Factors influencing pore-pressure prediction in complex carbonates based on effective medium theory

A calculation model based on effective medium theory has been developed for predicting elastic properties of dry carbonates with complex pore structures by integrating the Kuster-ToksC'z model with a differential method. All types of pores are simultaneously introduced to the composite during the differential iteration process according to the ratio of their volume fractions. Based on this model, the effects of pore structures on predicted pore-pressure in carbonates were analyzed. Calculation results indicate that cracks with low pore aspect ratios lead to pore-pressure overestimation which results in lost circulation and reservoir damage. However, moldic pores and vugs with high pore aspect ratios lead to pore-pressure underestimation which results in well kick and even blowout. The pore-pressure deviation due to cracks and moldic pores increases with an increase in porosity. For carbonates with complex pore structures, adopting conventional pore-pressure prediction methods and casing program designs will expose the well drilling engineering to high uncertainties. Velocity prediction models considering the influence of pore structure need to be built to improve the reliability and accuracy of pore-pressure prediction in carbonates.