The overburden gradient, at a depth of 12,000 ft, is calculated to be 0.98 psi/f

Question

The overburden gradient, at a depth of 12,000 ft, is calculated to be 0.98 psi/ft...

The overburden gradient, at a depth of 12,000ft, is calculated to be 0.98psi/ft and the pore pressure at the same depth is 5600psi. The horizontal stresses assumed to be isotropic. Given the following data: a) Calculate the magnitude of horizontal stress over the three formations using Elastic model and assuming Biot's coefficient of 1.0. b) Estimate the fracture initiation pressure around a vertical wellbore, assuming the rock tensile strength is insignificant. c) In which formation, fracture propagation requires less pressure? d) What would be your comments regarding the propagation direction for a hydraulic fracture from this well?

Explanation / Answer

given

depth= 12000 ft

overburden gradient=0.98 psi/ft

pore pressure=5600 psi

a)For formation of A the stress is calculated by

Horizontal stress=(depth×gradient)×poisson's ratio×Biot's ratio

=(12,000×0.98)×0.38×1=4468.8psi

For formation B the stress is calculated as

Horizontal stress=(12,000*0.98*0.19*1)=2234.4psi

For formation C the stress is calculated

Horizontal stress=(12,000*0.98*0.33*1)=3880.8psi

b)The fracture initiation pressure around vertical wellbore=pore pressure*biot's coefficient=5600*1=5600psi

c)In formation B fracture propagation requires less pressure as the stress is also less at that formation.

d)The propagation direction of hydraulic fracture is not constant along a single direction and it occurs from horizontal to vertical.

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