Some Applications Of Differential Temperature Logging

The value of a temperature log plotting temperature versus depth, has long been recognized. For years cement tops have been logged with simple temperature devises making use of the large exothermic reaction that occurs during cement hydration. Cement top information is needed prior to perforating and completing a well for obvious reasons. Additional sophistication of the use of temperature gradient logs included a method of evaluating formation productivity. However, it is not the purpose of this paper to go into the theoritical uses of this log, but rather the practical applications of the log in the field. As the demand for more accurate recordings of temperature changes became apparent, the gradient temperature log was complimented by a differential curve. The first differential tool employed two sensing elements set a fixed distance apart, and balanced so that the reading from each sensor would cancel the other as long as both sensors were submerged in a fluid of the same temperature. When the leading sensor detected a change in temperature the electrical over balance would be recorded at the surface. This system was a good start in the right direction; however, a newer more accurate A Priori Differential Temperature system was developed.

The case histories that follow take advantage of the very sensitive A Priori Differential Temperature Logs, and they cover a variety of different well situations.

There is nothing new in the idea of differential measurements; however, the idea of applying a set of values to differences has many advantages over measuring absolute values which can be cumbersome and hard to read. The differential measurement is accomplished by determining the difference in temperature between two points rather than the absolute value of either one. The differential temperature tool has one sensor located in the bottom of the device which makes a measurement and compares this measurement with another measurement made with the same sensor slightly below the original measurement. This is done continuously by storing the original measurement with a capacitor memory system. The log produced from this system is actually a first derivative of the temperature gradient log.

The differential temperature log has proved itself to be a valuable tool in fracture treatment stimulation work from the Panhandle field north and east into the Anadarko Basin. Other areas are reporting success in the use of this type of log for better control of fracture work. Prior to the use of this log oil and gas wells were fracture treated with a predetermined volume of frac fluid and a predetermined injection rate with some flexibility depending on the surface treating pressure encountered. When the job was completed it was assumed that all of the zones of permeability were treated, that the frac fluid stayed in zone, and that if the well didn't circulate to surface that there was no channeling up. The only thing wrong has been that some wells respond profitably to fracture stimulation and others producing from the same interval, and many times the same subsea elevation, fail to pay out the cost of treatment. There are various possible reasons for poor results with the most obvious being to start with a dry reservoir. Other reasons include, poor cement bond with the related channeling up and/or down, and incomplete treatment of the exposed production interval. The fact that a well has all of the possible pay zones open through either perforations in set through wells or open hole completed wells doesn't guarantee good treatment throughout the zones.