The use of Self Healing Cement to Ensure long Term Zonal Isolation for HPHT wells subject to Hydraulic Fracturing Operations in Pakistan
Authors: Muhammad Ali , Taoutaou Salim , Shafqat Ali Usman , Mustafa Noor Sameer , Salehpour Amir 
Published: March 2015

OMV (PAKISTAN) Exploration G m.b.H (along with its JV partners ENI, MND, OGDCL, PPL, and GHPL) has been actively in drilling and stimulating HPHT wells in Pakistan since 1991.

Hydraulic fracturing of HPHT wells is one of the most technically challenging operations for well cementing. The challenge is to preserve long-term integrity in these wells with temperatures exceeding 330°F, pressures exceeding 10,000 psi during the fracturing operations, and the presence of corrosive gas (CO2 and H2S).

Understanding these technical challenges is a key to designing cement slurries, especially for HPHT wells subject to hydraulic fracturing. Because cementing these wells using conventional cement and practices has not been effective, introducing new technologies and newer cementing techniques has been necessary to ensure the long-term sealing of the cement sheath in these wells.

This study covers the long-term zonal isolation challenges that HPHT wells subject to hydraulic fracturing are facing and the new technologies and techniques used to seal these wells by introducing self-healing cement using case studies from actual jobs.

Introduction

More than 90% of wells have been drilled in these two development fields with an average production of more than 500 MMcf/d from both Sawan and Miano. Most of the production was from northern part of the field, where permeabilities were good. The largely unexplored Sawan South (Figure 1), Miano Hot Sand, and Gambat areas have low permeability (recognized as tight gas) sand reservoirs with permeability ranging from 0.06 mD to 8 mD. The maximum prefrac gas rates was 3 MMscf/day from a well drilled in Lower Goru B Interval 1, where the production enhancement technique "hydraulic fracturing" was evaluated to enhance and sustain the gas supply for the plant. Successful production enhancement not only depends on acquiring accurate fracturing parameters (fracture closure pressure, fluid leakoff and efficiency, near-wellbore tortuosity, perforation friction, and far field fracture complexity) prior to the propped fracture treatment to achieve optimum treatment results, but also depends on the resistance of cement sheath from the deteriorating impact of downhole stresses.

These enormously (Maximum Mohr-Coulomb Stress: 4,210 psi) high stresses are generated in the set cement sheath due to the change in downhole pressures and temperatures during normal pressure testing to test the integrity of production packers or stimulation activity during production stage (i.e., acid stimulation and hydraulic fracturing treatment) 1

Conventional cement undergoing these stresses will fail in traction or compression to propagate tangential or radial cracks in the set-cement matrix for premature screenouts, fracturing into a water transition zone, creating interzonal communication, or finally leading to a path for hydrocarbons to migrate up the surface. Based on the above stresses, a flexible cement with enhanced set-cement mechanical properties was designed to overcome predefined stress cycles and taking into account the formation's compressive strength to preserve well integrity for a longer period of time. However; in the event the downhole stresses change, the flexible cement can no longer ensure the seal and will fail. To remedy to this, self-healing cement was used as a secondary barrier; when the self-healing cement came in contact with hydrocarbon, the sealing mechanism was automatically triggered without surface intervention.