Keith W. Katahara

Piezoelectric transducer with displacement amplifier

A displacement amplifier interconnects a piezoelectric actuator with a displacement member and comprises respective bellows members of differential diameter defining first and second expansible chambers which are liquid filled and respond to linear displacement of a stack of piezoelectric disks to provide an amplified linear displacement to the displacement member. The transducer is particularly useful as a signal transmitting or receiving transducer for acoustic wave generation or measurement for wellbore logging tools.

Fluid substitution in laminated shaly sands

This paper describes a simple method for fluid substitution in sediments composed of alternating thin layers of shale and sand with thicknesses below the resolution of welllogging tools. Simplicity comes from using the fact that both density and compressional compliance (inverse of the stiffness) are linear functions of shale fraction. On a compliance-density crossplot, any laminated sand point lies on a straight line between the shale and sand endpoints. The method is robust because shale and sand endpoints are required to fall on well-behaved trend curves. Fluid substitution is insensitive to choice of the trend curves.

Magnetostrictive transducer for logging tool

An acoustic wellbore logging tool transducer formed of a composite plate member having at least one plate of a magnetostrictive material which responds to a magnetic field to elongate or contract to effect displacement of said plate member within a cavity formed in a housing section of the logging tool. The logging tool cavity may be exposed to wellbore fluid in direct contact with the plate member whereby acoustic pressure pulses may be delivered directly to the wellbore fluid by the transducer element. Spaced apart electromagnets are disposed in the housing section on opposite sides of the cavity for effecting controlled displacement of the plate member to generate acoustic waves in a liquid-filled wellbore.

Selecting which stress to use in velocity‐stress models for pressure estimation

Simple relations between shale velocity and vertical net stress (total vertical stress minus pore pressure) have been successfully used to estimate pore pressures. However questions have recently been raised as to whether there are better stress variables to use, such as mean stress in place of vertical stress, or a poroelastic effective stress instead of a simple net stress. When these questions are considered in a soil-mechanics context, there are no clear advantages to using something other than vertical net stress. The process of fitting the velocity-stress model to well data tends to force model errors to zero. For now, more rigorous models have little practical advantage over simple models because they introduce additional parameters that cannot be easily determined.

Effect of mineral Vp/Vs on rock Vp/Vs

Summary Effective medium modeling shows that the Vp/Vs ratio of the solid grains has significant effects on rock Vp/Vs ratios. For a given pore aspect ratio, Vp/Vs changes more with porosity for limestones than for sandstones, essentially because the mineral Vp/Vs ratio is higher for limestones than for sandstones. Adding dry pores causes Vp/Vs to decrease for both sandstones and limestones, but adding water-filled pores causes Vp/Vs to increase for sandstones, and to decrease for limestones for pore aspect ratios down to about 10 −2 .