The formation Consider a formation rock of volume equal to 150 cc with a porosity of 0.18. compressibility of this sample is 12X10 psi. You are asked to calculate the percent change in the pore volume and to calculate the new porosity if the fluid pressure in the rock is reduced from 10000 psia to 1000 psia. For your calculations use the approximate compressibility equation for small c: values b. use exact compressibility equation a.