Wednesday, November 9th, 2011

The Impact of Supra-Sub Salt Geomechanics on Trap’s Integrity and Drilling

Selim S. Shaker, Geopressure Analysis Services

Abstract

In the Gulf of Mexico (GoM), geopressured Tertiary-Quaternary sediments are caused mainly by the compaction disequilibrium phenomenon.  The principal, intermediate, minimum stresses (stress cage) and lithology are substantially control this process.  Salt’s unique petrophysical properties contribute to considerable geomechanics modification of the prospective geological setting and consequently the trap’s pore pressure profile integrity.
The salt’s low density is responsible for retarding the principal stress (overburden) gradient below the salt and, conversely, enhancing it above.  The negligible permeability of salt creates a perfect seal.  The salt’s ductile nature, that creates variable structural forms, substantially dictates the path and magnitude of the subsurface stress cage. Consequently, it impacts the integrity of the oil and gas trap’s sealing and retention capacities. Moreover, drilling programs can be substantially altered due to the presence of salt masses.

Six newly developed supra and sub-salt geopressure models were established based on numerous bore-hole data drilled in the GoM.  In each case history, these models substantiate the premise that the interaction between salt and sediment is an essential part of risk assessment and drilling appraisal.
Prospects above the salt, such as swells, domes, canopies, etc. are characterized by:

• High pressure gradient associated with relatively large sealing capacity (Auger and Crosby fields)
• Drilling challenges (kicks, shallow water flow) due to the high principal stress.  Several casing points and high mud pressure gradient are needed to drill through this zone.  For example, Mackerel prospect (MC 619 #1) needed four casing shoes and mud weight increased from 9# to 14# to drill 6000’ above the salt.

On the other hand, prospects below the salt are characterized by:

• Large reserves trapped by less faulted, sealed sub salt swells i.e. Crosby field covers four blocks
• Weak to moderate sealing capacity due to the principal stress retardation. Mackerel prospect’s subsalt objectives were deemed wet
• Low mud density gradient needed to reach objective targets
• Drilling hurdles, such as loss of circulation and high torque, especially in the salt-sediment gouge zones at the emerging frontier areas (Jack and Atlantis prospects)

While a great deal is known about salt body delineation from geological and geophysical data, this study addresses salt related risk assessment from a geopressure standpoint.

Speaker Biography

Selim S. Shaker is Director of and Consulting Geologist for Geopressure Analysis Services Inc. (G.A.S). He received a B.Sc. (honor), M.Sc and Ph.D.in Geology and also received a diploma in Hydrogeology from Prague University (UNESCO).

With over 35 years in the oil industry, he started his career in Egypt as well-site, stratigrapher and structural geologist. During his last 30 years of service his main function as Exploration Geologist was prospects generation in onshore and offshore the Gulf of Mexico. Internationally, he also evaluated several exploration projects in NW Australia, Libya, Algeria, N. Sea and China. Dr. Shaker established G.A.S. to focus on evaluating the implication of geopressure compartmentalization, seal integrity and salt interaction on leads and prospects worldwide. Pre and Post drilling risk assessment of a prospect is his specialty.

He is an active member of AAPG, SEG, HGS, GSH, and AADE. Dr. Shaker is the Co-Chair of the AAPG Deep Water Workshop. He has published over 40 papers and articles regarding geopressure, risk assessments and drilling.