Optimization of Drawdown Pressure About the Ultra-Deep Fractured Carbonate Reservoir

Shuang-gui, LI (Northwest Oilfield Branch Engineering Technology Research Institute) | Yu, Chen (Beijing Jinhui Botai Technology Co. Ltd)

OnePetro 

Abstract

The original stress equilibrium of the formation is disturbed after borehole creation, and stress concentration is formed around the wellbore. During production, if drawdown pressure is too high, it will cause wellbore collapse and associated accidents. On the contrary, if the drawdown is too small, the production cannot meet the development needs. The ultra-deep fractured carbonate formation is tight and the in-situ stress is high. Dependent on the development of horizontal fractures and the degree of fracture filling and cementation, the distribution of UCS along the wellbore is quite different, leading to a larger variation of critical drawdown pressure. Meanwhile, different wellbore trajectories will result in different stress concertation around the wellbore and influence the critical drawdown pressure. Furthermore, UCS of the rock will be reduced when the rock is soaked in completion fluids for a long time, and the critical drawdown pressure will be reduced. In view of these problems, this paper carried out a series of core tests, and developed a model to predict the critical drawdown pressure of uncased wellbores in fractured carbonate reservoirs. The influences of completion-fluid immersion and well trajectory on critical drawdown pressure is investigated. The results show that the critical drawdown pressure of vertical wells is less than that of horizontal wells. The critical drawdown of wellbores along the minimum horizontal principal stress is larger compared with wellbores along the direction of maximum horizontal stress. Completion-fluid immersion can cause a 4-5% reduction in critical drawdown pressure.

Introduction

The original stress balance is broken after drilling, and there is a stress concentration around the wellbore. When the rock strength of the borehole wall is exceeded, wellbore instability occurs. After drilling horizontally in ultra-deep carbonate reservoirs, open-hole completions are usually used. Due to the existence of a large number of low-angle natural fractures, the distribution of rock strength is uneven. Under the action of the ground stress, if the drawdown pressure is set too high, the wellbore will collapse, and the wellbore rock will clog the tubing after falling off, result in a substantial decline in output, increase operating costs; and set too low will lead to the results of production can not meet the development needs. At present, most of the studies on drawdown pressure are for sandstone reservoirs, and there is little research on the extreme drawdown pressure of ultra-deep carbonate reservoirs. Due to the large difference in physical properties between ultra-deep carbonate reservoirs and sandstone reservoirs, the study in this paper is of great significance to the rational development of the same type of carbonate reservoirs and the design of drawdown pressure.