil and gas reservoirs. The emphasis is therefore on gaining significant information about possible productive zones, and the drilling of the well itself in most cases merely a means to this end. While drilling, an exploratory well often encounters unexpected geological conditions and new potential reservoirs. Obviously, the best time to acquire pertinent information about such reservoirs is when they are freshly penetrated and least disturbed by mud filtrate. Also, optimum drilling speed and minimum drilling cost can only be obtained if the exact nature of the changing formations being drilled is known at all times. Because of these considerations, the search for a reliable and diagnostic method of gaining such information while drilling and not afterward is as old as the drilling industry itself. The following brief summary of the important stepping stones in the industry's search for better evaluation of the formations penetrated in exploratory wells will serve as a background for the discussion of the subject material of this paper. Early Developments in Intermittent Formation Evaluation of Rotary Wells Logging while drilling in the days of cable tools was no problem. In effect, the formations were being continuously sampled and production tested as the cable-tool bit drilled ahead. With the advent of rotary drilling, however, the industry suddenly found that its ability to "see" downhole while drilling was greatly impaired. The drilling fluids generally served to hold back formation pressures, and the analysis of cuttings obtained from the mudstream became much less diagnostic. In order to get some indication of the variations in lithology, the speed of drilling was often watched closely, and correlation of drilling breaks with the cuttings obtained could sometimes serve as a guide regarding the formations being penetrated. Coring The introduction of the rotary core barrel to the industry during the 1920's provided for the first time a means of obtaining a good sized sample of the formations penetrated. Its universal acceptance as a standard drilling tool was limited, however, by its ability to retain only a core before requiring time-consuming and costly recovery operations, and by the reduced drilling speeds generally required while coring. Drillstem Testing Introduction of drillstem testing, also during the 1920's, provided the operator with a means of obtaining actual samples of formation fluids from the penetrated formation without the need for setting pipe, and was another step forward in the search for improved formation evaluation. Electrical Logging Although the formation evaluation problem in rotary wells was greatly aided by the development of the rotary core barrel and the drillstem tester, the most important step forward in curing the inherent blindness of the rotary system came with the introduction of the electrical log around 1930, as a means of identifying and evaluating the formations penetrated. JPT P. 487ˆ