Status of Drill-Stem Testing Techniques and Analysis H. K. VAN POOLLEN THE OHIO OIL CO. MEMBER AIME LITTLETON, OHIO Abstract if the theoretical PI is low; but if the theoretical PI is Downloaded from http://onepetro.org/JPT/article-pdf/13/04/333/2237597/spe-1647-g-pa.pdf by guest on 28 September 2021 high, chances for stimulating the well are still good (either This paper is a compilation of the latest drill-stem testing 2 4 by removal of the skin surrounding the wellbore - or by techniques. New tools are described, together with present­ further penetration of the productive zone) . day interpretation techniques, and their limitations are given. The importance of proper times for shut-in and The reservoir pressure aids reservoir engineers in their flow periods are stressed. A number of suggestions for the reserve calculations. It is important in exploration studies future are given, and a rather complete list of references dealing with entrapment of oil under hydrodynamic con­ in the field of drill-stem testing has been supplied for use ditions. by interested readers. Trends in Drill-Stem Testing With more and more emphasis on detailed interpretation, Introduction the trend in drill-stem testing has been toward: (1) the use A drill-stem test (DST) is a temporary completion. It of double closed-in pressure tests;* (2) more accurate consists of a combination of a packer arrangement and pressure recording devices; (3) more accurate and detailed drill pipe or tubing. Valves are present in this arrange­ reading of pressure charts; (4) on-the-spot pressure eval­ ment to open and close the tool, and pressure and temp­ uation of tests and calculations; (5) calculations by means erature recording devices are employed. Upon completion of digital computers; and (6) new tools enabling up-hole of the test, the entire arrangement is withdrawn from the testing, continuous testing and testing while drilling. well. The purpose of a DST is (1) to determine whether or Basic Tools not to complete the well and (2) to obtain reservoir or Drill-stem testing can be divided into two main cate­ aquifer information for exploration applications. The DST gories - open-hole and hook-wall. Since the operating and will render a wealth of information - e.g., a sample of mechanical principles of the two strings of tools are the the fluid itself, the actual productivity index, the reservoir same, only the open-hole string will be discussed. pressure, the theoretical productivity index and the amount Trends in modern drill-stem testing have led to more of well bore damage. versatile and, consequently, more complete tools.'-1O The The fluid will show whether the well can be completed various components that make up a test string can be as­ as an oil well, a condensate well or a gas well; or, it will sembled in any number of combinations. Only the parts of show if the formation should be abandoned (for being a the string considered most important for interpretation will water producer or for being dry). Tests can be run on the be discussed (Fig. 1). sample to determine the hydrocarbons present, viscosity Starting at the bottom of the string, the most commori of the mixture, API gravity, paraffin content, pour point testing tools and their main function are as follows: or gas-oil ratio. The water produced can be analyzed for salinity and electrolytes present to aid in stratigraphic The blanked-off pressure recorder (outside gauge) pro­ correlation, and the resistivity of the water will aid the vides a step-by-step graphic story of the drill-stem test. By logging engineer in his evaluation of the electric logs. The comparison with other gauges in the string, it will reveal DST further aids in determining gas-oil or oil-water con­ the proper function of the testing string. No flow of fluids tacts, and the proximity of pinchouts or faults. from the formation passes by this gauge; consequently, all pressures are recorded directly from the annulus below Two different productivity indices (PI) can be obtained the packer. during the DST - (1) the actual PI and (2) the theoreti­ cal PI. Normally, the actual PI is lower than the theoretical The perforated anchor supports the testing string and PI. A high actual PI shown by a given well probably will keeps the packer seated in the well bore. The anchor is be the deciding factor in favor of completing that well. provided with small holes (approximately 3/16 in.) to If the actual PI is low, however, the well possibly should allow passage of formation fluids and to screen debris that be abandoned. In all probability, it should be abandoned might otherwise plug the fluid passages in the tool. 2References given at end of paper. Original manuscript received in Society of Petroleum Engineers office ~'Where air chambers originally were used for the determination of Nov. 22. 