Formation Evaluation in Pressure-Depleted Reservoirs

SORENSON, RAYMOND P., Anadarko Petroleum Corporation, Oklahoma City, OK

Wells drilled within the Mid-Continent frequently penetrate reservoirs that have undergone significant pressure loss due to offset production. Recognition of this pressure depletion, through wireline and mud-log interpretation, is crucial to proper completion (or plugging) decisions and evaluation of reservoir continuity between wells. Reservoirs exhibiting symptoms of low pressure can still be capable of commercial production but sometimes require alteration of standard drilling and completion procedures.

Locally excessive hydrostatic overbalance causes penetration rates to slow significantly in a depleted zone, with expected drilling breaks absent or reversed. Slow drilling in turn causes mud-log gas increases, which are a function of hydrocarbon-bearing rock volume drilled per unit of time, to be small or absent. The depth of origin of cuttings is commonly misinterpreted, and objective sandstones are frequently logged as absent due to lack of a normal drilling break. Chromatographic composition of gas readings remains essentially unchanged,

allowing evaluation of fluid types and contacts, although the small gas concentrations can strain instrument resolution limits. Lost circulation due to excessive overbalance is a common problem in exhausted reservoirs, resulting in drilling difficulties, loss of cutting and mud-log gas data, and subsequent completion problems. Unlike the mud-log data, which routinely paint a dismal picture of reservoir potential, wireline log effects are less consistent and can generate unwarranted optimism. Low-pressure zones are commonly characterized by excessive mud cake, an exaggerated neutron-density gas effect because of the low fluid density, and an altered resistivity invasion profile.

AAPG Search and Discovery Article #91008©1991 AAPG Mid-Continent Section Meeting, Kansas Geological Society, Wichita Kansas, September 22-24, 1991 (2009)