Perched water, the concept and its effects on exploration and field development plans in sandstone, and carbonate reservoirs

Abstract

Perched-water conditions have been observed infrequently in oil and gas fields in zones above fine-grained zones that may be located a few to 100 meters above the oil water contacts. Although infrequent in scale of number of fields globally, perched-water zones are important to consider when evaluating reserves and planning field development. The study described in this paper was conducted to examine the perched-water conditions and quantitatively evaluate factors that control perching behavior and its impact on field development plan. The differences between perch-water and hydrodynamically-tilted contacts have to be understood and clearly identified in a given reservoir by studying the geological controls. The geological controls that shape contacts either perched or hydrodynamically tilted do play vital role in field development plans because of their structural differences leading to pressure communication or lack of it. A perched-water zone in the subsurface is defined as a water-saturated zone that is above or not directly connected to the regional oil water contact. Perching phenomena may occur in a permeable layer overlaying a relatively impermeable layer. A perched-water zone develops when saturated conditions above a low permeability layer are needed to move infiltrating water vertically through this layer. Perched water is mainly controlled by geological structure as well as changes in geological facies. Identification and differentiation of perched water contact from hydrodynamically-tilted one requires the understanding of geological settings from seismic data, trapping mechanism and pressure data analysis from RFT/MDT pressure measurements. Perched water contacts for sandstone reservoirs from Nile data, Egypt and carbonate reservoirs in Sarawak basin, Malaysia are presented in this study in a quantitative context. Perched water may lead to the incorrect interpretation of geology and petroleum system of an area. Multiple hydrocarbon/water contacts are interpreted as isolated pressure systems more often than not that leads to erroneous interpretations of compartmentalization with the ensuing downgrading of reserves and the reduction in economic viability of a field. With perched water, it is the various hydrocarbon/water contacts that are the anomaly and the whole complex is within one pressure system. Once perched water is recognized the economic viability of an area can be enhanced The construction of the MDT pressure data confirmed the presence of perched water which was different than the oil water contact or free water level for the same zone. A lot of change in reserve volume can be corrected based on our understanding the hydrodynamic system and presence of perched water system. Ramifications of the identification of perched-water contact for reserve calculations for the case studies are evaluated and presented based on hydraulic analysis of the water zone systems for each case. This study focuses on the importance of understanding the hydrodynamic system and the financial impact of perched water system on field economics.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015