Determining Subsurface Stratigraphic Position by Fluid-Entry Analysis During Air Drilling
Authors: Mathias J. Paul 
Published: December 2007

The most common methods of determining subsurface stratigraphic position while drilling with mud are difficult or impossible to use in air-drilling operations because drill cuttings may be powdery, and the extremely fast drilling rate remains unchanged as different formations are penetrated. Moreover, air-drilled holes are commonly completed in 4 to 10 days and logs are run only at total depth. These factors demand a wellsite specialist and a rigorous attempt to monitor tops, if formation depths cannot be accurately predicted.

A different type of subsurface stratigraphic surveillance, applicable only to air-drilling, has proved successful. Rather than attempting to monitor proved successful. Rather than attempting to monitor the rock characteristics of marker zones, the method monitors their fluid content. Since air compression is stopped and the well is opened to the atmosphere while drillpipe connections are made, drilling with air is like conducting a series of 30-ft formation tests. This fact, together with the fact that bottom-hole air pressures rarely reach 500 psi and bottoms-up times pressures rarely reach 500 psi and bottoms-up times last only a few minutes, makes it possible to observe the type and depth of fluids feeding in from penetrated formations. By comparing fluid-entry depths with logs, the fluid content of specific formations in an area can be determined. It follows that this information may provide a means of fixing the geologic position of a well by monitoring fluids that enter the wellbore during drilling. Thus, by comparing, the nature and distances between fluids entering the wellbore with known fluid-entry characteristics for the area, the subsurface stratigraphic position can be deduced. This technique is termed fluid-entry analysis.

The method has been applied by Continental Oil Co. on the thickly faulted Douglas Creek Arch in northwestern Colorado and has provided sufficiently accurate subsurface control to allow substantial savings in spite of otherwise unpredictable formation depths. In air drilling to the Mancos "B" gas reservoir, it became evident that specific up-hole formations contributed fluids to the wellbore in a fairly predictable manner. From this, a system was devised to predictable manner. From this, a system was devised to monitor subsurface position during drilling.

In planning a well, tops are predicted from the structure contour map and surface elevation. The well is drilled with air or mist, and depths at which water and gas occur, as well as increases in water and gas, are noted. Fluid-entry data are compared with known fluid-entry characteristics and the original predictions are revised as information is received. predictions are revised as information is received. After the adjusted total depth is reached and a flow test is taken, the well is logged. The only data immediately required are formation productivity and tops, so specially trained personnel are not required on location. The company drilling foreman reports this information and a completion decision is made.

Following are the results of using this procedure during the drilling of 40 wells:

By not drilling 2593 ft of useless hole in 9 wells that were structurally higher than predicted, $11,000 was saved. Ten wells were drilled deeper than originally planned and were stopped at the correct depth planned and were stopped at the correct depth without interrupting drilling, saving approximately $21,000. Only one log was required and it was not necessary to re-enter and deepen any well after logging. Specially trained personnel were required on location for only 17 man-days as compared with an estimated 100 man-days if conventional surveillance methods had been attempted. Using the method, casing, was not set in any well that was later found noncommercial.

Gas feed-in from the Mancos "B" alone might be sufficient to bottom the well properly however, should it be necessary to mud up before reaching the Mancos "B" because of water flow, or should the well not produce gas or flow less than expected at the predicted Mancos "B" depth, the lack of up-hole predicted Mancos "B" depth, the lack of up-hole fluid-entry data would dictate that drilling be halted and that a log be run to determine where the well is geologically. This could require a geologist or engineer at the wellsite and additional expense if it were necessary to re-enter the well and deepen after logging, or if the well had already been drilled too deep.

Fluid-entry analysis can be applied wherever two or more marker formations contribute fluids consistently over a large enough area to allow application of the method to a number of wells. The drilling fluid must be air or air mist and fluid feed-in should be of low enough pressure and volume to prevent having to mud up, at least until enough interval has been drilled to determine up-hole stratigraphy.

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