Experimental study on the effect of abnormal high pressure on the elastic properties of glutenite and a novel pressure prediction model: a case study from Triassic Formation in north slope of Mahu Sag, Junggar Basin

Several high-efficiency oil and gas reservoirs have been found in the Permian and Triassic glutenite reservoir in Mahu Sag, Junggar Basin. The exploration practice shows that abnormal high pressure is widely developed in the glutenite reservoir, especially in the slope area near the hydrocarbon generation center. Due to the existence of abnormal high pressure, the elastic parameters of glutenite reservoir in slope area of Mahu Sag exhibit some special properties. In this paper, we conduct a comprehensive study of the variation law of glutenite elastic parameters with abnormal high pressure based on the petrophysical tests of 63 rock samples, the measured formation pressure from more than 10 wells, and conventional logging and mud logging data in Mahu area. The results show that the glutenite in the slope area of Mahu depression is affected by the mineral composition and the abnormal high pressure, and its elastic characteristics are quite different from those of conventional sandstone reservoirs which are usually represented by the Casatgna sandstone line and the Li QingZhong sandstone line. Compared with the above two kinds of sandstones, the samples having the same densities in the study area have lower velocity. The main reason for the low velocity is that the development of abnormal high pressure hinders the cementation of rocks. In the abnormal high-pressure area, the cementation of samples is generally poor, and thus the velocity of samples is low. Moreover, the influence of this phenomenon on P-wave and S-wave velocity is different, and it has a greater influence on S-wave velocity. In order to quantitatively study the influence of abnormal high pressure on the P-wave and S-wave velocity in glutenite reservoir, a systematic petrophysical experiment is designed to measure the velocity of samples under the conditions of fixed formation pressures and fixed differential stresses. The influence mechanism of abnormal high pressure on the elastic properties of glutenite reservoir in slope area of Mahu Sag is then explained experimentally. In order to improve the accuracy of the formation pressure prediction in the study area by using seismic data, we summarize the variation laws of different elastic parameters with formation pressure and differential stress, and developed an improve method for calculating the effective stress coefficient. Then, a novel model is established to predict formation pressure for glutenite reservoir in Mahu area. Finally, the novel model is applied in the well M131 zone at the slope of Mahu Sag. From the comparison of the predicted and measured pressure coefficients, we found that the predicted results have a good agreement with the measured data at the drilling position. In addition, the predicted pressure coefficient distribution also conforms to the formation mechanism of abnormal high pressure in the slope area of Mahu depression.