[PDF] Special Applications of Drill-Stem Test Pressure Data

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Special Applications of Drill-Stem Test Pressure Data

JOHN P. DOLAN

MEMBER AIME

CHARLES A. EINARSEN

JUNIOR MEMBER AIME

GILMAN A. HILL

MEMBER AIME

PETROLEUM RESEARCH CORP.

DENVER, COLO.

T. P.4667

ABSTRACT

'} his paper discusses how the following formation characteristics can be determined mathematically from drill-stem test pressure charts: true formation pres sures, effective permeability of the entire section tested. well productivities, wellbore damage_ and possible de tection of barriers (faults, pinchouts, etc.).

This paper also presents

a practical method for im mediate determination of effective permeability and well bore damage from successful double shut-in pressure tests. A list of recommendations for improving the re liability of drill-stem test pressures is also presented.

INTRODUCTION

A drill-stem test is a temporary completion designed to sample the formation fluid and to establish the pos sibility of commercial production. Early pressure record ing devices were used merely to verify proper opera-. tion of the testing tool. Until recently the accuracy of the pressure gauges has been insufficient for any re liable quantitative use of the recorded pressures. In view of the need for more reliable formation evalua tion and as a result of the recent interest in explora tion work employing the concept of hydrodynamic en trapment,'",3" better pressure recording gauges are now in use.

These devices can record pressure within 1 per

cent above 1,000 psig and can detect differential pres sures as low as lh psig. In addition to formation pressure, several other res ervoir characteristics can be determined from DST charts; namely, well productivity, formation permeabil ity, wellbore damage, and the possible existence of bar- Original manuscript received in Society of' Petroleum office on May 1. 1957. Revised manuseript received Sept. 18, 1957. Paper presented at Third Annual Joint Meeting of Rocky MountaiL Petroleum Sections in Billings, Mont" May 23-24. 1957. lReferences given at eru-I of paper.

SPE 851-G

;HS riers l faults, pinchouts, changes in permeability, etc.). This paper presents a practical method to interpret DST pressure charts for formation pressures and many other reservoir properties, a method that has been de veloped in analyzing approximately 4,000 DST charts during the last five years.

The techniques used are a

composite of published articles on testing".'. together with well-known pressure bUIld-up analym methods.

THEORY

It has been shown'" that the following equation may be used for analysis of pressure build-up curves: q{L (to+O) )

P" -Pw = 162.6 kh log -0-' . . .(1

When this equation is applied to the curves obtamed in drill-stem testing, the assumptions and boundary con ditions are more nearly realized than in conventional flow and build-up tests on producing wells. Zak and Griffen' have recently discussed in detail the use of this equation in analyzing

DST charts.

One of the problems with DST curves is the lack of reservoir data for precise analysis. Therefore, it is neces sary to develop empirical rules and field methods for analyzing DST charts in quantity. For this reason, the empirical methods presented in this paper have been developed, and their derivation is found in Appendices A and B.

METHOD USED FOR ACCURATE READING OF

DRILL-STEM TEST CHARTS

In order to apply the. pressure build-up theory to DST charts, it is necessary to obtain a digitized expres ,ion for the pressure and time data recorded by the pressure gauge. These data may be either provided by the servicecompanv or interpolated bv projecting a

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photographic reproduction of the chart against a car tesian wall screen and converting the scale readings to pressure and time values based on the reported readings of key points.

The authors have used an optically linear opaque

projector and a standard cross-section-millimeter paper screen to tabulate intermediate pressure points between the key points normally reported on the charts.

Use of

this technique depends upon correct calibration of the gauges.

Comparing the measured mud pressure with mud

weight and the measured flowing pressure against recov ery weight are independent methods for checking gauge accuracy.

The variation between measured mud pres

sures and estimated mud pressures, which are calculated from mud weight and depth. usually check within 2 per cent, as shown in Fig. 1.