1960. Revised manuscript received Feb. 23. 1961. Paper presented initial closed-in pressures, present-day practice trends toward the method at 5PE Formation Evalu.. tion Symposium. Nov. 21·22. 1960. in Houston. of initial flow followed by initial shut-in. APRIL, 1961 SPE 1647-G Reprinted from the Aprii. 1961. Is.ue of JOUR:--lAL OF PETROLEUM TECH:--lOLOGY 333 The packer assembly provides a bridge in the well bore Then it is shut in to record the initial closed-in pressure. between the drilling fluid and the zone to be tested. The The initial flow and shut-in periods also may be estab­ hydrostatic head is withheld from the formation by means lished by means of a conventional tester valve. After the of the packer. packer is set, the by-pass closes and the tester valve opens. The flow-stream gauge (inside gauge.) also will give a This is the beginning of the initial flow period. To shut-in concise picture of the testing operation. This pressure the well, the pipe is lifted enough to close the tester valve recorder is placed in the flow stream above the packer and but not enough to unseat the packer. A special arrange­ below the tester valve. All pressure fluctuations must pass ment keeps the by-pass from opening." through the perforated anchor to reach this recorder. Important optional components of a testing string are The tester valve prevents entry of drilling fluids into the safety joints, jars, bottom-hole choke, reversing sub, etc. empty drill pipe while the pipe is being run into the hole. (Local conditions will govern location of the tools in the It also retains a sample of the formation fluids recovered string and their application.) while pulling out of the hole. Several improvements on the tester valve have been offered the industry in recent Testing Without Use of Anchor Pipe years. The air-chamber gauge generally is run inside the air It is frequently desired to test only a certain part of a chamber on tests where an initial closed-in pressure is formation. In this case, two packer arrangements are used taken and provides a means of checking the air chamber in such a manner that they "straddle" the zone of interest. The anchor pipe is blanked off. The lower packer separates for fluid entry prior to the opening of the tester valve. Downloaded from http://onepetro.org/JPT/article-pdf/13/04/333/2237597/spe-1647-g-pa.pdf by guest on 28 September 2021 the bottom of the hole from the formation, and the upper An auxiliary valve placed at a distance above the tester packer separates the formation from the annular space valve provides an air chamber into which compressed mud above the upper packer. In straddle-packer testing, long below the packer can expand when the tester valve is anchor pipes frequently are used, extending from the zone opened. This method allows the pressure beneath the of interest to total depth. packer to drop below reservoir pressure, followed by a It is possible now to perform straddle tests without the rapid build-up of the formation pressure prior to any use of anchor pipe.",5l The tool which replaces the anchor appreciable amount of production. pipe consists of mechanical slips mounted on a wedge­ Two types of auxiliary valves are available for the dual shaped body, a set of open-hole type drag springs, and a closed-in pressure procedure." One is the disc valve, which J-slot locking mechanism to hold the slips in the unset consists of a steel body with a fluid passage blanked-off by position while going into the hole. To set the tool, the drill an aluminum disc. This type of valve is opened by dropping stem is picked up slightly, rotated and lowered, releasing a steel bar to rupture the disc. Another valve is a rotation the J-slot locking mechanism and allowing the slips to type which has a three-position sleeve. The valve is run expand. After the slips are set against the wall of the hole, into the hole in the closed position and is opened by they provide support for the drill-pipe weight that must rotation. Further rotation will again close the valve and be applied to set the packers and open the tester valve. To provide a final closed-in pressure. release the slips, the drill stem is picked up, which reposi­ Also available to the industry is another rotation-type tions the locking mechanism and readies the tool for re­ auxiliary valve!"" This newer type of auxiliary valve offers setting. two advantages over the other equipment. It eliminates the additional piece of equipment necessary to reverse out the Testing Procedures recovery obtained on the DST, and the air-chamber is DST's can he classified as (1) conventional, (2) double eliminated.