A Logging-Based Method for Predicting High-Pressure Brine Layers
Authors: Zhang Xin , Lu Yunhu , Li Hongda , Ma Qingyuan , Xie Chidong , Zhan Qing 
Published: September 2023

ABSTRACT
Identifying high-pressure brine formations in the Kumgremu Group of the Tarim Oilfield is a difficult part of the drilling project. In this paper, we proposed a new method to identify high-pressure brine formations based on logging data. First, the response characteristics of the acoustic time difference, natural gamma, and natural potential to the high-pressure brine layer are analyzed. The high-pressure brine layer is mainly developed in the mudstone and sandstone, located in the inter-salt or sub-salt. The response characteristics of acoustic time difference curve of high-pressure brine layer are obvious, increasing around several tens of us/ft; the natural potential curve abnormal fluctuation is generally around 10-100mv; the natural gamma becomes small. Then, the empirical formula for predicting the high-pressure brine layer is given by the trial algorithm. The calculation results show that when the formula fitting result is greater than 0.7, the Kumgremu Group develops a high-pressure saline layer. The prediction results were validated in several wells and showed a prediction accuracy of 90% or more.

INTRODUCTION
High-pressure brine layers are widely developed in the sandy mudstone and tuff between the deep salt formations in the Tarim Oilfield, which presents a great challenge to drilling engineering (Wenhua et al, 2012; Yin et al, 2012; Jian et al, 2017). Inaccurate prediction of high-pressure brine layers can lead to serious accidents such as overflows and even blowouts and significantly increase drilling costs (Xinming et al, 2017; Qun et al, 2021). And accurate high-pressure brine identification can provide risk prediction for drilling wells, helping field personnel to take effective measures against complex incidents such as overflows. Therefore, accurate identification of high-pressure brine layers is particularly important.
The distribution pattern of the high-pressure brine layer is difficult to determine. High-pressure brine layers may develop in the upper part of the salt layer, the inter-salt and sub-salt strata. Studies have shown that if a gas display is found in the lower part of the thick salt layer, it can be used as a detection marker for high pressure in the salt layer (Meshcheryakov et al, 1966). For example, the sign of drilling encounters with ultra-high pressure reservoirs in the Shebelinka gas field in Ukraine is the drilling of gas encounters within hard gypsum formations. There is a clear correlation between high-pressure brine formations and tectonic weaknesses that act as conduits for the upward flow of fluids from subsalt strata (Ziberman, 1972). The area of the high-pressure brine layer can be traced by predicting the thickness of the weak zone. In addition, high-pressure brine layers are usually associated with faults and tectonics and often occur in areas with the strongest tectonic movements (Ziberman et al, 2001). Vertical seismic profiling (VSP) techniques with drilling have made important advances in predicting high-pressure brine formations. However, the disadvantages of these techniques are high cost and high loss of drilling time (Leaney et al, 2004). Pre-drill seismic imaging technology is being used to detect high-pressure brine formations. This technology is less expensive and can be used to replace expensive VSP (Brown, 2009).