The in-situ mechanical strengths of rocks are very difficult properties to determine or measure because of variation in stress states and rock anisotropies. On the other hand, drilling penetration rates in rocks which are related to some rock strengths can be measured with a higher degree of reliability in the field. From penetration rate and other drilling data, a rock's drilling strength may be determined and related to its mechanical properties. properties. Wireline electrical log data have been extensively used in reservoir characterization and sometimes give direct measurements of some elastic properties of the rocks. However, there is still properties of the rocks. However, there is still the need to evolve some universally acceptable rock strength determination method from wireline electric log data. This approach has the advantage of not requiring extensive drilling and laboratory data and also provides a realistic estimate of in-situ rock strength.
This paper discusses relationships between rock drilling strength and some wireline log properties and shows how a formation's drilling strength can be estimated from log data.
The use of sophisticated computer data acquisition systems and sensors in the drilling industry has enabled researchers to begin to quantify dynamic formation drilling strength or rock drillability with a higher degree of confidence than in the earlier years in the industry. This is of special significance in the use of drilling strength to optimize drilling operations. Extensive work by Warren and others show how rock strength (i.e., drilling strength) may be determined for roller cone bits and used in applications such as drilling simulation and bit selection. Drilling strength may be defined as the resistance to bit penetration during drilling. It is a function of penetration during drilling. It is a function of the rock properties.
Previous attempts have been made to correlate drillability, drilling penetration rate, rock mechanical properties with electric log data and elastic properties of rocks. Walker, et al., proposed some equations relating weight on bit, proposed some equations relating weight on bit, depth, in-situ compressive strength, porosity and average grain size to penetration rate for roller cone bits. But, the in-situ compressive strength used in their work requires the use of data such as weight on bit and angle of internal friction which come from drilling and rock mechanics tests, respectively.
Gestalder and Reynal related sonic velocity, Young's modulus, and specific disintegration to rock drillability by using Schreiner rock hardness tests. Results from their studies led them to suggest that it might be possible to predict rock drillability from sonic log data.
Elkington, et al. studied the relationships between the diametrical point load tests performed on different rocks and log properties and concluded that the neutron, gamma ray, and sonic logs have the greatest rock strength prediction potential of all the logs they analyzed.
Somerton, et al. extended the work by Gestalder and Reynal and proposed that a mineralogical factor should be used in correlating sonic velocities to rock drillability in carbonate rocks.