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Downhole Stiffness Measurement using Ultrasonic Transducer or Multifinger Style Caliper on Wireline Logging or While Drilling

IP.com Disclosure Number: IPCOM000246973D
Publication Date: 2016-Jul-19
Document File: 2 page(s) / 16K

Publishing Venue

The IP.com Prior Art Database

Abstract

A method for estimating the continuous stiffness of a rock formation is disclosed. A tool that can accurately determine the ID of the wellbore is conveyed down hole. One such embodiment is with an ultrasonic transducer and a pressure gauge is centrally conveyed into a wellbore in the formation. The caliper tool is ran into the wellbore on the first pass at hydrostatic pressure and the reflected ultrasonic signal captured to establish the baseline strain at initial pressure (0 psi), thereafter in subsequent passes the wellbore is pressurized from surface and more passes are made at higher differential pressure (e.g. 250 psi, 500 psi, e.t.c.) and the ID change is measured. The relative rock stiffness profile of the formation is then calculated from the known pressure (stress) and the change in distance (strain) to obtain continuous measurements of stiffness throughout the wellbore. Ultrasonic acoustic transducer uses a common methodology throughout the industry and relies on a rotating transducer which operates in "pulse echo" mode to measure a reflected ultrasonic signal of the borehole wall. This methodology is excellent for detailed borehole shape deformation analysis since it can acquire both amplitude and a travel-time. Amplitude and travel time can provide information regarding formation changes due to changes in stress (pressure) at the borehole walls. In order to estimate the strain we need to know the distance to the borehole wall, to compute the distance to the borehole wall, the borehole fluid velocity must be known and since the borehole fluid velocity can be continually measured with a separate pulse-echo transducer with a known spacing to a reflector (e.g. using CBIL tool), therefore the distance can be directly calculated from the travel time because the signals travels at a known velocity since the travel time (tt) uses the absolute time of arrival at the ultrasonic transducer. The borehole fluid velocity, cf, is found by: 𝑐𝑓=2π‘‘π‘‘π‘Ž where: d is the spacing between the transducer and reflector ta is the arrival time of the pulse echo signal. The distance, d, to the borehole wall is then found by: 𝑑=𝑐𝑓𝑑𝑑2 where: tt is the arrival time of the transducer. Another method to measure the inside diameter of the wellbore would be with a multifinger style caliper device that can accurately measure the ID changes in a wellbore. With this embodiment the calipers extend against the formation and the change in distance from the tool body to the formation is accurately measured. This will show the change in strain at various pressure which is related to the rock stiffness. Since the strain range in the elastic region to measure rock stiffness is >0.05in. and the detection capabilities of the ultrasonic transducer can be as small as 0.005 in, we should be able to capture the strain required.

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Title:Β  Downhole Stiffness Measurement using Ultrasonic Transducer or Multifinger Style Caliper on Wireline Logging or While Drilling

Abstract:Β  Β A method for estimating the continuous stiffness of a rock formation is disclosed. A tool that can accurately determine the ID of the wellbore is conveyed down hole. One such embodiment is with an ultrasonic transducer and a pressure gauge is centrally conveyed into a wellbore in the formation. The caliper tool is ran into the wellbore on the first pass at hydrostatic pressure and the reflected ultrasonic signal captured to establish the baseline strain at initial pressure (0 psi), thereafter in subsequent passes the wellbore is pressurized from surface and more passes are made at higher differential pressure (e.g. 250 psi, 500 psi, e.t.c.) and the ID change is measured. The relative rock stiffness profile of the formation is then calculated from the known pressure (stress) and the change in distance (strain) to obtain continuous measurements of stiffness throughout the wellbore.

Ultrasonic acoustic transducer uses a common methodology throughout the industry and relies on a rotating transducer which operates in β€œpulse echo” mode to measure a reflected ultrasonic signal of the borehole wall. This methodology is excellent for detailed borehole shape deformation analysis since it can acquire both amplitude and a travel-time. Amplitude and travel time can provide information regarding formation changes due to changes in stress (pressure) at the borehole walls.

In order to estimate the strain we need to know the distance to the borehole wall, to compute the distance to the borehole wall, the borehole fluid velocity must be known and since the borehole fluid velocity can be continually measured with a separate pulse-echo transducer with a known spacing to a reflector (e.g. using CBIL tool), therefore the distance can be directly calculated from the travel time because the signals travels at a known velocity since the travel time (tt) uses the absolute time of arrival at the ultrasonic transducer.

The borehole fluid velocity, cf, is found by: 𝑐𝑓=2𝑑𝑡𝑎

where: d is the spacing between the transducer and reflector

ta is the arrival time of the pulse echo signal.

The distance, d, to the borehole wall is then found by:

𝑑=𝑐𝑓𝑡𝑡2 where: tt is the arrival time of the transducer.

Another method to measure the inside diameter of the wellbore would be with a multifinger style caliper device that can accurately measure the ID changes in a wellbore. With this embodiment the calipers extend against the formation and the change in distance from the tool body to the formation is accurately measured. This will show the change in strain at various pressure which is related to the rock stiffness.

Since the strain range in the elastic region to measure rock stiffness is >0.05in. and the detection capa...