Deepwater Gulf of Mexico Turbidites— Compaction Effects on Porosity and Permeability

Abstract

The deepwater Gulf of Mexico is now a major area of activity for the U.S. oil industry. Compaction causes particular concern because most prospective deepwater reservoirs are highly geo-pressured and many have limited aquifer support; water injection may also be problematic. Thus, economic development may require significant drawdowns with ensuing compaction and its attendant problems.To address some of the issues associated with compaction, we initiated a special core-analysis program to study compaction effects on turbidite sand porosity and permeability specifically. This program also addressed a number of subsidiary but no less important issues, such as sample characterization and qual ity, sample preparation, and test procedures. These issues are particularly pertinent, because Gulf of Mexico turbidites are generall y unconsolidated, loose sands, and are thus susceptible to a whole array of potentially serious core-disturbing processes.One key result of the special core analysis program is that turbidite compressibilities exhibit large variations in both magnitude and stress dependence. These variations correlate with creep response in the laboratory measurements. The data suggest creep may be important because observed creep relaxation rates, although slow on a laboratory time scale, are much faster than typical reservoir drawdown rates.The effects of compaction on permeability are significant. To eliminate complicating effects caused by fines movement, we made oil flow measurements at initial water saturation. The measurements indicate compaction reduces permeability four to five times more than porosity on a relative basis.

Viscous creep in room-dried unconsolidated Gulf of Mexico shale (I): Experimental results

“…Laboratory experiments have shown that such timedependent deformation is evident in a wide variety of sediments and sedimentary rocks (Cogan, 1976;Rongzun et al, 1987;Dudley et al, 1994;Ostermeier, 1995;among others). The mechanism of timedependent deformation in such materials has been attributed either to pore fluid expulsion under drained condition (Cogan, 1976;Karig, 1993;Hornby, 1998) or to pore pressure redistribution under undrained condition (Holzer et al, 1973;Jones and Wang, 1981).…”

Application of rate‐and‐state friction laws to creep compaction of unconsolidated sand under hydrostatic loading conditions

“…[2] Given that the deformation of most unconsolidated sands has been observed to depend on both state (contact area or porosity) and deformation rate, it is necessary to include these effects when constructing an appropriate constitutive law [Yale et al, 1993;Ostermeier, 1995;Chang et al, 1997;Hagin and Zoback, 2004]. A natural choice for such a law is the rateand-state variable friction law, since it already contains the necessary terms and was empirically derived from laboratory observations in a concise mathematical form [e.g., Dieterich, 1978Dieterich, , 1979 and has a physical basis in thermally activated creep at high-stress asperity contacts [Nakatani, 2001;Rice et al, 2001;Beeler, 2004;Nakatani and Scholz, 2004].…”

“…For compaction of quartz sand in friction experiments [e.g., Richardson and Marone, 1999], strain rate decays according to a power law function of porosity during a hold. In volumetric creep tests on unconsolidated sands from the upper terminal zone of the Wilmington field (and others from the Gulf of Mexico), strain rate decays according to a power law of time, under conditions of hydrostatic stress [Ostermeier, 1995;Chang et al, 1997;Hagin and Zoback, 2004].…”

Viscous deformation of unconsolidated reservoir sands—Part 1: Time‐dependent deformation, frequency dispersion, and attenuation

“…Ostermeier (1995) observed creep strain under constant hydrostatic loading conditions in saturated Gulf of Mexico turbidite sands and characterized his observations using a standard linear solid viscoelastic model. Dudley et al (1994) observed similar behavior in saturated Gulf of Mexico turbidite sands under uniaxial strain conditions and modeled the creep strain using a power law function of time.…”