Pre-Khuff Exploration Data Gathering Strategy-Learnings and Best Practices from Abu Dhabi Offshore Fields

摘要

Drilling deep exploration wells to Pre-Khuff formations requires considerable investment and therefore data gathering through logs and cores should be of highest quality to maximize value of information. The abrasive nature combined with high downhole temperatures and bit sticking present a uniquely hostile environment. In this paper, we discuss challenges in data acquisition and propose best practices for logging and coring programs tailored for Pre-Khuff exploration targets. The oldest Pre-Khuff stratigraphic unit penetrated offshore is Silurian Qusaiba equivalent. Apart from Permo-carboniferous Unayzah, the other stratigraphic units encountered in Abu Dhabi Offshore are Permian Basal Khuff Clastics (BKC), Early Carboniferous Berwath, Silurian Tawil /Sharawra Formations. In recent years, a number of deep exploration wells penetrated Pre-Khuff in various offshore fields, often under challenging high temperature and high pressure conditions. Considering extensive data gathering programs, this includes mud logging, LWD, wireline logging, and core extraction followed by detailed core analysis. Log data acquired through wireline is frequently impaired by wash outs and bad hole conditions, hence posing a challenge to assess rock properties reliably. Cores were successfully acquired as whole core and sidewall core and extensive core analysis programs were conducted. The key objectives for data gathering in Pre-Khuff are lithostratigraphy, sedimentology and age dating, petrophysical properties and prove hydrocarbon presence as well as geomechanical properties for hydraulic fracturing of the formation for subsequent well testing operations. Palynological analysis at well site is important for age determination. Correlation of mud logging results with open hole logs and Borehole image logs proved essential to determine hydrocarbon presence. The complex lithology mix is best resolved with elemental spectroscopy logging. LWD log acquisition is preferred over wireline because of a reduced risk of stick and pull and minimum invasion. However, some useful measurements such as Dielectric log are currently only available through wireline and industry should develop this tool for LWD. NMR tools generally cannot cope with the high temperature environment. Hence, service industry is encouraged to provide higher spec NMR tools. Due to low matrix permeability, core analysis for crushed rock or retort methods are preferred to obtain permeability measurements through pulse decay and pressure decay methods. Recommendations are given for optimal data gathering to ensure maximum value of information and best possible data quality. Logging operations should be a combination of Logging-While –Drilling and Wireline operations. Core acquisition will have to exclude BKC (Basal Khuff Clastic) to limit the risk of core jamming. A typical core analysis program to assess petrophysical and geomechanical properties for Pre-Khuff is proposed, leading to a successful risk assessment prior to hydraulic fracturing of the formation. Further lithostratigraphic and age-determination techniques are included in the scope.