Experimental study of formation damage in horizontal wells

Abstract

The influence of overbalanced pressure and long mud circulation time on the formation damage during drilling operations were studied by using an innovative ultrasonic method to measure mud invasion depth. Experiments were conducted on Berea sandstone core samples in a constructed Leak-off apparatus. Regain permeabilities and skin factors were determined by damage data generated through leak-off experiments and mud invasion depths were measured by an ultrasonic method. The results showed a decrease of regain permeabilities and increase in skin factors with overbalanced pressure and mud contamination time. Whereas the mud invasion depth decreases with increase in overbalanced pressures till it reaches a critical pressure. Beyond that, invasion depth increases with overbalanced pressure. A strong physical relationship between overbalanced pressure and pore & particle size distribution is observed which affects the mud fines and filtrate invasion depth. It is found that the permeability damage is more dependent on the severity of the formation damage than invasion depth. It is recommended to drill the well under low overbalanced pressure with high rate of penetration to produce maximum regain permeabilities and minimum skin factors. An example of application of the results to horizontal wells is given at the end to illustrate how the method can be applied to horizontal wells.