Contiuation exam 2009

Problem

The following well data are given:

Oil gravity:0.85

Gas gravity:0.6

Viscosity of separator oil:4 cP

Reservoir depth:3214 m

Reservoir temperature:70 C

Initial reservoir pressure:355 bar

Measured length of production tubing:3600 m

Inner diameter, of production tubing:110 millimeter

Expected future oil price: 2000 kr/Sm3

The expected oil price will be used internally in the company for all assessment of field economy.

The well produces without choking. The data following have been measured:

Time / 1/6-2006 / 1/6-2007 / 1/6-2008 / 1/6-2008
Oil production / 630 Sm3/d / 560 Sm3/d / 500 Sm3/d / 620 Sm3/d
Gas production / 1.2.105 Sm3/d / 1.1.105 Sm3/d / 0.9.105 Sm3/d / 1.2.105 Sm3/d
Separator pressure / 40 bar / 40 bar / 40 bar / 35 bar

(You may not find the above data contain all the information you would like to have. If so, explain/justify additional assumptions you may choose to do.)

Compressibility: c, is defined as

V: fluidvolume

dp: pressure change

dV: volume change

a) Estimate the productivity index

b) Derive the relation for “Exponential decline”. (This result is given in the Formulae).

c)The well is expected economical as long as the production exceeds 100 Sm3/d. How long may the well be expected to produce naturally against separator pressure 40 bar

d)Lowering the separator pressure may increase the economic productive life. Give your estimate and judgment of the effect of lowering the separator pressure to 30 bar.

e)Another alternative is to stimulate thewell. It is assumed that this may increase the productivity index by 50%.Give your estimate and judgment of how such stimulation, rather than changing separator pressure, may affect the economic productive life.

f)Estimate and compare income during the 2 first years: - after lowering the separator pressure; and - after stimulation.

g)It has also been proposed to reduce the separator pressure to1.5 bar. This may increase production life further, but may also have additional and negative effects. Identify and evaluate additional effect and give your recommendation to the management. (Do you recommend the proposal? Which additional effect may be expected? How, negative effects be taken care of.)

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