[PDF] A Review of Drill-Stem Testing Techniques and Analysis

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FEATURE ARTICLE

A REVIEW of DRILL-STEM TESTING TECHNIQUES

and ANALYSIS

W. MARSHALL BLACK

JUNIOR MEMBER AIME

HUMBLE OIL & REFINING CO.

HOUSTON, TEX.

Abstract

The present techniques of using

the drill-stem test as a formation evaluation tool are discussed. The basic drill-stem test operation is di vided for discussion into three phases: planning the test, performing the test, and interpretation, both qualitative and quantitative.

The use of small

bottom chokes and large top chokes is suggested in order to permit quan titative interpretation for gas-oil ratio, productivity, and permeability. The importance of measuring chloride content on a suite of samples taken from a recovered column of salt wa ter is illustrated.

Introduction

A drill-stem test is a temporary

completion of the well. Drill-stem tests are usually made for one or both of the following reasons: ( 1) to determine the producible fluid content of a formation, and (2) to determine the ability of a formation to produce.

Drill-Stem Testing Methods

The drill-stem test, or temporary

completion, . can be made either in open hole or inside casing through perforations. A drill-stem testing pro gram can be planned for a well so that the tests will be made in accord ance with one of three general meth ods:

1. Test possibly productive inter

vals in open hole as the zones are penetrated; normally, this method is used in conjunction with coring and

Original manuscript received in Petroleum

Branch office on Sept. 15. 1955. Revised man

uscript received May 1'6. 1956. Paper pre sented at Formation Evaluation Symposium.

Oct. 27-28. 1955. Houston. Teo<.

JUNE,1956

SPE 589-G

ether-cuts may be used to detect hy drocarbon shows.

2. Test possibly productive inter

vals in open hole after drilling deep er or reaching total depth; normally, this method requires that a cement plug be set for each test, unless strad dle packer testing is employed. Side wall cores and logs are commonly used to detect the shows. 3.

Test possibly productive inter

vals through perforations after casing has been set; log and core data may be used in selecting the intervals.

Drill-stem testing is widely used to

confirm or prove the presence and! or the producibility of oil and gas that is detected by the other services. The testing program in a well can follow anyone of the methods of drill-stem testing outlined in the preceding sec tion; however, the method of testing cored shows as the prospective pays are penetrated is probably most wide ly used at present. Under this meth od, a test will usually be made after penetrating a few feet into the pros pective zone, and if the results are favorable, subsequent tests may be made in search for fluid contacts.

Testing programs during

the early phases of field development are as important as the coring and logging programs for delineation of the res ervoirs and for establishing or con firming the gas-oil and oil-water con tacts.

Trends in Drill-Stem Testing

Since the early days about three

fourths of all drill-stem tests have been performed in open hole prior to setting oil string casing. This pre dominance of open-hole testing defi nitely places drill-stem testing in the category of exploratory forma-tion evaluation. Presently, tests in side casing are about 91 per cent mechanically successful as compared with 81 per cent 10 years ago, and conventional open-hole, wall packer testing is mechanically successful about 87 per cent of the time as compared with 72 per cent 10 years ago.

The Drill-Stem Testing Tool

Modern drill-stem testing tools are

highly versatile and consequently are complex. The various components may be assembled in innumerable combinations, either to provide spe cial information or to provide for emergencies that may develop. The following paragraphs briefly outline the functions of the more common tool components.

The three basic mechanisms or

components of a drill-stem test tool are as follows: (1) the tester valve, (2) the by-pass valve, and (3) the packer. These three component mechanisms will be found in some form in any good drill-stem test tool.

The functions of each of the basic

components in the assembly are as shown below.

FUNCTIONS OF BASIC COMPONENTS OF TOOL

1. The Tester or Retaining Valve

o. To prevent drilling mud from entering empty drill pipe while funning in. b.

To aid in preventing drilling mud from enter

ing drill pipe while pulling out and, conversely, to aid in retaining formation liquid recovery in the drill pipe. c. To open the tool, permitting passage of forma· tion fluids into the empty drill pipe after the packer is set.

2. The By·Pass or Equalizing Valve

o.

To permit mud under hydrostatic pressure to

flow downward throug h the packer mandrel at the conclusion of the test into the hole below the packer.

This action equalizes the pressure above

and below the packer, making it easier to pull loose. b.

To provide additional area through which the

drilling mud can pass around the packer while running in and pulling out of the hole.

Note: The new "hydraulic testers" ore unitized

teater valves and by·pass valves; the respective functions of these are unchanged.

21 Downloaded from http://onepetro.org/JPT/article-pdf/8/06/21/2237729/spe-589-g.pdf by guest on 08 October 2023

3. The Packer

o. To bridge the hole at Q point immediately above (and also below on straddle tests) the zon,e to be tested, thus permitting this zone to be of hydrostatic mud pressure when the tool IS opened and isolating the zone from other forma tions.

Important auxiliary components of

the drill-stem test tool are as follows: the disk valve, the shut-in pressure valve or tool, the formation or bot tom choke, the anchor pipe, and the pressure recorders.

In addition to

these, a circulating valve, a safety joint, and sometimes a set of jars may be included in the test tool or in the drill pipe or tubing string.

FUNCTIONS OF AUXILIARY COMPONENTS IN

TYPICAL

TEST TOOL STRING

1. The Disk Valve

o. To aid in preventing drilling mud from enter ing the drill pipe while running in. b. To permit the packer to be set firmly and tester valve opened before the tool is finally opened by dropping a go-devil to rupture the disk valve (as used with certain tool assemblies).

