No abstract

Rotliegendes sandstones have been found to display a wide range of petrophysical and mechanical properties stress sensitivity behaviour. The extent of stress related changes has been found for permeability, electrical conductivity and Young's modulus to be related by the depositional facies. It has also been noted that the relationship between some rock properties which are thought to exist in situ can not be replicated unless measurements conducted at near in situ stress levels are employed. These results lead to the conclusion that the structure reflected by properties measured at low confining pressures is not representative of that which exists in situ.

There is a growing awareness among petroleum engineers of the value of rock mechanics based analytical approaches to wellbore stability and sand production problems. It is readily accepted that the key to successful engineering lies in the acquisition of reliable data. Geomechanical properties testing is often perceived, however, as being expensive. All to often only limited numbers of samples are tested. This paper demonstrates how porosity can be employed as a geomechanical index to enable rock mechanics materials properties to be estimated using general and field specific correlations. Such an approach can be used to extend the application of a limited number of calibration samples. General correlations exist between porosity and the uniaxial compressive strength and Young's modulus of sandstones and carbonates. Most of the work reported in the literature has concentrated on correlating porosity with absolute values of mechanical properties. Strength is, however, a stress sensitive property, increasing as confining stress increases. It is shown that porosity correlates with the constants required to construct theoretical and empirical failure criteria which describe such behaviour. Porosity also reflects other geomechanical characteristics of sedimentary rocks including failure mode, fracture intensity, drilling rate and perforation penetration. The utility of porosity as a geomechanical index enables existing core and log data to be utilised to extend the use of limited core material.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractCalcium carbonate is a commonly occurring oilfield scale which proves to be difficult to manage. Carbonate scale prediction can be improved by considering the kinetic chemistry in addition to the conventional thermodynamics based predictive techniques. To improve the performance of a predictive model, a thorough understanding of the physical influences on carbonate scale kinetics is required. The work presented in this paper includes new data, experiment technique improvements and preliminary investigations into the efficiency of scale inhibitors. The new data acquired from the experiments described in the paper has been incorporated into our existing kinetic scale prediction model and a combined thermodynamic and kinetic computer model has been developed. An example is given which demonstrates how the combined model can be used to predict oilfield downhole carbonate scale profile and study the impact of production parameter changes, such as increasing water cut.

This paper presents the results of a study into the potential for sand production from a Rotliegendes reservoir rock. The approach is purely mechanical in that debris will be produced from the rock when the principal stresses applied are greater than the yield strength. An analytical solution to the stress concentration around a hole in an hydrostatically loaded rock has been developed by Wilson and modified by Smart et al. This approach predicts the extent of a zone of yielded material around the hole. For application in non-hydrostatic stress conditions and to accommodate the geometry of a perforation adjacent to a borehole, finite element analysis is used to determine the extent of the yield zone as no analytical solution is possible. Correlations developed from laboratory mechanical testing of Rotliegendes samples are used to predict the area of the yield surface produced in yielded rock and the mass of debris generated from that surface. The debris created may act as a source of solids which may be produced with the reservoir fluids. The study reported here was limited to debris creation only.