Best Practices and Lessons Learnt from a Comparative Study of LWD Formation Pressure Measurement in Static and Circulating Wellbore Conditions in ERD Wells

Abstract

Pressure measurements in low permeability reservoirs are generally supercharged and cannot be used for mud weight optimization, selection and placement of in-flow control devices (ICD) in lower completion design and future reservoir management. The paper focusses on analyzing the effects of static and circulating wellbore conditions on the formation pressure data acquired using logging while drilling (LWD) tools in extended reach drilling (ERD) wells. Supercharging is commonly observed in low permeability zones mainly due to poor quality mud-cakes having low thickness and high permeability. Thus, pressure measurements using LWD tools read erroneous values almost equivalent to the annular circulating pressure. To reduce the effective overbalance and emulate wellbore environment for wireline logging, pressure measurements were performed in static conditions using LWD tools powered by batteries instead of downhole turbines. This paper discusses formation pressure datasets acquired using LWD tools in eight wells and thereby recommends best practices from the lessons learnt through the pressure testing campaign. By reducing the effective overbalance from circulation (ECD) to the static mud weight, the effect of supercharging on formation pressure measurements was reduced. However, in lower mobility ranges, less than 5mD/cP, the supercharging effects were prominent in static wellbore conditions as well. In such cases, using a sampling tool to pump out and relieve the supercharged pressure in the near-wellbore invaded zone is the best method to acquire accurate and reliable formation pressure data in such low mobility zones. ERD wells require formation pressure measurements done using LWD tools to mitigate pipe sticking risks and acquire formation evaluation data in a single pass resulting in saving rig time. While deploying batteries to acquire accurate formation pressure data in static wellbore conditions in every operation is a robust solution, it should be weighed against the formation testing objectives, borehole stability and differential sticking risks to avoid further down time in resolving these issues.