RESULTS

PRESSURE EXTRAPOLATION

Experience in plotting a large number of DST charts on semi-logarithmic paper has shown that a straight line is usually obtained when the indicated kh/ fL is greater than 10 md ft/cp. In the ranges of kh/fL less than 10 md ft/cp, curved plots are usual. Curved developments also occur when non-radial flow is present. Fig. 2, which is an example of the result that can be expected. illustrates the comparison of formation pressures ob tained by DST 44 days before completion and by ex tended pressure surveys. Fortunately in this case, two surveys were available for comparison. There are several causes of error in extrapolation to original pressure. Aside from a multiplicity of tool, packer, and gauge troubles, which can usually be identi fied, there is the problem of low-capacity (kh) forma tions. The production of even a small quantity of fluid is frequently enough to draw the formation pressure down, so that a prohibitively long shut-in time is nec essary to obtain a usable build-up curve. The initial shut-in pressure technique is used to minimize the effects of excessive fluid production. Entrapped mud pressure is bled off, presumably just enough to equalize the formation pressure, by opening the formation into a limited air chamber sealed off from the main drill pipe. This technique is very useful in medium-to-good perme ability formations, since a level formation pressure is quickly obtained. In the low-capacity (kh) formations, even the initial shut-in will fail to develop in reasonable .2' a. 5000
W- e:: en 4000
ff3 e:: 0.. 0 3000
2000

1000 2000 3000 4000 5000 6000

FIG. I-MEASURED MUD PRESSURES COMPARED

WITH ESTIMATED MIJD PRESSURES.

Ve)L. 210, 11)-.57

time. Mud leakage from the annulus also produces ab normal pressures. Again it is the low-capacity (kh) for mation which is susceptible to mud-leakage effect. The measurement of pressure in the low-capacity (kh) forma tion is a continuing problem that merits considerable attention. _

Closely related to low-capacity

(kh) is the question of proper shut-in time. Other things being equal, the error in extrapolation is proportional to the amount of log (t + (})/(} remaining at the end of the shut-in period.

Fig. 3 illustrates

the relationship between extrapolation error and shut-in times.

One of the greatest causes of

non-usable DST pressure charts is insufficient shut-in time relative to the flowing time and capacity (kh) of the formation.

The lower the capacity of the formation, the

longer the shut-in time must be to obtain an accurate extrapolated pressure.

EFFECTIVE PERMEABILITY

The effective formation permeability may also be

determined within limits from the DST chart by using the well-known methods·,'··l1,l2 for pressure build-up curves.

The use of an average production rate determ

ined from the total recovery divided by the flowing time is generally sufficient for use in the formula kh

162.6 .

(2) Unless the flowing curve is approximately straight, indicating constant production rate, Eq. 2 will not be strictly correct (see Appendix

A). Fortunately, accuracy

requirements on permeability are not strict, and the approximate value obtained from a DST is useful. Since the permeability so determined represents the average effective value for an entire drainage area, it may in fact be a better value than the permeability reconstructed from isolated core plugs from the section. In vugular and fractured porous zones, the effective permeability of the drainage area is all important and cannot be measured except by testing. __ __ __

CD OST, 44 DAYS BEFORE COMPLETION

(ZJ PRESSURE SURVEY, 24 DAYS AFTER COMPLETION () PRESSURE SURVEY, 99 DAYS AFTER COMPLETION -2800 90

PRESSURES CORRECTED TO A DATUM PLANE _ 80

----70 (j) /60 -----.--:-. --2750 _______ 01 40 . I ~ 20 10 <---.. -2700

FIG. 2-DST PRESSURE BUILD-UP CURVE COMPARED

WITH CONVENTIONAL PRESSURE BUILD-UP CURVES.

FLOW PERIOD (t) = I HOUR

SHUT-IN PERIOD (9)

100 70 50 30 20 108 6 4 3 2 1.5

FIG. 3-THE EFFECT OF SHUT-IN TIME ON THE

ACCURACY OF EXTRAPOLATED PRESSURE.

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FIELD METHOD FOR CALCULATING EFFECTIVE

PERMEABILITY

A practical field method for estimating effective

permeability is illustrated in Fig. 4.

It is necessary to

have a successful dual shut-in pressure test, in which the initial shut-in curve is nearly leveled out. The final shut-in pressure need only be developed to about three fourths of the way between the final flowing pressure and the initial shut-in pressure. The procedure is as follows: Extend the initial shut-in pressure curve until it inter sects the pressure ordinate where (t + ()/() = 1. Con nect this point with the final shut-in pressure point which has been plotted according to the value of (t + () / 0 from the open time (t) and shut-in time ((). Extend this line until it intersects the pressure ordinate where (t + ()/() = 10. Using this f::,P across one logarithmic cycle, calculate the effective permeability (kh/ft) accord ing to

Eq. 2.

As a specific example and referring to Fig. 4: DST: open 45 minutes. Shut in 15 minutes. Recovery: 540 ft water in 4l1z in. drill pipe. Sand thickness: 20 ft. Esti mated fluid viscosity:

1 cpo ISIP = 1,800 psig. FSIP

1,620 psig. Average production rate:

540 X .0142 X 1,440 D

q = 45 = 245 B/ , . 45 + 15 (1+0)/0= 15 =4.0

Connecting the ISIP with FSIP and extending this

line until it intersects the pressure ordinate where (f + 0)/0 = 10,

P," = 1,500 psig

p, -p", = 300 psig/cycle kh 162.6 (245) p. 300 k -= 6.65 mu/cp ft k 7 md ±.

