[PDF] Status of Drill-Stem Testing Techniques and Analysis

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Status of Drill-Stem Testing Techniques and Analysis

Abstract

H. K. VAN POOLLEN

MEMBER AIME

This paper is a compilation of the latest drill-stem testing techniques.

New tools are described, together with present

day interpretation techniques, and their limitations are given. The importance of proper times for shut-in and flow periods are stressed. A number of suggestions for the future are given, and a rather complete list of references in the field of drill-stem testing has been supplied for use by interested readers.

Introduction

A drill-stem test

(DST) is a temporary completion. It consists of a combination of a packer arrangement and drill pipe or tubing. Valves are present in this arrange ment to open and close the tool, and pressure and temp erature recording devices are employed. Upon completion of the test, the entire arrangement is withdrawn from the well. The purpose of a DST is (1) to determine whether or not to complete the well and (2) to obtain reservoir or aquifer information for exploration applications. The DST will render a wealth of information -e.g., a sample of the fluid itself, the actual productivity index, the reservoir pressure, the theoretical productivity index and the amount of well bore damage. The fluid will show whether the well can be completed as an oil well, a condensate well or a gas well; or, it will show if the formation should be abandoned (for being a water producer or for being dry). Tests can be run on the sample to determine the hydrocarbons present, viscosity of the mixture, API gravity, paraffin content, pour point or gas-oil ratio. The water produced can be analyzed for salinity and electrolytes present to aid in stratigraphic correlation, and the resistivity of the water will aid the logging engineer in his evaluation of the electric logs. The DST further aids in determining gas-oil or oil-water con tacts, and the proximity of pinchouts or faults. Two different productivity indices (PI) can be obtained during the

DST -(1) the actual PI and (2) the theoreti

cal PI. Normally, the actual PI is lower than the theoretical PI. A high actual PI shown by a given well probably will be the deciding factor in favor of completing that well. If the actual PI is low, however, the well possibly should be abandoned.

In all probability, it should be abandoned

Original manuscript received in Society of Petroleum Engineers office Nov. 22. 1960. Revised manuscript received Feb. 23. 1961. Paper presented at 5PE Formation Evalu .. tion Symposium. Nov. 21·22. 1960. in Houston.

THE OHIO OIL CO.

LITTLETON, OHIO

if the theoretical PI is low; but if the theoretical PI is high, chances for stimulating the well are still good (either by removal of the skin surrounding the wellbore 2-4 or by further penetration of the productive zone) . The reservoir pressure aids reservoir engineers in their reserve calculations.

It is important in exploration studies

dealing with entrapment of oil under hydrodynamic con ditions.

Trends in Drill-Stem Testing

With more and more emphasis

on detailed interpretation, the trend in drill-stem testing has been toward: (1) the use of double closed-in pressure tests;* (2) more accurate pressure recording devices; (3) more accurate and detailed reading of pressure charts; (4) on-the-spot pressure eval uation of tests and calculations; (5) calculations by means of digital computers; and (6) new tools enabling up-hole testing, continuous testing and testing while drilling.

Basic Tools

Drill-stem testing can be divided into two main

cate gories -open-hole and hook-wall. Since the operating and mechanical principles of the two strings of tools are the same, only the open-hole string will be discussed. Trends in modern drill-stem testing have led to more versatile and, consequently, more complete tools.'-1O The various components that make up a test string can be as sembled in any number of combinations. Only the parts of the string considered most important for interpretation will be discussed (Fig. 1). Starting at the bottom of the string, the most commori testing tools and their main function are as follows: The blanked-off pressure recorder (outside gauge) pro vides a step-by-step graphic story of the drill-stem test. By comparison with other gauges in the string, it will reveal the proper function of the testing string. No flow of fluids from the formation passes by this gauge; consequently, all pressures are recorded directly from the annulus below the packer. The perforated anchor supports the testing string and keeps the packer seated in the well bore. The anchor is provided with small holes (approximately 3/16 in.) to allow passage of formation fluids and to screen debris that might otherwise plug the fluid passages in the tool.

