Stabilizing Highly Fractured, Middle Creataceous Formations by Using Organic Fibres While Drilling an Exploration Well in Eastern Abu Dhabi
Specifically selected and manufactured organic fibres were used to bridge microfractures and stabilize shale and mudstone formations while drilling the Fiqa, Shilaif, Mishrif, Maddud and Nahr Umr formations and during a 5-day logging interval at the Mishrif formation.
A newly developed method for testing invasion depth in 20/40 gravel pack sand, in order to monitor the fluids’ bridging capability, was developed for drilling and logging the unstable Middle Cretaceous formations. The information from the tests was actively used to monitor and maintain the concentration of organic fibre materials in the drilling fluid to reduce invasion and to stabilise the highly fractured and layered shale and mudstone. A treatment programme was established around the data collected, using drilling fluids from the active system with a high concentration of fibres which was streamed back into the active system accordingly. The depletion rate was recorded and compared with the various formations during drilling.
The drilling fluid chosen for theproject was a water-based system, with 14%NaCl to balance the activity (AW) and formulated with a polyamine clay inhibitor. The section was planned with an extensive logging program in the Mishrif formation. Drilling had to be slow and controlled for parts oftheNahrUmr formation to pick the casing setting point in top Shuaiba formation.
The hard and brittle shales found in this group of formations have been problematic for drilling in the Arabian Peninsula for a long time. The Nahr Umr formation is particularly known for its loose structure and frequent hole collapses which are caused by fluid penetration along pre-existing fractures and laminated surfaces. Oil-based drilling fluids are the preferred systems of choice to reduce the capillary effect of invading fluids, and the consensus is to reduce the exposure time for the formations as much as possible.
As a water-based drilling fluid was chosen, a program was set up to focus primarily on a rapid build-up of a filter cake to bridge fractures and reduce filtrate invasion. The chosen organic fibres are anionic in nature and therefore have an affinity to the broken edges of clay platelets with a proven ability to create a network with particles that bridge the formation.
The 12 ¼” section was drilled without stability issues and without remedial back reaming to the logging depth. The top part of the 12 ¼” section was kept open for 5 days during logging. The Nahr Umr formation was drilled with a controlled, low ROP to identify the setting depth for the 9 ⅝” casing in top Shuaiba formation. Total open hole time was 4 days after logging.
The casing was cemented without losses or other operational issues. The use of specific organic fibres to stabilise highly fractured shale formations presents a low cost and efficient method for dealing with a high-cost problem associated with significant NPT.
Real time data collection and close monitoring of the fluids bridging capability using a fluid-invasion test kit proved to be an effective method for responding quickly to changes in hole stability, formation strength/integrity and fluid invasion.
A quick mix and bleed method of additions based on the data collected is presented as a key finding. The novel treatment programme is shown to give practicing engineers a system and toolkit that works when dealing with the known problems associated with the Nahr Umr shale formation.