Previous Talks

Wednesday, October 8th, 2014

Using Sonic-Density Crossplots in Shales to Distinguish Fluid Expansion as a Pressure Mechanism

Keith Katahara, Hess Corporation

Abstract

In this talk I will discuss patterns in sonic-density crossplots that might arise from different overpressure mechanisms.  In particular, fluid-expansion mechanisms can give distinctly different patterns when clay content is considered as an additional variable.  For this talk, fluid expansion will be defined to be a process in which pore pressure increases with a corresponding decrease or unloading in effective stress.  I will restrict attention to non-source-rock shales, and I will assume that the solid matrix of the shales remains basically unchanged during fluid expansion.  (For overpressure caused by hydrocarbon generation, assume that water-phase overpressure is transmitted into the shale from a neighboring source formation, such as a coal, but that hydrocarbons do not enter the shales.)  I will discuss examples from the North Malay Basin and the Gulf of Mexico.
 

Speaker Biography

Keith Katahara is a Sr. Geophysical Advisor at Hess Corporation.  He earned a Ph.D. in geophysics from the University of Hawaii and then spent a few years on the research faculty there.  His industry experience before Hess includes stints at ARCO-Vastar-BP, at Spinnaker-Hydro-Statoil and at Devon.  He is interested in rock physics, quantitative seismic analysis, pore-pressure estimation, and related aspects of geomechanics.

Wednesday, May 9th, 2012

Fluid flow in salt and implications for subsalt pore pressure

Keith Katahara, Hess Corporation

Abstract

This talk will review selected public domain evidence for fluid flow in salt and discuss implications for pore pressure below salt. The evidence includes:
  1. Barker and Meeks (2003, SPE 84552) document cases where formation integrity tests (FITs) in salt are taken to pressures well above overburden. Superficially, this might be taken as evidence that fluids do not flow through salt, even at very high pressures. Of course during drilling, pressures above overburden are imposed only for times that are very short compared to geological times.
  2. Netzeband et al. (Marine Geology 230, pp. 249–273,2006) and Gradmann et al. (Marine and Petroleum Geology, v. 22, pp. 597-611, 2005) show seismic sections of the Messinian salt in the Mediterranean Sea offshore Israel. They see features that imply fluid flow through the salt.
  3. The mud log from Mississippi Canyon 860 (OCS-G 18301) #1 (Bob North). This well penetrated several thousand feet of salt. The mud log mentions oil encapsulated in the salt virtually from top to base. There are very few mentions of clastics, so it seems unlikely that the hydrocarbons have been entrained during salt movement or suturing. The oil seems to be of varying composition, implying multiple episodes of migration through salt.
  4. Schoenherr et al. (AAPG Bulletin, v. 91, pp. 1541-1557, 2007) see evidence for 2 different episodes of oil migration through the Ara salt in Oman. They cite work by Lewis and Holness, (1996, Equilibrium halite-H2O dihedral angles: High rock-salt permeability in the shallow crust?, Geology, 24, 431-434) showing that pore water forms connected pore networks in a certain pressure and temperature range that may occur in deeply buried salt. Once a connected pore-water network forms, oil or gas can invade the pore space if its pressure exceeds the water pressure by the capillary entry pressure. Salt becomes dilatant via microcracks when pore fluid pressure exceeds one of the principal stresses, and permeability then increases rapidly.
  5. Lux et al. (in Lux, Mikley, Wallner & Hardy, Basic and Applied Salt Mechanics: Proc. 6th Conf. Mech. Behavior of Salt, Taylor and Francis, London, 2007) describe and model both laboratory and field tests in which a cavity in salt is filled with brine and sealed. They find that brine infiltrates salt slowly through a network of microcracks. Significant flow may take several days, years or decades to initiate and the flow front may take years to move a few meters. This is consistent with the industry FIT tests mentioned above. Fluid may not infiltrate salt significantly on a drilling time scale, but it can flow through salt over geological time. Note that these experiments and models are at temperatures and pressures where brine should not spontaneously form a connected pore network.

Taken together the evidence indicates that fluids can migrate through salt. Brine migration occurs when pore pressure at base salt slightly exceeds the principal stress. Thus subsalt pore pressure cannot exceed overburden by very much over geologic time periods. 

This limit can be reached where the base of salt surface is concave down, because then the overlying sediments will not hydraulically fracture or leak allowing pore fluid to escape and pressure to bleed off before it reaches overburden. Alternatively, if sediment density exceeds salt density, overburden stress will increase towards the salt edge as salt thins. That could mean that fracture stress increases enough toward the salt edge to prevent pressure from bleeding off even if base salt is not concave down.
 

Speaker Biography

Keith Katahara is a Sr. Geophysical Advisor at Hess Corporation. He earned a Ph.D. in geophysics from the University of Hawaii and spent a few years on the research faculty there. His industry experience before Hess includes stints at ARCO-Vastar-BP, at Spinnaker-Hydro-StatoilHydro and at Devon. He specializes in rock physics, AVO analysis and pore-pressure estimation. He has also worked intermittently on some topics in geomechanics.

Wednesday, January 13th, 2010

Lateral earth stress and strain

Keith Katahara, Devon Energy Corporation

Abstract

A popular method for estimating in-situ horizontal stress assumes that the horizontal elastic strain is zero. This talk discusses whether and when this model might be useful. Clearly the model is not applicable in tectonically active areas where there is lateral extension, compression or wrenching. In young passively subsiding basins, sediments are mechanically compacted with little lateral strain, so the model might be expected to work there.
 
However, existing experimental data on uniaxial mechanical compaction show clearly that even if total horizontal strain is zero, the elastic strain is far from zero because there is an equal but opposite inelastic strain that balances it. The horizontal elastic strain is thus determined by the inelastic rock properties, so the elastic strain will generally be nonzero and non-uniform from one formation to another.
 
There is one geological situation in which elastic strains may tend towards zero. Consider a passive basin that is neither subsiding nor eroding. Over geologically long time periods, rocks may creep and thereby approach an equilibrium stress state. To the extent that such rock masses in the earth are thermodynamically closed systems, equilibrium occurs at minimum energy. I show here that the condition of zero lateral elastic strain minimizes the elastic strain energy. This could be a quasi-equilibrium condition in the earth if there is a relaxation mechanism that can reduce the elastic energy to a minimum without also relaxing shear stresses to zero. If such a mechanism exists, it might produce zero lateral elastic strain in strata that are tectonically quiescent.
 

Speaker Biography

Keith Katahara is a Senior Advisor at Devon Energy in Houston.  He earned a Ph.D. in geophysics from the University of Hawaii and spent a few years on the research faculty there.  His industry experience before Devon includes a stint at ARCO-Vastar-BP and another at Spinnaker-Hydro-StatoilHydro.  He specializes in rock physics, AVO analysis and pore-pressure estimation.  He worked on geomechanics, including in-situ stress profiling, while at ARCO in the mid 90's.  His interest in stress estimation was revived 2 years ago when he joined Devon where hydraulic fracturing is used extensively.