2References given at end of paper.

air chambers originally were used for the determination of initial closed-in pressures, present-day practice trends toward the method 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 Downloaded from http://onepetro.org/JPT/article-pdf/13/04/333/2237597/spe-1647-g-pa.pdf by guest on 08 October 2023

The packer assembly provides a bridge in the well bore between the drilling fluid and the zone to be tested. The hydrostatic head is withheld from the formation by means of the packer. The flow-stream gauge (inside gauge.) also will give a concise picture of the testing operation. This pressure recorder is placed in the flow stream above the packer and below the tester valve. All pressure fluctuations must pass through the perforated anchor to reach this recorder. The tester valve prevents entry of drilling fluids into the empty drill pipe while the pipe is being run into the hole. It also retains a sample of the formation fluids recovered while pulling out of the hole. Several improvements on the tester valve have been offered the industry in recent years. The air-chamber gauge generally is run inside the air chamber on tests where an initial closed-in pressure is taken and provides a means of checking the air chamber for fluid entry prior to the opening of the tester valve. An auxiliary valve placed at a distance above the tester valve provides an air chamber into which compressed mud below the packer can expand when the tester valve is opened. This method allows the pressure beneath the packer to drop below reservoir pressure, followed by a rapid build-up of the formation pressure prior to any appreciable amount of production. Two types of auxiliary valves are available for the dual closed-in pressure procedure." One is the disc valve, which consists of a steel body with a fluid passage blanked-off by an aluminum disc. This type of valve is opened by dropping a steel bar to rupture the disc. Another valve is a rotation type which has a three-position sleeve. The valve is run into the hole in the closed position and is opened by rotation. Further rotation will again close the valve and provide a final closed-in pressure. Also available to the industry is another rotation-type auxiliary valve!"" This newer type of auxiliary valve offers two advantages over the other equipment.

It eliminates the

additional piece of equipment necessary to reverse out the recovery obtained on the DST, and the air-chamber is eliminated. This valve is opened to flow for a period just long enough to bleed-off the pressure below the packer.

DRILL PIPE -----i4-

AUXILIARY VALVE-------',* .. u.

ANDIOR ,

CIRCULATING VALVE

AIR CHAMBER GAUGE I

(OPTIONALl

TESTER VALVE AND --:.:>+.

BY -PASS VALVE

FLOW -STREAM GAUGE--i

OPTIONAL TOOLS

(JARS,SAFETY JOINT

AND ETC.)

PACKER ASSEMBLY ---,-:_

PERFORATED

I 1:;"

BLANKED-OFF GAUGE

.. J I I I I I I I I ACE J --F---- G __ B 0 __ H_ Fig. I-The main components of a typical drill-stem testing string. 334
Then it is shut in to record the initial closed-in pressure. The initial flow and shut-in periods also may be estab lished by means of a conventional tester valve. After the packer is set, the by-pass closes and the tester valve opens. This is the beginning of the initial flow period. To shut-in the well, the pipe is lifted enough to close the tester valve but not enough to unseat the packer. A special arrange ment keeps the by-pass from opening." Important optional components of a testing string are safety joints, jars, bottom-hole choke, reversing sub, etc. (Local conditions will govern location of the tools in the string and their application.)

Testing Without

Use of Anchor Pipe

It is frequently desired to test only a certain part of a formation.

In this case, two packer arrangements are used

in such a manner that they "straddle" the zone of interest.

The anchor pipe is blanked off.

The lower packer separates

the bottom of the hole from the formation, and the upper packer separates the formation from the annular space above the upper packer.

In straddle-packer testing, long

anchor pipes frequently are used, extending from the zone of interest to total depth. It is possible now to perform straddle tests without the use of anchor pipe.",5l The tool which replaces the anchor pipe consists of mechanical slips mounted on a wedge shaped body, a set of open-hole type drag springs, and a J-slot locking mechanism to hold the slips in the unset position while going into the hole.

