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Much less attention has been paid to subnormally pressured reservoirs than to overpressured reservoirs. This is probably because there are fewer spectacular [[drilling problems]] associated with subnormal pressures and underpressures. However, problems exist that can be serious.
Much less attention has been paid to subnormally pressured reservoirs than to overpressured reservoirs. This is probably because there are fewer spectacular [[drilling problems]] associated with subnormal pressures and underpressures. However, problems exist that can be serious.
If the reservoir pressure is much lower than the pressure in the [[drilling fluid]], severe formation damage can occur. The drilling mud filtrate penetrates the reservoir, causing swelling and migration of clays, which may plug the pore throats. Even a little water in the hole can kill a low pressure producing gas well. The water is drawn into the pores by capillarity and ruins the [[relative permeability]] to gas. In the case of low pressure gas sandstone reservoirs, it is desirable to set casing at the top of the reservoir interval and drill with gas, salt water, or oil-based mud to minimize formation damage.
If the reservoir pressure is much lower than the pressure in the [[drilling fluid]], severe formation damage can occur. The drilling mud filtrate penetrates the reservoir, causing swelling and [[hydrocarbon migration|migration]] of clays, which may plug the pore throats. Even a little water in the hole can kill a low pressure producing gas well. The water is drawn into the pores by capillarity and ruins the [[relative permeability]] to gas. In the case of low pressure gas sandstone reservoirs, it is desirable to set casing at the top of the reservoir interval and drill with gas, salt water, or oil-based mud to minimize formation damage.
Also, if the gas reservoir has a low pressure, there may be no indication of gas on the mudlog. The logs of many abandoned [[dry hole]]s should be reexamined to look for bypassed gas zones.
Also, if the gas reservoir has a low pressure, there may be no indication of gas on the mudlog. The logs of many abandoned [[dry hole]]s should be reexamined to look for bypassed gas zones.