It is critical to obtain the rock strength parameters along the wellbore. Rock strength logs are used to conduct different types of analysis such as preventing wellbore failure, deciding on completion design methods and controlling sand production. One source of data which is often overlooked in calculating rock strength is drilling data. To utilize the drilling data in calculating strength, correlations are developed from inverted rate of penetration models. From these models unconfined compressive rock strength can be calculated from drilling data. The rate of penetration models takes into account operational drilling parameters, bit types/designs and geological formation information. Results from various onshore and offshore fields verify that drilling based rock strength compares to other methods of estimating rock strength. The big advantage using drilling data is that rock strength can be calculated for all hole sections, less expensive onshore wells and from old wells, where electrical logs or preserved core samples do not exist.


It is critical to obtain the rock strength information along the wellbore. For instance, it is critical in obtaining the safe mud weight window to avoid wellbore instabilities and when planning the casing program. Sand produced during oil extraction is also to a great extent controlled by the compressive strength of the reservoir sandstone. Rock strength also controls the drilling rate of penetration (ROP) and bit wear, and is therefore important information to the drilling engineer during drilling operations. Information about the rock strength can be directly measured from rock mechanical tests, performed either at rig site or in the laboratory, or obtained indirectly from electrical log correlations. Well preserved core samples for conducting laboratory measurements are rare and logs are usually only available in the reservoir sections of the wells which will limit the availability of continuous rock strengths along the wellbore.

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