133 md ft

cp 'I here fore, from the reported data we are able to c:alculate the effective permeability. This calculation can be performed at the weI! immediately after remov ing the chart from the testing tool.

PRODUCTIVITY INDEX AND WELLBORE DAMAGE

Productivity index and damage ratio can also be

determined from DST data. Two values of productivity 320

108 6 4 3 2

P s,

INITIAL SHUT -IN PRESSURE

r a. /' PRESSURE (F'ROM DST CHART) p 10

TRANSMISSIBILITY = = 162.6 iP

9 0' in a. uS 0:: en en w 0:: a.

FIG. 4---TECHNIQUE FOR FIELD INTERPRETATION OF

EFFECTIVE PERMEABILITY.

index are obtainable. The first comes from the flow curve and is determined by the amount of fluid recov ered, the length of flowing time, and the pressure differ ential between the flowing pressure and the true forma tion pressure. The second value of productivity index comes from an analysis of the final shut-in curve. The first value of productivity index is affected by any kind of well bore damage, because during the flow period the fluid recovered had to pass through the damaged zone.

The second value

of productivity index is nearly inde pendent of damage because essentially no flow takes place during the final shut-in time. The ratio between these two values of productivity index is therefore indicative of wellbore damage. This damage is commonly caused by a mud filtrate water block on the formation face or pressure loss across per forations in the anchor or across the testing tool.

FIELD METHOD rOR CALCULATING DAMAGE RATIO

Although more precise methods are described in the literature,"'''''' the damage ratio may be calculated at the well site immediately after the

DST chart is recov

ered by using the following empirical equation (see

Appendix

B):

D.R. = .183 p, -(3)

f::,P

Following the same field method for permeability

determination, a f::,P, across one logarithmic cycle is determined. The final flowing pressure, Pi' is obtained directly from the chart. Fig. 5 illustrates the procedure used.

As a specific example:

DST: Open 45 minutes. Shut-in

15 minutes. Recovery: 100 ft mud with show of oil.

ISIP = 2,780 psi. FSIP = 2,720 psi. FFP = 50 psi.

P", = 2,680 psi. p, p", = 100 psi.

. _ .183 (2,780 -50) _ .. D.I. -(2,780 _ 2,680)---5. The above calculation indicates that approxImately live times as much fluid would have been recoYered had no damage occurred, or if the tool had remained open a sufficient length of time to sample the formation more effectively.

Had no damage occurred or had the

tool been left open longer, the next influx of formation fluid into the testing tool would presumably have been oil. The operator, at this point, may desire to retest the formation to prevent the possibility of passing up a potentially productive zone.

By determining the forma

tion damage immediately after the test, the operator's evaluation of the productivity of the tested interval may be considerably different from that based on recovery alone and may warrant retesting the zone, a change in the drilling schedule, or a change in completion interval. 1 T i FINAL FlOWIN6 PRESSURE.

DAMAGE RATIO. 0 R

P. -R

183_'_'

"p

FIr.. ;.--TEf:I!NlQUE FOR FIELD INTI:RPRETATLOl\

OF DAiI! AGE RATIO.

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In addition to the above field methods, permeability and damage effect on DST charts can also be qualita tively evaluated by visual inspection. Fig. 6 shows the effect of permeability and damage on the actual appear ance of the flowing and shut-in curves. These charts are facsimiles based on earlier analyzer studies of the process.",15

The chief effect of damage is in reducing

recovery.

The shut-in rate is also faster where a damage

effect exists.

BARRIER DETECTION

In principle, the detection of changes in transmissi bility, kh/p., in the vicinity of the wellbore can be determined by study of the pressure build-up curves.' When formation conditions are favorable, DST charts may be analyzed to detect nearby barriers. For example, in the case of a linear barrier fault, the expected result is a break in the linearity of the plot on semi-logarith mic paper.

The early part of the curve will have a

slope, f':"P, in psi/cycle exactly one-half the f':"P of the latter part of the curve. In practice, an exact one-half ratio is not measurable. Fig. 7 shows an actual DST curve indicating the possibility of barrier interference. The ISIP appears reliable. The permeability measured on the latter part of the shut-in curve is in agreement with core data on the interval tested. The test recovered

1,600 ft of free oil while the off-set well was tested dry

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