A method for predicting the pressure of unloading-type abnormal pressure formation

Ledong area in the South China Sea is a typical unloading-type high pressure area, where the conventional formation pressure prediction method is poorly applicable and on-site drilling operation faces the risks of well kick and well blowout. In order to establish a formation pressure prediction model applicable to Ledong area, this paper carried out laboratory acoustic testing experiments on artificial cores and established the functional relations between acoustic velocity and effective stress, lithology and shale content. It is experimentally indicated that acoustic velocity is in an exponential relation with effective stress and in a linear relation with the square root of porosity and shale content. Then, the modified Bowers formation pressure prediction model was established according to the linear superposition principle. The original model can be only used to predict the pressure of mud shale formations, but the modified Bowers model breaks this restriction and can be also used to predict the pressure of sandy mudstone formations. In addition, the modified Bowers model overcomes the theoretical defect of the unloading equation in the original model that the acoustic velocity approaches to the wave velocity of sea water when the effective stress approaches to zero infinitely. The field application shows that the average error of the prediction result in Ledong area by modified Bowers formation pressure prediction model is only 2.6%, indicating its good applicability in Ledong area.