Tapered Liner System for Zonal Isolation and Hydraulic Fracturing: An Application in the Kirthar Fold Belt Region
Authors: Hussain Muhammad Azhar , Abid Syed Asad , Khalid Muhammad Umar , Sarwar Muhammad Azeem , Shahid Sucklaj , Ahmad Hammad , Gorinak Mateusz , Shaikh Rashid Ali , Al Lawati Ibrahim 
Published: January 2024

This paper aims to describe the first ever successful deployment in Pakistan, of a 9-5/8″ × 7″ × 4-1/2″ Tapered Liner System with a provision of a lower PBR (rated at more than 16,000 psi) with a Vo rated Seal Stem Assembly to achieve zonal isolation and meet requisite design criteria for successful Hydraulic Fracturing of a tight reservoir zone. This insight empowers drilling Engineers to make informed decisions when selecting Liner Hanger Systems for Zonal Isolation and Hydraulic Fracturing ultimately optimizing drilling performance and wellbore stability as demonstrated in the paper.

The Pab sand Formation in the Kirthar Concession Block typically exhibits permeability less than 1 mD and is categorized as "Tight". This formation is drilled and hydraulically fractured to produce at commercial flowrates. Hydraulic Fracturing at pressures in excess of 16,000 psi Bottom Hole Pressure is required to achieve necessary formation stimulation. To solve these challenges, an innovative application of incorporating a 9-5/8″ × 7″ Liner Hanger assembly with the provision of tapering 7″ Liner string to 4-1/2″ string with a Lower PBR was considered.

A robust and compatible V0 rated Seal Stem Assembly stung into the lower Polished Bore Receptacle (PBR) above the 4-1/2″ liner segment would allow for DST or a Completions string to be run and completed as a monobore string. The Seal Stem Assembly would be a barrier containing the pressure loads generated due to various Production or Stimulation / Intervention cycles. The length of the seal assembly and the lower PBR were to be adjusted to allow for tube movement during these cycles.

The Tapered Liner system allows for Hydraulic Set or Wireline Set Packer to be set inside the 7″ Liner segment of the Tapered assembly as a barrier, protecting the Liner Top from the high pressure loads generated during Hydraulic Fracturing – mitigating for possible leak paths.

When this state of the art design was executed it has resulted in numerous deliverables to this efficient well plan. The Liner was rotated during the cement job allowing for excellent zonal isolation. The Seal Assembly was run in on a DST string and stung inside the lower PBR. The string was then pressure tested cyclically and the pressure was held successfully demonstrating robustness of the seal stem and the feasibility of the tube movement calculations. The subsequent operations included successful DST, Hydraulic Fracturing, and Well Completions operations.

In conclusions, this study has substantial implications for future drilling operations in Kirthar Block wells, as reservoir sections are mostly drilled in the 8-1/2″ hole section. Given the hole stability challenges in the formations drilled above reservoir, the Tapered Liner system further allows for a 6″ hole section in the reservoir zone, casing off challenging stressed formations separately in the 8-1/2″ hole section – as demonstrated further in this paper.