Effect of exposure time on the compressive strength and formation damage of sandstone while drilling horizontal wells

Worldwide increase demand for oil and gas led to an evolution in drilling techniques that maximize oil and gas production such as inclined, multilateral, and long horizontal drilling. The interaction of the drilling fluid and mud filtration with the penetrated formation is very critical and very important to be evaluated especially in the case of horizontal drilling where the drilling operation may take more than 20 days. This study aims to assess the effect of exposure time on the changes of the unconfined compressive strength (UCS) while drilling horizontal sandstone formation using water-based drilling fluid. Modified high-pressure high-temperature (HPHT) filter press cell was used to soak Buff Berea sandstone cores with barite-weighted mud under 300 psi differential pressure and 200 °F temperature for different periods. Nuclear magnetic resonance (NMR) was used to assess the change of the porosity while the scratch test was conducted to evaluate the changes in the UCS. The obtained results showed that UCS decreased significantly with increasing exposure time. For instance, the UCS was reduced by 18% after 5 days. NMR results confirmed that there was a corresponding increase in formation damage as shown by a progressive decrease in the rock porosity and permeability with time. The exposure time was found to be a controlling factor for both reducing the rock UCS and porosity. The statistical analysis showed that there is an inverse linear relationship (with 0.92 coefficient of determination R²) between the UCS values vs the extended exposure time. Meanwhile, an approximate logarithmic relationship (with R² of 0.98) for estimating the reduction in rock porosity with the exposure time was investigated.