低渗砂岩储层岩石应力敏感实验与理论研究

Terzaghi有效应力是外应力减去内压的差值，因此，增加同样大小的围压和降低同样大小的内压其有效应力的变化大小是一样的。实验室通过改变外压来改变净应力，和实际开采过程中围压不变降低孔隙流体压力改变净应力，测定的孔渗随净应力变化的结果是一样的。改变外压测定孔渗随净应力变化，得到的结论是低渗岩心比中高渗岩心对净应力和有效应力更敏感，而孔隙度对它们均为弱敏感。通过引入双重有效应力理论，对开采过程中有效应力随净应力的变化关系进行了重新校正和评价。结果表明，低渗岩心虽比中高渗岩心对有效应力更敏感，在衰竭开采过程中，净应力变化虽然很大，有效应力的变化率非常小，导致低渗透砂岩储层渗透率和孔隙度基本不变。

EXPERIMENTAL AND THEORETICAL STUDY ON ROCK STRESS- SENSITIVITIES IN LOW PERMEABILITY SANDSTONE

Terzaghi effective stress equals the outer stress (the confining pressure) of rock minus its inner pore pressure. The value of effective stress by increasing the outer pressure is the same as that of decreasing the inner pore pressure of the same value according to his theory. Therefore, measurement of permeability and porosity stress-sensitivity can be done through increasing the outer stress in laboratory instead of decreasing its inner pore pressure. The results from the experiment show that the low permeability sandstone reservoirs have strong permeability stress-sensitivity and low porosity stress-sensitivity. Because the effective stress is equal to the net confining pressure, it reduces the permeability significantly as reservoir pressure decreases. The paper introduces the double effective stress theory. The effective stress is not equal to the net confining pressure in light of the theory. It equals the outer stress subtracts the product of the inner pore pressure and the porosity, resulting that the effective stress varies little as reservoir pressure decreases or the net confining pressure increases greatly. The increment of effective-stress is much smaller than that of the net confining pressure, it results in the change of permeability and porosity of low permeability sandstone reservoir are very small during during depletion production.