Previous Talks

Wednesday, December 12th, 2007

Modeling the Mechanical and Phase Change Stability of Wellbores Drilled in Gas Hydrates

Richard Birchwood - Schlumberger


Within the last five years, gas hydrates have attracted a great deal of attention from governments and the private sector because of their enormous potential as an energy source. Although estimates of reserves vary widely, some analysts believe that the amount of carbon stored in methane hydrates worldwide is comparable to the current global coal, oil and conventional gas reserves combined. However drilling and producing reservoirs containing gas hydrates is a technically challenging and potentially risky endeavor. Dissociation of gas hydrates during drilling can possibly result in the uncontrolled release of gas into the wellbore leading to blowouts. Overpressured gas may exist below the gas hydrate stability zone. Producing operations in and around gas hydrate zones can significantly reduce the strength of sediments leading to borehole failure, subsidence, casing collapse, and loss of support for surface installations. Predicting these risks requires the use of advanced geomechanical and thermal models.

In this presentation, we show how such models were devised, benchmarked, and applied in support of a recent expedition in the Gulf of Mexico organized by the Chevron Joint Industry Participation Program (JIP) Gas Hydrates Project with the co-sponsorship of the U.S. Department of Energy. During the expedition, several wells were drilled to shallow depths below the mudline at deepwater locations in Atwater Valley and Keathley Canyon. Methods were developed to predict the mechanical and phase change stability of boreholes drilled in soft sediments containing gas hydrates. A semi-analytical elastoplastic Mohr-Coloumb formulation was used to evaluate the mechanical stability of the wellbore. The formulation was benchmarked against a finite element code. Satisfactory agreement was obtained. However some discrepancies were observed, due to the fact that the two codes used slightly different formulations of the Mohr-Coulomb failure criterion. The robustness of the assumptions underlying the semi-analytical formulation will be discussed.

Models of mechanical failure and downhole temperature were constructed from seismic and log data for the wells in Atwater Valley and Keathley Canyon. Mechanical failure models were based on property correlations devised from triaxial test data. The formation thermal properties required for borehole temperature simulation were estimated using existing effective medium methods. The sensitivity of borehole temperature to various factors such as circulation rate, ROP, ocean current velocity, and the geothermal gradient was investigated. Mechanical and thermal model predictions were compared with LWD caliper, image, and temperature logs in three JIP boreholes. In all cases, the predictions made by the mechanical failure models were consistent with deformation features observed in image logs. An excellent match was obtained between modeled and measured downhole temperatures in Atwater Valley. However for reasons that remain unknown, temperatures observed in Keathley Canyon were generally lower than those predicted by the model.

Mechanisms of borehole instability in Atwater Valley were analyzed in great detail. Remarkable images of drilling induced fractures showed evidence of significant horizontal stress contrast less than 100 ft below the mudline. Time-lapse analysis of LWD caliper and gamma ray logs revealed that coarse-grained solids were falling into the BHA annulus from uphole causing packoffs. Sand influxes from shallow water flows and possibly creep may have been chiefly responsible for these problems. Measures for improved borehole stability in unconsolidated sediments will be discussed.

Speaker Biography

Richard Birchwood is a geomechanics specialist in the Schlumberger Data & Consulting Services Geomechanics Group. His current projects and research interests relate to wellbore stability, stress testing, sand production, acoustics, and gas hydrates.

Richard holds a B.Sc. in Engineering Mathematics from the University of London and M.S. and Ph.D. degrees in Mechanical Engineering from Cornell University. Prior to his Houston assignment in 2004, Richard was based in Caracas where he served as Schlumberger’s geomechanics specialist for Venezuela and Trinidad & Tobago from 2001. Before joining Schlumberger, Richard held faculty appointments in Mechanical Engineering at Binghamton University (1993-1995) and in Civil Engineering at The City University of New York (1995-2000).