An Investigation and Laboratory Study on Water Loss and Invasion Depth of Drilling Fluid in the API Standard and Comparison with Sand Bed

Currently, to measure drilling fluid loss, API standard test is used by filter paper with a pore opening of about 2.7 micron, which perform permeability reservoir role; this is usually different than the real well conditions and, in many cases test, the results are quite different from the real conditions. API standard test shows only the amount of fluid loss in a static state and cannot investigate fluid loss invasion depth. In this study, a sand bed sized with a mesh of 20-40 (0.29-0.51 mm) and pressed under a force of 1 kg is used as a filter paper replacement in order to measure drilling fluid loss in the real formation and identify fluid invasion depth. Then, different fluids are used and in comparison with filter paper a significant difference between fluid loss and its invasion depth in presence of sand bed is seen. Hence it can be concluded that fluid loss measured by filter paper does not identify reservoir specification; therefore, more practical methods such as sand bed filtration should be used to investigate this phenomenon. The function of some materials as fluid loss controller and formation damage prevention can also be evaluated by this method while they cannot be evaluated by API laboratory filtration test method.