Page 26 - A Practical Companion to Reservoir Stimulation

RESERVOIR AND WELL CONSIDERATIONS

EXAMPLE A-8
Figure A- I0 contains the total pressure drop from the
Pressure Drop Across Damaged Zone
reservoir and into the well and the ratio A~J,~,,~/A~,~,,~,. For skin
effects larger than 10, which is a value encountered often in
It is often confusing as to what portion of the total pressure pressure transient analysis, the ratio is approximately 60% or
drop (from the reservoir proper and across the sandface) is more. This calculation indicates the importance of skin re-
caused by the near-wellbore zone. This is the zone character- moval.
ized by the total skin effect. Calculate the total Ap and the ratio This calculation also demonstrates the scope of pressure
Ap,km/Ap for a range of skin effects using the well and transient measurements. While Eq. 1-67 is for steady-state
reservoir data in Example A-5. Use the steady-state relation- conditions, Eq.1-39 is for transient flow. As can be seen, the
ship for a 40-acre spacing. Assume a rate equal to 200 STB/d. only difference is the replacement of the term In rJr,, by pn, the

Solution (Ref. Sections 1-3 and 1-3.1) latter.being invariably smaller. The Ap,~r,r remains constant as
The total pressure drop is given by Eq. 1-67, whereas the ratio suggested by Van Everdingen and Hurst (1949). Thus, mea-
surements of fractions of pounds per square inch are cur-
APddP is rently used to detect pressure transients, while the total pres-
sure may be already penalized by hundreds of pounds per
___- S (A-21) square inch as a result of the near-wellbore condition.
AP\l,ll -
AProrul h(re/G 1 + s

3000

2500 1 .o

2000 0.8

I='
v)
Q 55
Y
5 1500 0.6 3
P
Q aa
a
1000 0.4

500 0.2

0 10 20 30 40 50
Skin Effect

Figure A-10-Total pressure drop and ratio of pressure resulting from skin to the total pressure drop for Example A-8.

A- I5