10,323
edits
Changes
Jump to navigation
Jump to search
Line 113:
Line 113: − +
→Subnormally pressured reservoirs
==Subnormally pressured reservoirs==
==Subnormally pressured reservoirs==
Subnormal reservoir pressures, that is, pressures less than 0.43 psi per ft of depth, are very common.<ref name=pt03r12>Dickey, P. A., Cox, W. C., 1977, Oil and gas reservoirs with subnormal pressures: AAPG Bulletin, v. 61, n. 12, p. 2134–2142.</ref> The cause of abnormally low reservoir pressures is not well understood. If a reservoir containing either gas or oil is isolated and then subjected to uplift and erosion, the removal of overburden causes an elastic rebound of the solids and an increase in volume of the pores. The elastic dilation of sandstones is about 7 × 10<sup>–6</sup> volumes per psi. Water expands only 3 × 10<sup>–6</sup> volumes per psi, so that the pressure of the pore water in the aquifer and the enclosing shales will drop, possibly sucking some of the water out of the aquifer. Most low pressure reservoirs are in areas where there has been uplift and erosion since the sediments forming the reservoir were deposited and lithified.
Subnormal reservoir pressures, that is, pressures less than 0.43 psi per ft of depth, are very common.<ref name=pt03r12>Dickey, P. A., Cox, W. C., 1977, [http://archives.datapages.com/data/bulletns/1977-79/data/pg/0061/0012/2100/2134.htm Oil and gas reservoirs with subnormal pressures]: AAPG Bulletin, v. 61, n. 12, p. 2134–2142.</ref> The cause of abnormally low reservoir pressures is not well understood. If a reservoir containing either gas or oil is isolated and then subjected to uplift and erosion, the removal of overburden causes an elastic rebound of the solids and an increase in volume of the pores. The elastic dilation of sandstones is about 7 × 10<sup>–6</sup> volumes per psi. Water expands only 3 × 10<sup>–6</sup> volumes per psi, so that the pressure of the pore water in the aquifer and the enclosing shales will drop, possibly sucking some of the water out of the aquifer. Most low pressure reservoirs are in areas where there has been uplift and erosion since the sediments forming the reservoir were deposited and lithified.
===Drilling problems with subnormally pressured reservoirs===
===Drilling problems with subnormally pressured reservoirs===