2. The Shut-In Pressure Valve or Tool

o. To permit the test tool to be closed at the con clusion of the flow period with reduced likelihood of unseating the packer or letting pressure equal ize around the packer through the by-pass. b. To aid in preventing drilling mud from enter ing the drill pipe while pulling out and, converse ly, to aid in retaining the formation liquids recov· ered within the pipe.

3. The Formaton or Bottom Choke

a. To restrict the volume of formation fluids that flow through the drill pipe to the surface. b. To hold some backpressure under the packer, which reduces the hydrostatic load on the packer, and to reduce the amount of pressure drawdown in the formation. c. To allow quantitative drill-stem test interpreta tion.

4. The Anchor Pipe

a. To support the open-hole wall packer at the desired place in the bore hole. b. To aid in screening out cuttings or junk that might plug the choke or foul other tool com ponents.

5. The Pressure Recorders

a. To provide measurements of hydrostatic mud pressure, formation flowing pressures upstrea!," from the formation choke, and formation shut-In or bottom-hole pressure. These pressure measure ments are necessary for complete test interpreta tion and formation evaluation; therefore, the pres sure recorders are, in a sense, among the most important components of the tool. b. To provide a graphic record of the proper or improper functioning of the test tool.

6. The Circulating Valve

a. To permit test recoveries to be pumped out of the drill pipe by reverse circulation into a pit or tank. b. To provide a means of conditioning the mud in the annul us and thus make testing a safer op eration.

7. The Safety Joint

a. To provide a means of releasing the drill pipe and tool from a stuck packer or anchor.

8. The Jar

a. To increase the possibil ity of freeing (] stuck tool. (The jar used for this purpose is usually a special hydraulic tool designed to deliver impact blows.) b. To facilitate setting the tool for measuring for. motion shut-in pressure when a rotating shut-in pressure valve is not used. (The jar used for this purpose is a simple telescoping slip joint arrange ment.)

9. The Surface Control Head

a. To permit control of fluid flow from the drill pipe at the surface through means of valves and chokes.

Planning the Test

The Basic Decisions

Detailed consideration must be

given to a number of factors in plan ning a drill-stem test in order to in sure that the desired information will be obtained and to increase the prob ability of a mechanically successful test. Decisions must be made before hand on the following: ( 1) service company to be employed; (2) 22
amount of hole to test; (3) packer size or sizes; (4) location of packer seat; (5) top and bottom choke sizes; (6) probable length of flowing and shut-in period and use of dual shut-in periods; (7) type of pressure gauges, manner of placement in the tool, and optimum pressure capacity and clock speed; (8) use of, type, and location of circulating sub, safety joint, and jar; (9) use of water cush ion and amount; (10) method of handling test production at the sur face; and (11) special packer ar rangements.

Amount of Hole to Test

In most instances, a more conclu

sive test can be obtained by testing the shortest section practical.

In thin

sands, where it is desired to locate the gas-oil and oil-water contacts, a test zone of 2 to 5 or 10 ft is often used in open hole. Where producing zones of greater thickness are en countered, it may then be feasible to test more hole per test. This is par ticularly true in long limestone sec tions where the location of the porous zones may not be known, and it is usually desired to determine the over all fluid content and productivity of a certain interval. If the volume of the hole below the packer is too great, the drilling fluid may fill the pipe to such an extent that a low formation pressure will be insuffi cient to cause entry of any appre ciable quantity of formation fluids against the backpressure. Also, the source of water produced from a long interval is indefinite.

Selection of Packer Size

W'all Packers

The open-hole wall packer does

not enjoy the controlled conditions of usage of the hookwall packer; it is frequently required to seal off in plastic formations and in a hole whose diameter is known only ap proximately. Successful use of rub ber in wall packers requires that the stresses be kept low enough that the rubber will act entirely in the elastic or solid phase; that is, it must return to its original shape when the load is taken off. This must be done by keeping the clearance between the packer and the wall of the hole as small as practical, by keeping the axis of the packer parallel to and coincident with the axis of the hole, and by choosing the packer seat in the least plastic formation possible.

It is important to have a straight

true-to-gauge hole and a sufficiently heavy, rigid anchor pipe.

Rathole testing is employed

wh.::re a core hole, or hole of reduced diam eter, is drilled ahead for exploratory purposes. Successful use of conven tional double-end wall packers re quires a very close fit to the hole size. Because· of this, a reduction in hole size or rathole for the last 300 to 500 ft of hole, including the test zone, permits greater packer clear ance while running in and out in the full hole. Ratholing is largely confined to soft formation areas.

It has been found that the ratio

of hole size to packer size largely governs the amount of packer com pression that will occur at pressure differentials up to

5,000 psi and that

leakage or rupture of the rubber element will occur if the ratio of hole size to packer size is such that complete mandrel travel is attained.

A differential pressure of

5,000 psi

will produce complete compression when ratio of hole size to packer size approaches 1.25;

5,000 psi dif

ferential pressure will cause about

50 per cent compression when ratio

of hole size to packer size is about 1.08 or 1.10. In the commonly drilled hole sizes, the 1.08 ratio pro vides a reasonable balance between clearance in true-to-gauge sections of hole and the excess expansion available should the packer seat yield or be washed out.

Somewhat larger clearances can be

used with the new "expanding shoe" packers, and in areas where a re duced size hole or rathole need not be used, this type of wall packer has excellent application. These packers were developed inquotesdbs_dbs35.pdfusesText_40

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