A Four-Pressure Profile Prediction Method for Horizontal Wells in Conglomerate Formations
Authors: Yan Mingfei , Jin Yan , Lu Yunhu , Lu Zongyu 
Published: August 2024

The Manan conglomerate reservoirs are widely distributed in the Xinjiang oilfield in western China. This reservoir is characterized by complex reservoir space and strong heterogeneity of pressure distribution, which leads to the frequent occurrence of wellhore leakage, collapse and other complex situations in the pre-drilling stage, bringing serious challenges to the drilling engineering. Therefore, exact prediction of its pressure profile is of great significance for the stability of the wellbore and the speed of drilling. This paper proposes a method for predicting pressure profiles in horizontal wells in conglomerate reservoirs based on the particle discrete element method. Firstly, a small number of borehole cores were utilized to carry out triaxial compression experiments to obtain the typical mechanical parameters of conglomerates and clarify the damage characteristics. On this basis, a discrete geometric model of conglomerate at the core scale is established to determine the rock strength of the conglomerate, taking into account the particle shape, size, and distribution of the conglomerate. Finally, combined with logging data, the four-pressure profile of horizontal wells in conglomerate reservoirs were calculated and compared with the results of actual drilling. The results show that this method can effectively determine the wellbore destabilization section of horizontal wells in conglomerate reservoirs, while reducing the quality sensitivity of logging data. The study provides a new means of pressure prediction for the development of heterogeneous conglomerate reservoirs, which can notably instruct adjustment of the drilling fluid density to ensure safe and rapid drilling.

1. INTRODUCTION

The geological conditions of the Mahu conglomerate reservoir in the Xinjiang oilfield in western China are extremely challenging. The window of safe density for drilling in the Upper Urho Formation is narrow, and the prediction of formation pressure is difficult. In the previous period, due to the prediction of formation pressure coefficient error of 21.1-57.8%, 11 wells out of 16 wells drilled were complicated, and the number of complications reached 35 times, mainly in the aspects of well leakage, well intrusion and wellbore instability. Due to the high pressure coefficient (1.44-1.74sg) of the conglomerate formation and the large geological regional variability, the conventional pressure prediction method based on sedimentary compaction theory cannot meet the accurate prediction of conglomerate formation pressure. Meanwhile, since the uncertainty of the main controlling factors of wellbore instability, the model based on the mechanics of continuous medium cannot predict the collapse, leakage and rupture pressure of discontinuous conglomerate formation with strong heterogeneity and random distribution of gravel. Therefore, in order to improve the drilling wellbore stability in conglomerate formations, it is necessary to propose a new method to accurately predict the pre-drilling four-pressure profile in conglomerate formations.