Abstract

KOC planned to drill a challenging horizontal well in Ratqa field where there are serious limits to build deviation, reservoir navigation and hole instability. The candidate well was targeted into early Cretaceous Zubair reservoir, a highly laminated sand/shale sequence. Objective of this horizontal well is to target two highly depleted thin reservoirs (UCH and LCH) within Zubair, with a thick shale layer in between. While navigating the wellbore optimally through sweet spots within the reservoir units is a challenge, the risks of shale instability and differential sticking are other expected challenges during drilling. The target reservoirs sequence is around 8-10ft thick and there is 8ft thick interbedded shale separating the two sands. How to land the well properly inside the upper channel with optimum exposure of two sand units is one concern, and stability of interbedded shale at high angle is another concern. In order to mitigate these issues, pre drill geo-mechanical modelling was performed to predict the require mud weight and real time geo-mechanical monitoring was also carried out in conjunction with geo-steering. The resistivity contrast of these sands is around 70Ωm versus 10Ωm, which would give 9ft – 10 ft depth of detection (DOD) for azimuthal resistivity tool. This service would help to place the well optimally inside the reservoir and also provide early warning before exiting reservoir. The well was landed 7ft TVD inside the upper channel and continued building to 90° inclination in the lateral section with mud motor in order to build faster and also to prevent any stuck up with high configuration tool. The re-logged section confirmed that the wellbore already reached the base of upper channel. It was then decided to continue drilling to lower channel by crossing the interbedded shale using 9.5ppg mud weight in order to keep options open for running Inflow Control Devices (ICD). However, tight hole, over pull, pack-off, bit stalling, bigger size cavings etc. indicated poor wellbore condition. As drilling with lower mud weight already initiated shale instability it was then attempted to gradually increase the mud weight to avoid further deterioration of hole condition. It was then decided to sidetrack the section with a mud weight of around 12+ppg, as recommended by the pre-drill model. In addition, mud formulation was adjusted to block micro-fractures/invasion into shale to reduce differential sticking risk. Real-time wellbore stability monitoring was carried out including LWD log responses, cavings characterization, hole condition monitoring, etc. The wellbore was built to 89° inclination upon entering the lower channel as per correlation and was maintained inside the lower channel. The lateral section achieved 1,120ft long footage with a 78% exposure inside the sweet spot. Integrated real-time geo-steering and geo-mechanics successfully helped placing the well optimally in the target sands without hole instability.

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