To set the tool, the drill

stem is picked up slightly, rotated and lowered, releasing the J-slot locking mechanism and allowing the slips to expand. After the slips are set against the wall of the hole, they provide support for the drill-pipe weight that must be applied to set the packers and open the tester valve. To release the slips, the drill stem is picked up, which reposi tions the locking mechanism and readies the tool for re setting.

Testing Procedures

DST's can

he classified as (1) conventional, (2) double closed-in using air chambers and (3) double closed-in with a flow period preceding each shut-in.

Conventional DST

The conventional DST consists of four periods (Fig. 2) -(1) going in the hole, (2) flow into tool, (3) shut-in period, and (4) coming out of the hole. Detailed chart descriptions have been treated adequately in the litera ture.",I2,52 The increase in pressure during Period 1 is caused by the weight of the mud column. When the packer is set and the tester valve is opened, this weight is removed and replaced by the weight of the column in the drill pipe, being either air or a cushion. The increase in pressure during

Period 2 results from the increasing weight

of fluid flowing into the drill pipe.

At the end of this period, a valve is

closed at the tool and further increase in pressure results from the restoration of formation pressure. Next, the packer is released and mud pressure takes over, which de creases again during Period 4 when the tool is run out of the hole.

Double Closed-in DST Using Air Chamber

The principle of measuring the initial shut-in pressure is to produce a very small amount of fluid from the forma tion followed by a shut-in period long enough to allow for pressure equalization.

The pressure measured after this

shut-in period is the "true" reservoir pressure.

JOURNAL OF PETROLEUM TECHNOLOGY Downloaded from http://onepetro.org/JPT/article-pdf/13/04/333/2237597/spe-1647-g-pa.pdf by guest on 08 October 2023

1 0:: (J) (J) W 0:: a.. I -TIME-

2-Demonstration of important pm·ts of pressure charts

and water "ushion: (l) going into hole, (2) flow into tool, (3) shut-in period, and (4) coming out of hole. At first (and still at the present), an air chamber was incorporated in the conventional testing string.

The air

chamber is the volume inside the testing string between the tester valve and an auxiliary valve. Fig. 1 shows the rela tive location of the valves and the resulting pressure charts.

When the packer

is set and the tester valve is opened, the mud which was trapped below the packer is allowed to expand into the air chamber.

The flow will stop as soon

as the air chamber becomes filled with both the mud and the formation fluid.

The pressure will build up, and within

a short time it will be very close to the original reservoir pressure. After the initial shut-in pressure has been taken, the auxiliary valve is opened and the test continued as if it were a conventional DST. The air-chamber method of measuring initial shut-in pressure has limitations.

To obtain a correct initial shut-in

pressure representative of the reservoir pressure, the air chamber should be calculated precisely. A reasonably ac curate measurement of the borehole diameter is required. The compressibility of the mud should be known, and some idea should exist concerning the order of magnitude of the reservoir pressure. Also, the hole should be very clean. The bottom section of the hole may be partially filled with cavings or cuttings which will not carry the load of the testing string (false bottom), resulting in sliding of the packer and the air chamber's becoming relatively too small.

When using

an air chamber to obtain an initial shut-in pressure, it is wise to clean the hole thoroughly and to rely on the experience of the tester in any particular area. It next becomes important to evaluate the validity of measured initial shut-in pressure.

Fig. 3 illustrates a number

of examples of initial shut-in pressures obtained by means of an air chamber. Fig. 3a demonstrates a perfect measurement. A definite curvature in the beginning may be noticed, making it definitely differ ent from Fig. 3c. It demonstrates how the pressure below the packer was released and a build-up of reservoir pressure followed, ending with a flat portion. Fig. 3b is also a good measurement; however, it may be difficult to distinguish it from Fig. 3c because of the small amount of curvature. In this case, the air chamber was somewhat small for the prevailing conditions. Another reason may be the speed of the clock used in this test. To see the curvature, the initial

APRIL, 1961

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