Models for prediction of in-situ rock behaviour often suffer from lack of relevant input data, especially rock strength data. Rock mechanical testing of cores provides only discrete data points. This paper presents a method which produces a continuous plot of uniaxial compressive strength. The method is based on correlating standard log parameters with laboratory measured rock strength parameters with laboratory measured rock strength data. When a correlation has been established, it can be applied to new wells in the same field, giving a strength plot by computer processing of the necessary logs.
The method has been applied to two wells in a North Sea field, resulting in a statistically significant correlation. Proper depth adjustments between core depth and log depth are essential. It is further in-portent that the lithologies of interest are represented among the core material.
Mechanical instabilities in sedimentary rocks are related to the yield or failure strength of the rock. The strength of the rock is an important factor in stability problems during drilling, such as caving or sloughing and plastic flow. Stability problems during drilling are becoming more common the number of deviated wells increase. As the deviation angle increases, the risk of caving or plastic flow increases. When producing hydrocarbons, particle (sand, chalk) production is a all known phenomena in poorly consolidated or soft formations.
Other properties, especially the in-situ stresses, are required to make proper stability analyses. The strength of the rock is however the primary factor controlling stability in normally stressed formations. Much effort has been put into determining the strength (or rather maximum flowrate) of poorly consolidated sand reservoirs. Proper knowledge of the strength enhances the Proper knowledge of the strength enhances the possibility of selecting the most favorable possibility of selecting the most favorable perforation intervals and estimating the maximum perforation intervals and estimating the maximum drawdown or flowrate before sanding occurs.
There is however no direct method available for determining in-situ strength of the rock. The strength is normally obtained by testing core samples. Even if large uncertainties my be introduced due to the changes used on the core (stress relief etc.), there is no better method available today. This does however only give discrete points, and there is a need for a continuous strength record. Many studies have been devoted to finding relations between the uniaxial compressive strength and the dynamic Young's modulus. Most relations of this kind are however based on content rocks, and it may be dangerous to extrapolate to the weaker rocks. Using dynamic elastic moduli also requires that the shear wave velocity is recorded. This is not included as a standard logging parameter today. In addition the shear wave can be difficult to register in poorly cemented formations. Anderson et al apply an empirical relation between Young's modulus and the uniaxial compressive strength, based on work by Deere and Miller, to North Sea chalk. It is however doubtful if a universal correlation exists which can be applied to all fields and all types of lithologies. Anderson et al therefore recommend "that this be adjusted on the basis of local knowledge, and laboratory measurements of the uniaxial compressive strength".