A new log interpretation method is presented for determining mechanical properties of friable sands. Factors considered in the method include overburden weight, fluid pressure, and additional earth loads caused by local geological conditions. Log data needed for the interpretation include conventional sonic, density, induction, and open-hole gamma ray.

Introduction

Certain in-situ mechanical properties of friable sands must be measured to determine if such formations will remain stable under different wellbore conditions. Stability problems that can then be resolved include estimating (1) how fast oil or gas may be produced without casing a sand problem,1,2 (2) if there is sufficient cementation between adjoining sand grains to avoid a possible sand problem when water production occurs,1,3 and (3) the formation fracture pressure gradient.4 Specific mechanical properties that enable such estimates are sand strength, strength of cementation between grains, bulk compressibility, and Poisson's ratio. This paper presents techniques for calculating these properties.

The means for determining the in-situ mechanical properties of friable sands is through the use of well-log data. The accuracy of these properties determined from log data should be better than the properties determined from log data should be better than the properties determined by direct strength tests with core samples. Weakly cemented materials probably would disaggregate during the core recovery operation, and no strength of cementation would be detected in direct strength measurements. Relief of overburden load results in core expansion and subsequent breaking of weak bonds.

Logs used include conventional sonic, density, induction, and open-hole gamma ray. The conventional sonic log is used in preference to a full-wave sonic log for calculating the mechanical properties of friable sands. The full-wave log can provide both compression and shear-wave travel times if the formation grains do not slip by each other as the acoustic signals pass through. Many of the rock properties could then be calculated directly5,6 from such measurements. Grain slippage apparently occurs readily in many friable sands; the shear-wave signal is dampened severely, and it is difficult to record reliable data. Techniques are developed in this paper that make it possible to substitute conventional sonic-log data for full-wave log data for calculating the mechanical properties of friable sands.

Other methods have been reported1,4,7 for using log data to measure the mechanical properties incorporated all the following factors that can affect the mechanical properties:

  1. overburden weight gradients at different depths,

  2. other earth loads caused by local geological conditions,

  3. cementation,

  4. fluid saturation,

  5. fluid pressure,

  6. different bulk compressibilities, and

  7. the change in elastic modulus values 1 of friable sands at different load conditions. These factors are considered in the calculation technique reported here.

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